Turkish Marine Research Foundation (TUDAV), Istanbul, Turkey
P.P.Shirshov Institute of Oceanology, RAS, Moscow, Russia
The state of zoobenthos community structure and functioning may be considered as one of the most conservative indicators for assessing the structural and functional changes and thus its ecological health. In the 1960s, the northwestern shelf was used to be represented by very rich fauna and nourishing place for economically valuable fish species. The anthropogenic disturbances made this biocoenosis less vulnerable to the environmental changes in the 1970-1980s, and diminished its benthic populations particularly in the discharge regions of Danube, Dnieper, and Dniester Rivers. As a result, the zoobenthos community structure shifted to the dominance of smaller size hypoxia tolerant groups and opportunistic species that resulted in an increase in total zoobenthos abundance but decrease in total biomass. Degradation of benthic communities has further been intensified by other forms of pollution, impacts of exotic invaders and their unsustainable exploitation. Regarding to exotic invasions, wide diversity of biotopes and low species diversity of the Black Sea has provided favourable conditions for exotic invaders, which find unoccupied ecological niches without competitors and/or predators. The rate of alien species introductions has been constantly increasing and degrading benthic community structures.
The main characteristic features of the northwestern benthic ecosystem during the intense eutrophication phase may be summarized as follows (Gomoiu, 1992; Zaitsev and Mamaev, 1997): drastic decrease of the specific diversity; simplified zoobenthic community structures; decreasing abundance and biomass of benthic populations; reduction of biofilter strength of the system due to the loss of filter?feeder populations; qualitative and quantitative worsening of benthic biological resources, especially mollusks; flourishing of some opportunistic forms (especially worms causing sediment bioturbation);? invasion by some exotic species (Mya, Scapharca, Rapana etc.); severe disturbances in all benthic populations. The present chapter evaluates the status and trends of the Black Sea zoobenthos macrofauna after the 1980s and assesses its present state by focusing mainly on the western and northwestern littoral zones.
During 1973 ? 2005, nearly 4500 benthic stations have been executed in the NWS and along the coast of Crimea mostly in the shallow coastal zone, with less frequent sampling at depths deeper than 40 m in the 2000s. The quantitative development and long-term changes of macrozoobenthos were studied using the database of the Benthos Ecology Department of IBSS (Sevastopol) and referenced published materials. The taxa role and species importance were evaluated by the ?Index of functional abundance? (IFA) and ?Density indices? (DI): ; , where Bi , Ni? and pi are wet biomass (g m-2), abundance (ind. m-2) and occurrence (0 ? 1) of i taxon respectively. The approach to classification of bottom communities was based on the biomass determination of separate species dominating at stations (Vorobyov, 1949).
Bottom macrozoobenthos community of the Black Sea northwestern shelf (NWS) and of the Crimean Peninsula coastal zone have experienced major population changes and morphological anomalies in the 1970s and 1980s. Most notable changes were encountered in the north-northwestern coastal area including the Karkinitskiy Bay (Povchun, 1992; Black Sea biological ?, 1998; Shurova, 2003; Sinegub, 2006) and to a lesser extent along the western and southern coasts of the Crimean Peninsula (Zaika, 1990; Zaika et al., 1992; Petrov, Zaika, 1993; Kisseleva et al., 1997; Revkov et al., 1999; Zaika, Sergeeva, 2001; Makarov, Kostylev, 2002).
Fig. 8.1. Relative contribution of basic zoobenthos groups in the NWBS and at the Crimea coast during 1967 ? 2005 (without taking into account Oligohaeta and Turbellaria).
Bottom zoobenthos fauna of the northern and northwestern coastal zone of the Black Sea has the Mediterranean-Atlantic origin (Mordukhay-Boltovskoy, 1972). It includes 419 species in the NWS (Sinegub, 2006) and nearly 600 species along the coast of Crimea (Revkov, 2003a). However, the recent studies did not focus in sufficient detail on the taxonomy of Porifera, Coelenterata, Nemertini, Turbellaria, and Oligochaeta groups. They have only shown an increase of Oligohaeta species number from 4 before 1967 up to 29 species (Shurova, 2006). For convenience, the Turbellaria, Oligohaeta and Insecta (larvae) groups are excluded from the taxonomical structure of the macrobenthos fauna listed in Table 8.1. The list comprised 363 species in the NWS for 1967 ? 2005 as compared to 299 species before 1967 and 271 species during 1973 ? 2003. The most numerous group in the NWS was Crustacea (30 %) followed by Mollusca (23 %), Polychaeta (23 %) and ?Varia? (24 %) (Fig. 8.1). The Crimean coastal zone has richer bottom fauna including 574 macrozoobenthos species (Table 8.1) that was formed by 28% Molluscа, 26% Polychaeta, 27% Crustacea and 19% Varia (Fig. 8.1).
These regionally-averaged values, however, differ considerably in the different parts of the NWS and Crimean Peninsula (Fig. 8.2). For example, 209 species were identified in the Dnepr-Bug estuary at 2061 stations, 161 on the shelf between Danube and Dnestr at 674 stations, 166 in Karkinitskiy Bay at 115 stations, and 107 in the Central part of the NWS at 46 stations (Sinegub, 2006). The southern Crimean coastal zone near Karadag comprised 367 macrozoobenthos species comparable to 358 species in the Sevastopol Bay (Revkov, 2005). In Dnepr-Bug and Danube-Dnestr estuary areas, Crustacea were the dominant group and constituted 39 ? 40 % of the total species number (Fig.8.2). The ?Varia? group had the largest share (24 %) in the Sevastopol Bay.
When the marine forms of main taxa (Porifera, Coelenterata, Bryozoa, Polychaeta, Molluscа, Crustacea, Echinodermata, Tunicata) are only considered in waters with average salinity of 18 ?, the Crimean fauna is represented by 484 macrozoobenthos species before 1975 versus 531 species during 1980 ? 2005 (Table 8.1). Therefore, the number of benthic species in the Crimean coastal zone has not decreased during the last decades. Instead, it was enriched due to 1) expansion of some species, 2) introduction of new forms, previously observed only in the pre-Bosporus region, 3) introduction and population outbreaks of alien species, 4) more detailed analyses of some systematic groups.
Fig. 8.2. Species numbers (in %) of basic zoobenthos groups in different regions of the Ukrainian sector of the Black Sea. The data used for the NWBS correspond to 1973 ? 2003. For Sevastopol and Karadag, the measurement period covered observations prior to 1973.
So far, the group of Hydroids of Crimea was replenished by the species Coryne pusilla (Gaertner, 1774), Eudendrium annulatum Norman, 1864, E. capillare Alder, 1857, Opercularella nana Hartlaub, 1897 and Stauridia producta Wright, 1858; in the group of polychaetes new for the Crimean fauna in 1980 ? 2005 became Caulleriella caput?esocis Saint?Joseph, 1894, Euclymene palermitana Grube, 1840, Glycera gigantea Quatrefages, 1865, Hypania invalida (Grube, 1860), Nereis rava Ehlers, 1868, Notomastus latericeus Sars, 1851, Pectinaria belgica (Pallas, 1766); new for the science species were described, such as ? Nerilla taurica Skulyari, 1997 and Vigtorniella zaikai (Kisseleva, 1992); crustaceans group was replenished by Chthamalus montagui Southward, 1976, Colomastix pusilla Grube, 1861, Cumopsis goodsiri (Van Beneden, 1868), Pseudocuma graciloides G.O. Sars, 1894, P. tenuicauda (G.O. Sars, 1893), Schizorhynchus scabriusculus (G.O. Sars, 1894), Orchestia platensis (Kroyer, 1845), Parhyale sp., Micropythia carinata (Bate, 1862). Of Pantopoda this is Anoplodactylus petiolatus (Kroyer, 1844); of Bryozoa these are Electra crustulenta (Borg, 1931), Schizoporella linearis (Hassall, 1841) and Victorella pavida Kent, 1870. The most numerous additions appeared among mollusks: Anadara inaequivalvis (Bruguiere, 1789), Clausinella fasciata (Costa, 1778), Mya arenaria Linnaeus, 1758, Doridella obscura Verrill, 1870, Hydrobia aciculina (Bourguignat, 1876), H. procerula Paladilhe, 1869, Melaraphe induta (Westerlund, 1898), Mutiturboella cornea (Loven, 1846), Pontiturboella rufostrigata (Hesse, 1916), Pseudopaludinella cygnea Anistratenco, 1992, P. leneumicra (Bourguignat, 1876), Pusillina obscura (Philippi, 1844), Thalassobia rausiana (Radoman, 1974), Th. coutagnei (Bourguignat in Coutagne, 1881), Tricolia pulchella (Recluz, 1843), T. tricolor (Bucquoy, Dautzenberg et Dollfus, 1884), Steromphala crimeana Anistratenco et Starobogatov, 1991, Bittium jadertinum (Brusina, 1865), B. scabrum (Olivi, 1792), Cerithium spinosum Philippi, 1836, C. gracilis Philippi, 1836, Truncatella desnoyersii (Payraudeau, 1826) and T. truncatula (Draparnaud, 1805). The ?enrichment? of the gastropods took place mostly due to their taxonomical revision.
In parallel with the enrichment of the Crimean macrozoobenthos fauna, some traditionally rare species have not been observed since 1980s. This may be due to their small populations and inadequate sampling, as well as due to difficulties in identification of some specific groups (for example, Nemertini, Porifera, Turbellaria, Oligohaeta), and insufficient analysis of various biotopes. The species which were not observed along the coast of Crimea in 1980 ? 2005 included for Anthozoa: Synhalcampella ostroumowi Wyragevitch, 1905; for Crustacea: Palaemon serratus (Pennant, 1777), Chelura terebrans Philippi, 1839, Eurydice pontica (Czerniavsky, 1868), Jaera hopeana Costa, 1853 and Limnoria tuberculata Sowinsky, 1884; for Pantopoda Ammothea echinata (Hodge, 1864); for Mollusca: Cuthona amoena (Alder et Hancock, 1842), Doris ocelligera (Bergh, 1881), Embletonia pulchra (Alder et Hancock, 1844), Eulimella scillae (Scachi, 1835), Limapontia capitata (Muller, 1773), Parhedyle tyrtowii (Kowalewski, 1900), Pontohedyle milaschewitschi (Kowalewski, 1901), Pseudovermis paradoxus Perejaslawtzeva, 1890, Tergipes tergipes (Forskal, 1775) and Trinchesia foliata (Forbes et Goodsir, 1839); for Bryozoa: Aetea recta Hincks, 1880 and Bowerbankia caudata (Hincks, 1877). Most of these species were marked earlier as rare.
During 1980 ? 1990s along the coast of Crimea, the highest species diversity (242 species) was found at the coastal and relatively shallow depths of 11 ? 20 m where mollusks were the most diverse group (81 species). Crustaceans and annelids (74 and 80 species, respectively) had the highest diversity at the depths of 0 ? 10 m, and the fauna of miscellaneous species (35) - at the 21 ? 30 m depth range. The presence of more various bottom fauna under the conditions of the rather shallow coastal zone was accompanied by a greater variety of habitats. For the whole observation period, in Crimean waters 55 macrozoobenthos species were found at depths of 100 m and deeper. These were 19 species of the group Annelida, 18 Mollusca, 7 Arthropoda, 4 Coelenterata, 3 Echinodermata, 2 Ascidiacea and by one species from Nemertini и Porifera. More than half of them are ?occasional?, and only 26 species can be attributed to ?usual? for these depths (Revkov, 2003c). According to M.I. Kisseleva (2004) singular specimens of polychaetes Aricidea claudiae, Nephtys sp., Melinna palmata, Heteromastus filiformis, Terebellides stroemi, Oriopsis armandi were registered at the depth of 200 m off the southern coast of Crimea.
In deep waters (40 m and deeper) 3 ? 5 fold ?visual? reduction in macrozoobenthos species diversity was found in the 1980s as compared to the 1960s (compare the curves 1 and 2 in Fig. 8.3, Zaika 1990). However, in 2003 new data for the period 1980 ? 1990s were digested, essentially modifying the understanding of benthos communities development at depths 50 ? 120 m. Undoubtedly, the new data showed no real reduction in the total number of benthic species in deep waters, but decrease in their occurrence on the Crimean shelf. In other words, the detection of missing rare species became possible only after analyses of considerably greater number of stations (Fig. 8.3, line 3). At approximately equal number of stations executed at the coast of Crimea both in 1960s and 1980s (nearly 100 stations) the decrease in species population density in 1980s lead to the effect of seeming reduction in diversity.
In a similar way, 3 ? 4 fold seeming reduction of bottom fauna diversity was found on the northern Black Sea shelf during 1980 ? 1990s (Black Sea biological ?, 1998). In reality, decrease in population density of some species (rare chance to find them in the samples) occurred, but they had not disappeared from the local fauna completely.
Fig. 8.3. Changes in macrozoobenthos species diversity on the soft bottoms along the coast of Crimea during different periods.
In 2005 ? 2007, the total number of species in shallow NWS waters varied in the range of 30 ? 60, the lowest values found in the Danube Delta and Odessa Bay regions, and twice higher diversity in Yagorlytskiy and Tendrovskiy Bays (Fig. 8.4). Crustacea and Polychaeta had almost equal and highest contributions, Bivalvia came the second, and the Gastropoda group was represented by lowest number of species at all sites of observations. However, in terms of biomass, the Bivalvia group dominated entirely with: ? 500 g m-2 in the Danube Delta and Odessa Bay and around 1000 g m-2 in Yagorlytskiy and Tendrovskiy Bays, reaching 2000 g m-2 in the Yagorlytskiy Bay during 2007 (Fig. 8.5).
Fig. 8.4. Total number of macrozoobenthos species in different areas of the NW Black Sea:
?blue ? Bivalvia, black ? Gastropoda, green ? Crustacea, red ? Polychaeta (from Ukrainian National Report, 2007).
8.2.2. Biocenoses and quantitative development of bottom fauna: During 1983 ? 2003, nearly 19 types of bottom biocenoses were described in the NWS (Table 8.2), most of which were autochthonous (Sinegub, 2006). Heteromastus filiformis, Pontogammarus maeoticus, Paphia aurea, Orchestia cavimana, Anadara inaequivalvis, Irus irus and Donacilla cornea were rather new biocenoses. The biocenoses of Mya arenaria and Anadara inaequivalvis were formed by introduced species, and the biocenoses of Neanthes succinea and Heteromastus filiformis were formed temporarily as a result of near-bottom suffocation (Sinegub, 2006).
Using the results of benthos surveys carried out in 1980 ? 2004 along the coast of Crimea, about 50 bottom biocenoses can be described. According to the biocenotical classification suggested by Kiseleva and Slavina (1972), the biocenoses of Mytilus galloprovincialis, Modiolula phaseolina and Chamelea gallina were the most important and widespread ones presented on the maps as concentric zones along the coast. The other biocenoses were of more local origin occupying small areas in the region. The belt-community of Modiolula phaseolina extended from the mid-shelf (~50 m depth) to the shelf break (at 120 ? 135 m depths) where mollusks were found in the form of fine spots (Zaika et al., 1992). Modiolula phaseolina at depths more than 100 m was represented mainly by juvenile forms and its presence was even extended to the sub-aerobic zone at ~180 m (Yakubova, 1948; Kisseleva, 1985; Zaika and Sergeeva, 2001).
The lower boundary of aerobic benthos along the Crimean coast, defined by the position of oxic/anoxic interface, forms the Periazoic belt (depths about 115 ? 180 m). It is inhabited by a specific community of polychaete (Vigtorniella zaikai, Kiseleva, 1992), Protodrilus sp., specific hydroid and foraminiferan species, not studied in detail so far (Fig. 8.6) (Zaika, 1998). The periazoic belt was also found in the north-western part of the Black Sea (Bacesco et al., 1965). It is, however, not known whether the periazoic community forms ring-belt around the entire sea.
The belt of silt mussels Mytilus galloprovincialis was limited by the depth range from 30 ? 40 m to 50 ? 60. At depths less than 30 ? 40 m the Chamelea gallina belt community is located. Here the benthic habitat becomes more heterogeneous and higher number of factors impact on the distribution of benthic animals. Communities of this zone become more and more patchy. Regional differences in species composition are here more pronounced and in this zone (less than 30 ? 40 m) each benthic belt is a habitat of different local communities (Zaika, 1998).
Fig. 8.6. Benthic belts of the Black Sea shelf (from Zaika, 1998, with additions).
The list of leading species on the NWBS shelf is headed by Mytilus galloprovincialis, Mya arenaria and Neanthes succinea (Table 8.2), on the Crimean shelf ? by Chamelea gallina, Mytilus galloprovincialis and Modiolula phaseolina (Table 8.3). All three main biocenoses of the Crimean sea shelf are belts. In the biocenosis of Mytilus galloprovincialis, occupying large areas in the NWS, the greatest number of species (163) was registered. Significant part of the sampling stations in the Dnepr-Bug and the Danube-Dnestr marine areas (both belonging to the biocenosis of Mytilus galloprovincialis) was executed outside the suffocation zone in the depth range of 4 ? 10 m (Sinegub, 2006). At this depth range of the Dnepr-Bug marine area, the abundance and biomass of benthos exceeded 10000 ind. m-2 and 10 kg m-2, respectively. The lowest average values of abundance (1548 ind. m-2) and biomass (462.2 g m-2) were measured in the biocenosis of Mytilus galloprovincialis of the central NWS where the maximal length of mussels did not exceed 40 mm.
Leading species of biocenoses
Number of stations
Number of species
Average abundance, ind. m‑2
|Mytilus galloprovincialis Lamarck, 1819||1984 ? 2003||526||4 ? 45||163||2810||1486.7||95.3||NWBS|
|Mya arenaria Linnaeus, 1758||1984 ? 1999||244||6 ? 29||87||1630||217.1||82.1||DB, DD|
|Neanthes succinea (Frey et Leuckart, 1847)||1984 ? 2003||132||7 ? 29||46||1124||24.2||52.9||DB, DD|
|Heteromastus filiformis (Clapar?de, 1864)||1988 ? 2000||57||7 ? 25||25||352||2.8||65.7||DB, DD|
|Pontogammarus maeoticus (Sowinskyi, 1894)||1992 ? 2001||39||0 ? 1||9||8231||66.8||99.7||DB, DD|
|Cerastoderma glaucum Poiret, 1789||1988 ? 2000||31||1 ? 23||80||2025||86.7||60.4||DB|
|Mytilaster lineatus (Gmelin, 1791)||1988 ? 2000||28||1 ? 11||99||3774||415.1||42.0||DB, Kark.|
|Melinna palmata Grube, 1870 ?
?Nephtys hombergii Savigny, 1818
25 ? 35
|Paphia aurea (Gmelin, 1791)||1990||18||20 ? 31||29||210||41.2||49.3||Kark.|
|Nephtys hombergii Savigny, 1818||1984 ? 1990||16||2 ? 35||31||220||5.7||20.3||DB, DD,
|Orchestia cavimana Heller, 1865
(Syn. O. bottae M.-Edwards, 1840)
|1992 ? 1994||12||0||4||2108||12.3||95.9||DB|
|Lentidium mediterraneum (Costa, 1829)||1983 ? 1993||11||1 ? 6||30||9035||78.0||63.9||DB, DD|
|Chamelea gallina (Linnaeus, 1758)||1985 ? 2000||10||6 ? 26||65||1203||532.3||72.5||Kark.|
|Modiolula phaseolina (Philippi, 1844)||1985 ? 1986||6||49 ? 54||30||762||93.||59.2||Centr.|
|Melinna palmata Grube, 1870||1994 ? 1999||5||15 ? 19||15||974||48.5||73.0||DB|
|Anadara inaequivalvis (Bruguiere, 1789)||1992 ? 2003||5||6 ? 11||8||2533||198.6||87.4||DD|
|Irus irus (Linnaeus, 1758)||1988||3||2 ? 4||49||6567||1168.0||44.5||DB (Е)|
|Balanus improvisus Darwin, 1854||1983||3||1 ? 2||24||6251||213.7||73.6||DB|
|Donacilla cornea(Poli, 1795)||1992||2||0 ? 0.5||7||17800||88.6||80.7||DB (Т)|
Leading species of biocenoses
|Chamelea gallina (Linnaeus, 1758)||1981 ? 2004||157||1 ? 32||190||2547||494.9||75.8||Crimea|
|Mytilus galloprovincialis Lamarck, 1819||1980 ? 2001||86||1.5 ? 80||215||1767||670.6||77.6||Crimea|
|Modiolula phaseolina (Philippi, 1844)||1982 ? 1999||38||45 ? 110||68||596||31.2||63.4||Cr 2, 3, 4|
|Cerastoderma glaucum Poiret, 1789||1993 ? 2004||27||0.5 ? 17||106||3092||115.0||62.6||Cr 2|
|Terebellides stroemi Sars, 1835||1981 ? 1999||25||15 ? 136||49||338||5.4||64.7||Crimea|
|Pitar rudis (Poli, 1795)||1982 ? 1999||21||4 ? 70||111||1648||74.6||51.7||Crimea|
|Nassarius reticulatus (Linnaeus, 1758)||1982 ? 2001||21||1 ? 28||146||2218||60.7||47.8||Cr 1, 2, 4|
|Mytilaster lineatus (Gmelin, 1791)||1994 ? 2004||19||1 ? 16||127||5006||122.5||59.9||Cr 2, 4|
|Modiolus adriaticus (Lamarck, 1819)||1983 ? 2000||18||3 ? 40||104||2171||300.1||54.3||Crimea|
|Diogenes pugilator Roux, 1828||1983 ? 1998||10||2 ? 20||32||709||9.8||74.0||Cr 2 ? 4|
|Paphia aurea (Gmelin, 1791)||1980 ? 2000||8||4 ? 26||60||742||116.8||52.5||Cr 1, 2|
|Amphiura stepanovi Djakonov, 1954||1983 ? 1990||7||60 ? 106||24||414||8.5||37.9||Cr 3, 4|
|Abra ovata (Philippi, 1836)||1993||6||1 ? 12||40||2147||146.7||59.8||Cr 2|
|Nephtys hombergii Savigny, 1818||1987 ? 2001||6||10 ? 55||29||710||7.4||50.1||Cr 1?3|
|Parvicardium exiguum (Gmelin, 1791)||1989 ? 2004||6||6 ? 25||45||2113||81.9||49.2||Cr 2|
|Balanus improvisus Darwin, 1854||1994 ? 2001||4||12 ? 17||33||2385||17.9||43.5||Cr 2|
|Abra nitida milachewichi Nevesskaja,||1980 ? 1989||3||8 ? 35||30||276||47.1||61.1||Cr 1|
|Ascidiella aspersa (Muller, 1776)||1992 ? 1994||3||32 ? 52||29||636||266.3||54.4||Cr 2|
|Gouldia minima (Montagu, 1803)||1983 ? 1990||2||11 ? 30||17||570||212.2||60.4||Cr 3, 4|
|Loripes lacteus (Linnaeus, 1758)||2000 ? 2004||2||3||28||1364||39.8||79.4||Cr 2|
The transformation of NWS mussel settlements during the 1980s and 1990s was caused by periodic suffocations of bottom fauna, destruction of bottom zoocenoses, and silting of substrata under the impact of large-scale trawling (Shurova, 2003). All of these led to the development of a more simplified population structure of mussel settlements (Shurova, Stadnichenko, 2002), which did not change considerably during the last decade. In summary, the present status of NWS bottom fauna exhibits radical changes in species composition, abundance and biomass both at the species and community level (Sinegub, 2006). The benthos trophic structure was simplified by a sharp reduction in carnivorous and phytophage abundances, domination of detritovorous species by abundance and of sestonophages by biomass in the brackish waters areas of the NWS shelf (river influenced), which experienced the strongest damage by suffocations.
Fig. 8.7. Vertical profiles of the total zoobenthos biomass in the biocenosis Chamelea gallina (A) (according to 157 stations) and the biomass Chamelea gallina (B) (310 stations) at the coast of Crimea for the period 1980 ? 1990s.
Fig. 8.8. Vertical profiles of the total zoobenthos biomass in biocenosis Mytilus galloprovincialis (A) (according to 86 stations) and the biomass Mytilus galloprovincialis (B) (370 stations) at the coast of Crimea for the period 1980 ? 1990s.
In 1980 ? 1990s along the Crimea coast, the maximal average macrozoobenthos biomass in the biocenoses of Chamelea gallina (~520 g m-2) was registered within 0 -10 m depths (Fig. 8.7a) and of Mytilus galloprovincialis (~900 g m-2) within 10 ? 20 m depths (Fig. 8.8a). Previous locations of maximum macrozoobenthos biomasses in the biocenoses of Chamelea gallina and Mytilus galloprovincialis (1960 ? 1970s) were, however, at 25 m and 40 ? 45 m accordingly (Kiseleva, 1981). In 1980-1990s the maximal total macrozoobenthos biomass, allocated at depths of 10-20 m, did not coincide in space with the maximum of dominant species biomass observed in the biocenosis of M. galloprovincialis at 20-40 m depth (Fig. 8.8).
8.2.3. Long-term changes in quantitative development of the soft-bottom fauna: Analysis of long-term data for the period 1930s ? 2000s, (Chigirin, 1938; Kisseleva, 1981; Zaika, 1990; Zaika et al., 1992; Kisseleva et al., 1997; Black Sea biological ?, 1998; Revkov and Nikolaenko, 2002; Mironov et al., 2003; Mazlumyan et al., 2003; Sinegub, 2006; Revkov et al., 2008) evidences that the bottom fauna in the Ukrainian Black Sea has improved slightly (or, at least, has not worsened) during the last two decades in comparison with the 1970s. The areas below the ?offshore waters? and the ?Sevastopol Bay? of the western Crimea were used as examples to delineate the changes in the zoobenthos characteristics.
Fig. 8.9. Long-term dynamics of zoobenthos biomass in Sevastopol Bay ? with consideration of all macrozoobenthos (A), and without Mytilus galloprovincialis which is not a typical soft bottom species of the Sevastopol Bay (B).
Sevastopol Bay: The data from 1920s and 2000s reveal pronounced changes in the development and occurrence of certain benthic forms (Revkov et al., 2008). For example, the most common forms of macrozoobenthos carnivorous Nassarius reticulatus and Nephtys cirrosa in 1920s with the occurrences of 92% and 89%, respectively, were replaced in 2001 by the detritus-feeders Heteromastus filiformis (91%) and Cerastoderma glaucum (85%), with Nassarius reticulatus remaining as a sub-dominant form observed only at several near-shore bottom sites. The alteration of dominant forms indicates qualitative changes in the flow of organic matter in the benthic ecosystem. Moreover, a pronounced increase in the share of seston-feeders was also found in the 1980s (Mironov et al., 2003).
As shown in Fig. 8.9a, the total zoobenthos biomass decreased by half during the eutrophication period of the 1970s and 1980s as compared to the pristine state, and remained below 50 g m-2 until the early 1990s, and then experienced a marked increase to 300 g m-2 in the mid-1990s. The latter was followed by a decrease towards the background values in 2000s (Mironov et al., 2003; Revkov et al., 2008). When Mytilus galloprovincialis biomass was excluded from the total biomass data, since it was not a typical species for the soft bottom fauna of the Sevastopol Bay, the zoobenthos biomass tended to have a more gradual increase from ~20 g m-2 in the 1980s to 100 g m-2 at the end of 1990s and then a marked decline in the 2000s (Fig. 8.9b).
Offshore waters of the western Crimea: Similar long-term structural changes in the zoobenthos biomass and abundance were also observed in the offshore waters of the western Crimea. Here, obvious excess of the average biomass was observed in the 1990s at the depths of 1 ? 12 m (462?154 g m-2), 13 ? 25 m (476?97) and 26 ? 50 m (353?180). For the time being the data of 2000s testify to decreasing of the total zoobenthos biomass towards the level of 1950 ? 1980s (Fig. 8.10).
The most significant macrozoobenthos species (by Density index) on the soft bottoms during various periods of study were Chamelea gallina (1 ? 12 m depths), Chamelea gallina and Gouldia minima (13 ? 25 m), Mytilus galloprovincialis, Pitar rudis and Terebellides stroemi (26?50 m), Modiolula phaseolina and Terebellides stroemi (51 ? 103 m) (Fig. 8.11).
In terms of ?Density index?, Chamelea gallina abundance at the 1 ? 12 m depth range remained constant (about 25) up to 1980s and then experienced a sharp increase to 300 in the 1990s and decline afterwards to 137 in 2000s (Fig. 8.12). Chamelea gallina abundance, also dominating at the 13 ? 25 m depth range, changed from 53 in 1980s to 176 in 1990s and dropped to 102 in 2000s. Similarly, Mytilus galloprovincialis density index rose abruptly from less than 5 to 153 during the same phase at the 26 ? 50 m depth range, but its subsequent trend is not known due to the lack of data. Thus, Chamelea gallina and Mytilus galloprovincialis became the most optimal zoobethos forms in the 1990s. On the contrary, the density index of Modiolula phaseolina, which was the dominant species at the 51 ? 103 m range in 1950s, decreased from 53 to less than 1 in 1970s and then remained at this level during 1970s ? 1990s.
We assume that one of the possible factors determining the modern state of benthos at the western Crimea, at depths more than 50 m, is the ?near-bottom? trawling. The areas of most intensive trawling at the coast of Crimea are its western and southern parts with depths 45 ? 100 m where steady groupings of sprat are formed (Zuev, Melnikova, 2007). The underwater video observations executed in 2005 at the western Crimea (area off the river Kacha) have shown presence of essential anthropogenic pressure on the bottom seascapes, indicated by numerous traces of ?near-bottom? trawling (Boltachev, 2006).
During the period 1930s ? 1990s similar changes took place in the benthos along the south-western coast of Crimea. Here, obvious excess of the average biomass was observed in the 1990s at depths of 13 ? 25 m (800 g m-2) and in the 1930s at depths of 26 ? 50 m (667 g m-2). In all other cases it is possible to conclude practically comparable levels of benthos development on the mentioned depths during all periods of observations (Fig. 8.13). Such filter-feeding mollusks as Chamelea gallina, Spisula subtruncata, Paphia aurea, Mytilus galloprovincialis and Modiolula phaseolina are the most pronounced ?evolutioning? organisms, determining the revealed quantitative alterations in this region and the mode of functioning of benthic communities during all periods of observation.
Fig. 8.12. Long-term changes of ?Density index? values with respect to populations of Ch. gallina, M. galloprovincialis and M. phaseolina at the western coast of Crimea. The ?Density index? axes on the left and right are marked by L and R, respectively, in the figure.
In the Lisya Bay, located about 3 km to the west of Karadag (southeastern coast of Crimea), the number of species of all dominant trophic groups increased in 1973 - 1998, and the total species number changed from 56 to 93. Consequently, the average benthos abundance increased from 395 to 7066 ind. m-2 and the biomass from 35.66 to 778.44 g m-2 (Mazlumyan et al., 2003). These changes were further accompanied by reduction in soft bottom areas and expansion of the macrophytes zone. The latter affected the qualitative structure of macrofauna by increasing crustaceans and phytophilous species number. The role of filter-feeders increased markedly due to the domination of Chamelea gallina. Based on the survey conducted in summer 2008, preliminary results suggested that the total biomass of the Lisya Bay benthic fauna is approaching to the values of 1970s.
Fig. 8.13. Index of Biomass of benthic groupings per different years. Range of depths in groupings: a ? (sandy zone) ? 1 ? 12 m, b ? (silty-sand) ? 13 ? 25 m, c ? (mussel silt) ? 26 ? 50 m, d ? (phaseolina silt) ? 51 ? 110 m.
Thus, the tendency of sharp benthos biomass increase during the 1990s appears to be a common feature for many coastal waters of the northern Black Sea, related to the adaptation of the benthic ecosystem to increasing organic pollution. The filter-feeding molluscs Chamelea gallina and Mytilys galloprovincialis (in offshore waters) and Cerastoderma glaucum (in the internal part of the estuarine water areas) became the most abundant forms and altered the benthic assemblages structure along the Ukrainian coastal zone. Recent data from the 2000s testify to reduction in the total zoobenthos biomass towards the level of the 1970 ? 1980s, possibly due to a reduction in abundance of pollution resistant species.
?Almost 800 taxa of benthic invertebrates have been identified in Romanian coastal waters between 1960 and 1970 (Bacescu, et al., 1965), a major portion of which belonged to meiobenthos. The lack of taxonomic studies after 1970, during the period of serious ecological disturbances in the region, however resulted in a gap in the zoobenthic diversity studies.
The most widespread biocoenosis Lentidium (Corbula) mediterraneum in the 1960s northern Romanian coastal zone was represented by very rich fauna (over 100 taxa, mostly molluscs and meiobenthic species), high abundance (> 100000 ind.m-2) and biomass (> 50 g.m-2) (Bacescu et al. 1957, 1965). Because of this richness, the biocenoses used to represent a nourishing place for economically valuable fish species. The anthropogenic disturbances however made this biocoenosis less tolerant to the environmental changes in the 1980s and 1990s, diminished its populations, and dropped the abundances from more than 20000 ? 30000 ind.m-2 in the 1960?s to 3000 ind.m-2 in the 1980s. Similarly, the density of Spio decoratus, an important polychaete of this biocenosis, decreased from 30000 ? 50000 ind.m-2 to less than 1000 ind.m-2. At the same time, some new opportunistic species (e.g. polychaetes Neanthes succinea and Polydora limicola, bivalves Mya arenaria-soft clam and Scapharca cornea; syn. Anadara inaequivalvis) have appeared and started dominating eutrophic areas (Gomoiu, 1976, 1985; Tiganus, 1988).
The development of soft?shell clam Mya arenaria in sandy infralittoral zones of Romanian shallow waters has been an important ecological event. Following its settlement, Mya has become a mass species with the average density of 1037 ind.m-2 and biomass of 1936 g.m-2 in 1970-1971. It dominated other molluscs and replaced the aboriginal mass species Lentidium mediterraneum community sensitive to ecological changes in the 1970-1980s (Petranu and Gomoiu, 1972). As an opportunistic species with a high capacity for regeneration, Mya arenaria was able to take advantage of consuming increasing quantities of organic matter available in the environment.
The biocoenoses of coarse sands in the mediolittoral of southern zone was characterized by the bivalve Donacilla cornea (syn. Mesodesma corneum) and sometimes associated with the polychaete Ophelia bicornis in the 1960s. Both of these species have not been recorded in the subsequent decades, but the bivalve Donacilla cornea was registered again in 2004 (Micu and Micu, 2006). In addition to pollution, coastal engineering constructions (dams, barrages) have also caused scarcity of D. cornea population and the polychaete O. bicornis disappearance from the shallow water bottoms. Invaders from the upper infralittoral Idotea baltica, Gammarus subtypicus and G. olivii occupied their niche and became mass species.
Rocky substrata forming only 0.3% of the total sea floor area of the Romanian shelf have included ecologically important benthic communities, the biocoenosis of the Mytilus galloprovincialis being the most important.
Hard substratum constituted the most complex environment in the benthic realm with the greatest diversity of fauna, including over 40% of the total identified species and 2.5% of the whole fauna stock of the Romanian littoral.
In the present decade, a survey in the benthos rocky zones indicated a slight decline in biodiversity, mostly in the crustacean community, which has been observed since the beginning of the 1980?s. Twenty years ago, Jassa ocia and Erichtonius difformis accounted for 45% and 30% of the total abundance of amphipods, respectively. Research conducted between 1993 and 1998 revealed that E. difformis accounted for 12% while J. ocia 9% of the total amphipods abundance. Concerning the decapods, four crab species (Pachygrapsus marmoratus, Pilumnus hirtellus, Xantho poressa and Rhitropanopeus harissi tridentatus) were found in rocky zones. Both Pachygrapsus and Rhitropanopeus were still numerous, especially their juvenile individuals in the rocky zones in the southern littoral. Xanto poressa had a smaller distribution than in the 1980s. The large?size decapods species, such as Crangon crangon, the shrimps Palaemon elegans and P. adspersus constituted mass species in the past. Now, P. adspersus is considered as endangered and rare species.
?Apart from eutrophication and pollution, the main cause of these changes was the reduction in macrophyte fields, mainly of the perennial alga Cystoseira barbata habitat. The range of vagile fauna had shrunk and there had been severe reductions in populations of phytofile species (Tiganus & Dumitrache, 1995). Another negative ecological impact was the penetration of the predator gastropod Rapana venosa, originating from the Sea of Japan known to be a predacious enemy for the littoral malacofauna. It firstly appeared in the Danube estuaries and rapidly spread southward and became a common element in shallow waters both on sandy and rocky bottoms (Gomoiu, 1972). This gastropod species was found most abundantly and frequently on rocky bottoms between 4 m and 10 m isobaths with a maximum density (up to 10 ? 12 ind.m-2) at 8 m depth. Because of its high consumption of bivalves, especially Mytilus and Mya, it played a key role as natural biofilters.
The benthic communities of muddy bottoms have been influenced by numerous factors, including increased water turbulence and sedimentation. High load of alluvial deposits carried by the Danube River continually modified the substrata and induced instability.
Two subcoenoses in front of the Danube mouths at depths between 15 and 50 m included sandy-muddy type bottom (15?30m) and muddy type bottom (30 - 50m depth) with mussels Mytilus galloprovincialis. Hypoxia events associated with frequent and intense phytoplankton blooms caused mass mortality and impoverishment of many species of these subcoenoses (Gomoiu, 1981, 1983). Out of 32 species existed at 10-30 m depth range in 1975-1977, 22 remained in 1979, and only 14 in 1980 (Tiganus, 1982).
The muddy biocoenosis was considered to be the richest biocoenosis in the entire sector containing 50 different types of organisms among which Mytilus was the most dominant between 1960 and 1965. The presence of Phyllophora field in front of the Danube mouths at depths of 40 m played a significant role on the enrichment of the benthic fauna. When Phyllophora populations had declined to the point of extinction, the biodiversity started degrading during the 1976?1980; the most affected species became molluscs and crustaceans. Crustaceans reduced from 15 in 1977 to 2 in 1980 and molluscs declined from 20 to 4 over the same period. On the other hand, the populations of some opportunistic species have proliferated and become dominant in some communities such as: Mya arenaria, Neanthes succinea, Polydora limicola and Melinna palmata. For example, the polychaete worm M. palmata formed abundant populations and has become characteristic of communities at 15 ? 30 m depths (Gomoiu, 1981, 1985; Tiganus, 1982, 1988). The benthic communities on the sedimentary substratum have become more homogenous and large areas have been dominated by these opportunistic species.
Along the Romanian littoral, the Modiolus phaseolinus biocoenoses was the most characteristic species on the bottoms from 55-60 m to 120 m. It covered an area of 10,000 km2, which roughly corresponded to 40% of the total Romanian continental shelf (Bacescu et al., 1971). Its maximum development took place between the Sulina?Sf.Gheorghe (pre-Danubian area) and Mangalia (southern) sectors. Research conducted in 1970 indicated high density and biomass and a good trophic base for benthifagous fish. Measurements performed between 1970 and 1980 did not show any appreciable changes in the Modiolus phaseolinus muddy bottom, and it was the only stable biocoenoses as compared with shallow waters biocoenoses. The Modiolus biocoenosis was however degraded after 1990 that was identified by the reduction of macrozoobenthic organisms, particularly those less tolerant to pollution, from 36 in 1981-1982 to 33 between 1991 and 1995 and 23 in 2000-2001. As a result, opportunistic species were able to spread even in this community that have already dominated coastal communities and reduced total species number (Fig. 8.14). In general, the mean abundance and biomass of the deep benthic communities reduced from 7800 ind.m-2 and 233 g m-2 in 1981-1982 approximately five times in 2000-2001. This implies that the decline has begun in the early 1980s as a consequence of hypoxia (Gomoiu and Tiganus, 1990; Dumitrache, 1996/1997) even though this biocoenosis has not been further monitored after 2001.
Fig. 8.14. Change in species diversity in the muddy bottom biocoenosis of Modiolus phaseolinus during 1981?2001 that was the most characteristic biocoenosis along the Romanian coast from 55m to 120 m.
The pre-Danubian sector: The results of recent researches (Dumitrache and Abaza, 2004; Abaza et al., 2006a, 2006b) emphasized an improvement of the qualitative structure of the zoobenthic communities due to reduction in phytoplankton bloom frequencies and intensities. Taking into account the whole area from Sulina to Portitza (the northern sector of Romanian shelf), relatively high species diversity at depths between 15 to 50 m was registered. As compared with 20-to-24 species in the 1990s the macro-benthic fauna was represented by 26-to-44 species during 2000-2003, 49 species in 2005.? The increase can be even higher since the samplings in 2004 and 2005 only covered the northern Romanian sector between 5 m and 20 m depths (Fig. 8.15).
Regarding the quantitative structure, a slight recovery was noted in abundances of both shallow and medium depth species in 2004 and 2005 with respect to the 1990s: their abundances increased from 2591 ind.m-2 to 3140 ind.m-2 at 15-30 m depths and from 2128 ind.m-2 to 4453 ind.m-2 at 30-50 m depths. Worms constituting 95% of the total density dominated quantitative structure. Among polychaetes, most dominant species were Melinna palmata (55%), Neanthes succinea (52%), and Polydora ciliata (50%). The average total abundance at 5-20 m depth range increased to 12186 ind.m-2 in 2005 due to the improvement of the bivalve Lentidium mediterraneum populations (Fig. 8.16). The high percentage of young specimens (with 1-3 mm length), which settled on the substratum at 5 m depths, suggests process of recovery in response to the improvement in environmental conditions.
Regarding to the biomass, it is difficult to compare the data collected at different months and different depths. The data for 2000-2003 showed a lower biomass value (144.0 g m-2) compared with 1990-1999 (450.0 g m-2) at depths between 10 and 30 m (Fig. 8.17). In this particular case, the soft clam (Mya arenaria) populations flourishing at the beginning of the 1970s diminished during the recent years and represented only 25% of total biomass. The decrease can be related to mortalities caused by adverse effects of hypoxia or bottom trawling on benthic organisms. In 5-20 m depth range, the average biomass increased from 289.0 g m-2 in 2004 to 796.0 g m-2 in 2005. Highest biomass belonged to the mollusks due to their well-developed populations. Among the mollusks, Lentidium mediterraneum, Cardium sp. and Anadara inaequivalvis were dominant in weight; the last one is an opportunistic, self-acclimatized species, appeared and spread extensively through the highly eutrophicated marine environment. Biomass of mussels (Mytilus galloprovincialis) communities on the muddy bottoms between 30 m and 50 m depth increased two-folds as well, because of the well-developed mollusc populations. In this area the mussels were present in large numbers dominated by small and medium size populations.
The Constanta sector: Long-term investigations in the Constanta sector showed a recovery in terms of species diversity (Tiganus and Dumitrache, 1995). The species number reduced below 10 in 1995-1996 due to negative effects of intense and repeated phytoplankton blooms in spring-summer 1995. Beginning with 1997, weakening of intense algal blooms caused fast recovery and the biodiversity increased from 18 to 53 species in 2002 (Fig. 8.18).
Fig. 8.18. Change of species diversity in the Constanta marine sector between 1993 and 2002.
Fig. 8.19. Average zoobenthos abundance (ind. m-2, left) and biomass (g m-2, right) at 10-30m and 30-50m depth ranges in the Constanta sector of the Romanian shelf waters during 1990-99 and 2000-02.
From the quantitative point of view, the abundance increased 2-3 times in 2000-2002 with respect to the 1990s both at 10-30m and 30-50m depth ranges (Fig. 8.19, left). The revigoration tendency of the mollusk and crustacean populations was observed, even if the worms dominated density variation of entire macrozoobenthic population in this area. Similarly, in regards to the biomass the range of values between 286 g m-2 at 10-30 m depths and 361 g m-2 at 30-50 m depths obtained in 2000-2002 were slightly better than those registered in the 1990s (Fig. 8.19, right). In the Mytilus galloprovincialis mud community at 30-50m depths a slight recovery process of biomasses was observed; there are some zones where the mussel populations expanded under more favourable conditions.
The results of benthic ecological research in shallow bottoms (5-20 m) performed between 2003 and 2005 showed a slight reduction in the macro invertebrates fauna from 26 species in 2003 to 21 species in 2005. Similarly, the average abundance (13257 ind.m-2) was higher in 2003 than 2004 (6410 ind.m-2) and 2005 (9710 ind.m-2) (Fig. 8.20) that mostly dominated by Spio filicornis and Lentidium mediterraneum. The average biomass ranged between 327 g.m-2 in 2004 and 800-850 g m-2 in 2003-2005 (Fig. 8.22) that was due to a well-defined mollusks community dominated by Mya arenaria and Scapharca inaequivalvis (Abaza et al., 2006a, 2006b).
Fig. 8.20. Changes in the average abundance (ind m-2) and biomass (g m-2) of macrozoobenthic populations in the central (Constanta) sector at 5-20 m depth range.
The southern littoral zone: Species diversity in the southern sector of Romanian littoral zone between 15 to 50 m increased steadily in the present decade, and became almost double with respect to the 1990s (Fig. 8.21). The 2005-2007 period maintained relatively stable species number between 50 and 60. In particular, samplings from Tuzla to Vama Veche at depths to 20 m between 2003 and 2005 have revealed 73 different types organisms (Abaza et al., 2006a; 2006b). In the mud mussels? community at 30 m to 50 m depths, maximum 36 macrobenthic species have been identified in 2000-2002 that comprised the members of muddy bottoms biocoenoses as well as iliophylic and opportunistic species. In the subsequent three years, only areas down to 20 m depths have been monitored. The most representative species were the polychaetes Terebellides stroemi, Prionospio cirrifera, Nephthys hombergi, Exogone gemmifera and Phyllodoce maculata, the bivalves Mytilus galloprovincialis, and Modiolus phaseolinus and the amphipods, Corophium runcicorne, Microdeutopus damnoniensis, Iphinoe elisae and Phtisica marina. The frequency of these common species ranged between 66% and 100 %. Other two species recorded with 83% frequency were polychaetes Polydora limicola and Melinna palmata. The new environmental conditions promoted abundant populations of the opportunistic polychaete species M. palmata.
From the quantitative point of view, the benthic populations in the mud mussels? community at 30 m to 50 m depths has been subject to moderate interannual variations changing between 2000-3000 ind.m-2 since the early 1990s (Fig. 8.22, left). These populations were dominated primarily by worms and secondarily by mollusks and crustaceans. The mollusks however dominated the biomass after the mid-1990s although their biomass remained appreciably low, less than 200 g m-2 (Fig. 8.22, right). The abundance at 0-20 m depth range increased from 12377 ind.m-2 in 2003 to 14113 ind.m-2 in 2005. The biomass increased from 1000 g m-2 in 2003 to a maximum of 5596 g m-2 in 2004 and then reduced slightly to ~4500 g m-2 in 2005 (Fig. 8.23). Relatively high abundance in this littoral zone indicates a better capacity of rehabilitation as compared to further offshore.
Comparison of three regions: In terms of average biomass for the 2002-2006 period, the southern sector was three-to-four times superior to the others while the mean abundance is almost comparable to the central but twice better than the northern sector (Fig. 8.24a). The Shannon diversity index varied between 1.5 and 3.5 for all regions during 2002-2006, and implied moderate biodiversity for the central and northern sectors and slightly good biodiversity for the southern sector (Dumitrache et al., 2008). All three regions indicated better macrozoobethos characteristics when compared with the northwestern Ukrainian coastal waters (Fig. 8.7, 8.8). The organisms living in/on the sea bottom also suggested a rehabilitation tendency in terms of their diversity. The species number had a gradual increase in the Danube delta region up to 50 in 2004, comparable number in the central littoral zone and even better in the southern littoral zone (Fig. 24b). The eurioic forms (characterized by large ecological valence) however occurred with high frequencies in all three zones (Neanthes succinea, Polydora limicola, Melinna palmata, Ampelisca diadema and Mya arenaria). On the other hand, some species qualified as rare in the Black Sea Red Book, such as Apseudopsis ostroumovi, Caprella acanthifera and Xantho poressa were again identified in 2003.
Fig. 8.24a. Average abundance (ind. m-2) and biomass (g m-2) of macrozoobenthic populations during 2002-2006 at 0-20 m depth range of the northern, central, southern sectors of Romanian litteral zone.
The long-term changes in macrozoobenthic communities were examined by comparing recent data obtained along the standard monitoring network of the Institute of Oceanology, BAS (Fig. 8.25) during the summers of 1998-2002 (Stefanova et al. 2005, Todorova and Konsulova 2000) with the reference data from the ?pristine? period 1954-1957 of the Black Sea ecosystem (Kaneva-Abadjieva and Marinov, 1960) and the period of the most intensive anthropogenic eutrophication 1982-1985 (Marinov and Stojkov 1990).
The pool of samples collected during summers of 1998-2002 yielded 134 species and 5 taxa: Polychaeta (41 species), Crustacea (41 species) and Mollusca (38 species), Varia (3 anthozoans, 3 echinoderms, 4 ascidians, 2 pantopods, 1 phoronid, 1 cephalochordate), and the higher taxa included Turbellaria, Nemertini, Oligochaeta, Acarina, and Insecta. The hierarchical cluster analysis (Todorova and Konsuolova, 2000; 2006) differentiated five zoobenthic communities distributed on the Bulgarian shelf as given on the map shown in Fig. 8.26. Bathymetry and sediment type (Table 8.4) were identified as the important determinands of community structure and pattern.
Fig. 8.25. Map of the studied area with sampling locations and communities as differentiated according to the cluster analysis: S ? infralittoral sand community, C ? infralittoral silt community, M ? upper circalittoral silt community with Melinna palmata, T ? impoverished circalittoral silt community, P ? lower circalittoral clay community with Modiolula phaseolina.
The ?infralittoral sand community? (S) is distinguished by the typical psamophylic polychaetes Prionospio cirrifera Wiren, 1883 and Protodorvillea kefersteini McIntosh, 1869 and the clam Chamelea gallina Linne, 1758 that contribute mostly to between-groups dissimilarity. The community is dominated in the abundance by Prionospio cirrifera ? second order opportunist, tolerant to disturbance (Borja et al., 2000), Polydora ciliata Johnston, 1838 and oligochaetes ? first order opportunists tolerant to hypoxia, colonizers of organically enriched sediments (Pearson, Rosenberg, 1978, Gray et al., 2002). The community is the most abundant and diverse assemblage on soft bottom habitats of the Bulgarian shelf (see Table 8.4). This fact stresses the importance of sandy bottom habitats for marine biodiversity conservation as the sandy bank ?Cocketrice? (st. 511, Fig. 8.24) was declared as a protected area in 2001 by the Bulgarian Ministry of Environment and Waters. The bank was included into the network of European Marine Biodiversity Research sites established in order to address the climate change effects on species level (Warwick et al. 2003).
The ?infralittoral silt community? (C) is dominated in the abundance by Heteromastus filiformis Claparede, 1864; Neanthes succinea Frey & Leuckart, 1847; Hydrobia acuta Draparnaud 1805. These have major contribution to within-group similarity and discriminate the assemblage against other soft bottom communities. Heteromastus filiformis is first order opportunist, pioneer colonizer of organically enriched sediments (Pearson, Rosenberg 1978), Neanthes succinea is tolerant to disturbance by organic enrichment (Borja et al. 2000, Simbura & Zenetos 2002) and Hydrobia acuta is common in organically enriched fine sediments in the Black Sea and Azov Sea, tolerant to episodes of hypoxia and presence of H2S (Tatishvili et al., 1968).
Table 8.4. Habitat features, average ? conf. lev. 95 % of the number of species (S), abundance, biomass, and Shannon-Wiener community diversity H' index of soft bottom macrozoobenthic communities on the Bulgarian Black Sea shelf, summer 1998-2002.
|16-23||sand||37 ? 4||13500 ? 6291||2576 ? 2343||3.55 ? 0.29|
|12-26||silt||22 ? 6||6404 ? 2793||866 ? 1230||2.69 ? 0.36|
|28 ? 2||9923 ? 2206||420 ? 315||2.74 ? 0.15|
|64-93||clay silt||19 ? 3||1579 ? 381||21 ? 16||3.08 ? 0.39|
|25 ? 4||4581 ? 3316||84 ? 40||2.87 ? 0.49|
The ?upper circalittoral silt community with Melinna palmata? (M) is named after the terebellid worm Melinna palmata Grube, 1870 that ranks second in the abundance but has highest contribution to within-group similarity and is a key structural species. Its dense vertical tubes consolidate the sediment and determine the specific character of the habitat. The most abundant is Aricidea claudiae Laubier, 1967 considered as a species sensitive to anthropogenic disturbances (Borja et al. 2000, Simbura & Zenetos 2002). ?Melinna palmata silt? is one of the communities with widest spatial distribution on the Bulgarian shelf (Fig. 8.26).
In terms of species composition, the ?impoverished circalittoral silt community? (T) is transition between ?Melinna silt? and ?phaseolina clay? communities. The assemblage is dominated by Melinna palmata, but its average abundance is 4.5 times lower than ?Melinna silt community?. Increased occurrence of some species typical of deeper habitats such as Amphiura stepanovi D'yakonov, 1954 and Modiolula phaseolina Philippi, 1844 was observed in this community. Community impoverishment is manifested both in significant abundance/biomass decrease and species richness decline (Table 8.4).
The ?lower circalittoral clay community with Modiolula phaseolina? (P) is discriminated from the rest of the assemblages by the mussel Modiolula phaseolina with highest contribution to within-group similarity and dominant in the abundance. The habitat is characterized by bulk of dead shells and shelly detritus of the same species, hypoxia and increased salinity in comparison to coastal habitats. Other discriminating species are Amphiura stepanovi and Notomastus profundus Eisig, 1887.
Mussel beds typical of the Bulgarian shelf are not differentiated by the multivariate analysis of similarity as a distinct community assemblage. This is due to the continuous species composition alteration of mussel bed associations in correlation with bathymetry and sediment type.
The species richness decreased from shallow coastal sites to deeper offshore sites (Fig. 8.27). Species richness of benthic macrofauna had the second strongest negative correlation with silt-clay percentage in sediments after the strongest positive correlation with oxygen saturation in bottom water Todorova (2005). On the other hand, benthic diversity was weakly correlated with trophic supply. The observed spatial pattern of diversity is therefore basically driven by the depth gradient of decreasing oxygen concentration and hypoxia that are determined by regional hydrochemical characteristics, and further modified by sediment heterogeneity especially at the shallow habitats. The infralittoral sand habitat supported the most diverse zoobenthic community as evident by the highest average number of species and highest Shannon-Wiener index (Table 8.4). Silty and clay habitats were less diverse compared to sand. ?Upper circalittoral silt community with Melinna palmata? (M) was the richest in species among fine sediment habitats; however increased dominance of few polychaetes yields somewhat lower Shannon-Wiener index (Table 8.4). Minima of Shannon-Wiener index at coastal sites (st. 211, 301, 501) are due to the dominance of Melinna palmata and/or Heteromastus filiformis, while in the offshore area (st. 102, 204, 504) the observed minima are due to the dominance of Modiolula phaseolina (Fig. 8.26).
The abundance and biomass decrease from shallow coastal to deeper offshore area and from north to south (Fig. 8.27 and Fig. 8.28). The decrease along the depth gradient is related to the reduction in trophic supply offshore and significant hypoxia at benthic habitats deeper than 90 m, while the decrease from north to south along shallower coastal zone correlates with reduced primary productivity at increasing distance from the Danube discharge zone.
The abundance structure (Fig. 8.27) is commonly dominated by the polychaetes, except for the ?lower circalittoral clay community with Modiolus phaseolina? where the predominance of M. phaseolina increases the molluscs share. Most of the observed abundance maxima occur in the ?upper circalittoral silt community with Melinna palmata? due to M. palmata and Aricidea claudiae and in the ?infralittoral sand community? due to Prionospio cirrifera and Polydora ciliata. Extensive literature data showed that organic enrichment of sediments due to pollution and eutrophication resulted in an increase in abundance of opportunistic polychaetes largely due to their ability to continuously colonise the newly available sediment and thus overcome smothering and hypoxia episodes (Gray et al. 2002; Pearson & Rosenberg, 1978). Excessive abundance of polychaetes along the Bulgarian Black Sea coast suggests over-stimulation of benthic biota due to increased productivity of the marine ecosystem and organic enrichment of the sediments.
Considerable spatial variability of biomass that was caused by patchy distribution of the dominant species (Mytilus galloprovincialis) makes difficult determination of its average value within statistically acceptable limits (Table 8.4). The biomass structure (Fig. 8.28) is typically dominated by the bivalve mollusks, except for the ?impoverished circalittoral silt community?, which is dominated by the polychaetes due to almost complete absence of mollusks in the community composition.
Fig. 8.27. Distribution of average macrozoobenthos abundance on the Bulgarian Black Sea shelf and abundance structure at sampling stations, summer 1998-2002.
The ecological state of benthic macrofauna on the Bulgarian shelf is assessed according to the AZTI Marine Biotic Index (AMBI) (Borja et al. 2000) that provides an ?ecological state classification? in the range from 0 to 6 in terms of the percentages of abundance of the five ecological species groups according to their sensitivity to stress/pollution. The species are classified as very sensitive to organic enrichment and present under unstressed conditions (the group I), insensitive to enrichment and always present in low densities with non-significant variations with time (the group II), tolerant to excess organic matter enrichment with densities stimulated under organic enrichment (the group III), the second-order opportunistic species (the group IV), the first order opportunistic species (the group V). Opportunistic species are those that can take advantage of adverse conditions and thrive in locations where more sensitive species will not survive; they are capable of rapid colonisation and recovery. First order opportunists are species which first colonise the habitat after mass mortality episodes, while second order opportunists come next.
Fig. 8.28. Distribution of average macrozoobenthos biomas on the Bulgarian Black Sea shelf and biomass structure at sampling stations, summer 1998-2002.
The following threshold values are set to distinguish between five categories of benthic disturbance in consistent with the ecological state (ES) classification scheme established by the European Water Framework Directive (WFD): AMBI ? 1.2 Undisturbed community (High ES), 1.2 >AMBI ? 3.3 Slightly disturbed (Good ES), 3.3 >AMBI ? 4.3 ?Moderately disturbed (Moderate ES), 4.3 >AMBI ? 5.5 - Heavily disturbed (Poor ES), 5.5>AMBI ? 6 Extremely disturbed and azoic (Bad ES).??
At few coastal stations in the northern part of the shelf (st. 101, st. 200, st. 202) the ecological state is moderate (Fig. 8.29). Increased community disturbance probably reflects higher level of the eutrophication impact as the distance to the Danube nutrient source decreases. Offshore sites (except st. 603) manifest better ecological state (high at most of the stations, e.g. st. 204, 305, 504) compared to coastal sites. The pattern of improved ecological state offshore evidently reflects decreasing organic enrichment in the open Black Sea area. Despite the natural hypoxia, the environment at deeper offshore habitats is more stable and predictable and less exposed to anthropogenic impact compared to coastal sites, therefore the community is undisturbed or only slightly disturbed as implied by AMBI. The predominance of ecologically conservative bivalve M. phaseolina in the abundance/biomass also indicates low level of environmental impact. AMBI, in contrast to diversity indices, is independent of the habitat type, therefore more sensitive in reflecting the anthropogenic impact. On the contrary, the diversity indices may be used for ecological state assessment only if their deviation from reference values, expected under non-degraded conditions in similar habitat types, are known.
Fig. 8.29. AMBI values (mean ? st. error) at sampling stations (according to Fig. 8.26) on the Bulgarian Black Sea shelf, summer 1998-2002 and thresholds for five ecological state categories.
The ecological state classification provided by AMBI manifests lack of undesirable disturbance of benthic communities in the Bulgarian Black Sea area and gives an encouraging sign of ecosystem recovery after a period of severe decline during the 1980s.
Species composition: Total 57 taxa of macrozoobenthos organisms were found in 2001-2003 in the Bulgarian Black Sea. The number of species varied from 54 in 2001 to 47 in 2002 and 57 in 2003. These changes were related most probably to over-fishing of Mollusca and Crustacea species and the negative effect of bottom trawling activities on the bottom communities during the commercial harvesting of Rapana thomasiana.
The species composition comprised mainly Polychaeta, Mollusca, Crustacea and ?Diversa? groups. The majority of species (about 20) belonged to Polychaeta which included some dominant species (Melinna palmata, Nephthys homergii, Nephthys cirrosa) which were resistant to strong changes in environmental conditions. The second dominant group Mollusca was presented by 17 species, like Mytilus galloprovincialisand, Mactra subtruncata. Crustacea was mainly represented by their dominant species Ampelisca diadema. Polychaeta had more dominant share (43 %) during the first half of the years and slightly decreased towards the second half (35%). Crustacea (18 %-26%) showed increasing tendency from winter to autumn whereas Mollusca (30-31%) and Diversa (8-9%) species numbers remained steady throughout the year.
Comparison of the recent and historical data sets reveals decreased diversity of benthic macrofauna during the period of anthropogenic eutrophication (1982-85) in all key taxonomic groups (Fig. 8.30a). The polychaetes regained their species richness during the recent period (1998-2002); however, the recovery of the crustaceans, despite significant increase, was incomplete. The current mollusks? richness also exceeded the level of the ?pristine? period. Partly this is due to the immigration and naturalisation of several new settlers in the Black Sea such as the predatory gastropod Rapana venosa, and the bivalves Anadara inequivalvis (Bruguiere, 1789) and Mya arenaria Linne, 1758. Their expansion was determined by the rich trophic resources available to the predators and suspension-feeders and by hypoxia tolerance of both alien bivalves (Zaitsev & ?zt?rk 2001).
As the total number of species analysed depended on the sampling effort, the taxonomic structure is more objective indicator of community composition alterations. The observed changes are characterised by continuous increase of the mollusks? share over the three compared periods, increase of the polychaetes share during the eutrophication period and recovery in the recent period, decrease of the crustaceans share during the eutrophication period and incomplete recovery during the recent period (Fig. 8.30b).
The temporal trends in the taxonomic structure and species richness can be interpreted in the context of tolerance of crustaceans, polychaetes and mollusks to oxygen deficiency. The crustaceans are the most sensitive group to oxygen deficiency, the polychaetes are less sensitive and the bivalves are the most tolerant (Nilsson & Rosenberg 2000, Rosenberg et al. 1991). Recurrent hypoxia/anoxia, associated with extensive phytoplankton blooms during the period of anthropogenic eutrophication, probably caused the observed sharp decline of crustacean richness, whereas the mollusks and pollychaetes increased their relative share. The recovery of the crustaceans and polychaetes comparable to the ?pristine? state therefore suggests an improvement in hypoxia conditions during the recent period. However, the increase of mollusks share was probably caused by ample organic load to the bottom that caused episodes of oxygen deficiency that reduced other oxygen sensitive species (Moncheva et al. 2001).
Fig. 8.30. Alterations in (a) total number of species (S) and (b) taxonomic structure of benthic macrofauna over the ?pristine? period 1954-1957, the intensive anthropogenic eutrophication period 1982-1985, and the recent period 1998-2002.
Density and biomass: The average multi-annual abundance of macrozoobenthos during 2001-2003 was 1518 ind.m-2 that dropped to minimal values 50 ind.m-2 in 2002 at 20 miles offshore Cape Emine, and attained its maximal value 5520 ind.m-2 in front of Cape Emine in November 2001. The multi-annual average density in front of the Capes Galata and Emine was 1130 ind.m-2 for 1992-2000 and became 1037 ind.m-2 in 2001-2003. According to the average data for 2001-2003, the main share belonged to Polychaeta (65%) because of their successive outbursts, followed by Mollusca (15%) and then ?Diversa? and Crustacea (10%) (Fig. 8.31b).
When compared with the historical data (1954-57), the total average abundance did not rise during the eutrophication period 1982-85, whereas more than 10-fold increase was evident for the recent period (Fig. 8.31a). The overwhelming portion of this abundance increase belonged to Polychaeta. The change in abundance structure comprised the shift from predominant Mollusca species (60%) during the pre-eutrophication period to the current state of opportunistic polychaetes species (65%) (Fig. 8.31b). Thus, Mollusca share decreased by four times and Polychaeta share increased two-folds. High abundance of opportunistic deposit-feeding polychaetes during 1998-2002 indicates excessive organic load to sediments.
The average multi-annual macrozoobenthos biomass in 2001-2003 along the Bulgarian Black Sea coast was 452.253 g.m-2 and encompassed the range 0.31-9803.1g.m-2. The extremely high biomass was mainly due to high Mollusca Mytilus galloprovincialis abundance is some samples collected at some of its patchy sources along the Bulgarian coast. Furthermore, this mean biomass was almost identical to its 1992-2000 average value of 434 g.m-2. Mollusca biomass was slightly higher than the previous period which may be considered as a positive sign in the evolution of benthic community along the Bulgarian waters, and likely connected to the decreasing tendency of hypoxic conditions, decrease in the Rapana abundance due to its commercial harvesting, and diminishing density of Mnemiopsis.
Fig. 8.31. Alterations in (a) total abundance and (b) percent abundance structure for the ?pristine? period 1954-1957, the intensive anthropogenic eutrophication period 1982-1985, and the recent period 1998-2002.
According to Pearson and Rosenberg (1978) benthic succession model in response to organic enrichment, the total species number and biomass increase faster than the total abundance as organic load increases above background levels. For further increase in organic load, species diversity starts decreasing, biomass levels off, and abundance raises more rapidly. Additional organic load first causes a sharp peak in abundance with some corresponding increase in biomass and then a rapid drop in both abundance and biomass to background levels due to deteriorated oxygen conditions. Assuming that this model applies to the Bulgarian shelf benthic structure, the present state corresponds to the phase ?increase in abundance and level off biomass? prior to the collapse.?
Macrozoobenthic populations of the Turkish littoral and sublittoral zones have been investigated only partially so far. For the last 45 years, the studies of zoobenthic organisms carried out mostly within the Bosphorus-Black Sea junction region (Demir, 1952; Dumitresco, 1960, 1962; Rullier, 1963; Caspers, 1968; Kiseleva, 1981; Uysal et al., 2002). These studies were then extended more recently to the rest of the southern coastal waters (Kocataş and Katağan, 1980; Ateş, 1997;? Mutlu et al., 1992; 1993; Sezgin et al., 2001; G?nl?g?r, 2003; ?ulha, 2004; ?zt?rk et al., 2004; ?ınar & G?nl?g?r-Demirci, 2005; Kırkım et al., 2007; Sezgin& Katağan, 2007; Bilgin et al., 2007; Sezgin et al., 2007). On the basis these studies, macrozoobethos species richness along the Turkish coast and the indicator species list are given in Table 8.5 and Table 8.6, respectively. ?
According to the Table 8.5, out of 10 different groups, Polychaeta, Mollusca and Amphipoda accounted for 76% of the total abundance, followed by Decapoda, Isopoda, Echinodermata, Cumacea, Porifera, and others. 385 macrobenthos species were registered during 1980-2000, and this number increased to 419 in 2000-2007 (Table 8.5). Therefore, no evidence exists for the reduction of species richness in the Turkish Black Sea coastal zone during the last 25 years. Moreover, bottom fauna was enriched in 2000-2007 due to (1) introduction of some species that were? previously recorded only in the Bosphorus region, (2) introduction of alien species, (3) Mediterranization (climate change effects), (4) more detailed studies to cover neglected geographical locations or habitats,? (5) recovery of ecosystem health. However, contrary to the steady character of species richness, abundance and biomass of? some species were dramatically changed. The decline in populations of many benthic invertebrates (Crustacea, Mollusca, Polychaeta), which play a significant role in the food chain of the benthos consuming fish, has been clearly noted in the last two decades. The first visible changes in the structure of coastal benthic communities in southern coast of Black Sea were the incresae in density of some Mollusca species (such as Patella spp., Rapana, Chamelea) during the last 10 years. Moreover, the replenishment of juvenile bivalve populations was found to depend on the strength of Mnemiopsis-Beroe interactions in the pelagic zone and therefore subject to considerable interannual variations. Better resistance of Anadara ineaquivalvis to environmental stresses than the native species permitted its population to become a dominant group at the 10-30 m depth range.
A comprehensive zoobenthos survey conducted on soft bottoms along the Turkish coast in May-July 1999 (Kirkim et al., in pres) revealed that the depth range 10?25 m, ?mostly consisting of fine-to-medium sandy bottom sediment, was dominated by polychaete (M. palmata) and molluscs (C. gallina,? L. mediterraneum, L. divaricata). The total average abundance of zoobenthos was 1524 ind.m-2 and their biomass 109 g m-2 (Kırkım et al., unpublished data). At the 25?50 m depth range, the composition of bottom sediments slightly changed to sand-mud composition. The number of recorded zoobenthic species decreased to 74. and their ?total average abundance and biomass was 2134 ind.m-2 and 62.4 g m-2, respectively. Within 50-80 m depths, the bottom sediments consisted of the combination of mud, clay and dead shells. The species diversity was the poorest; a total of 52 species recorded among which polychaetes and some echinoderms the most abundant. The total average abundance and biomass of zoobenthos was 1171 ind.m-2 and 41 g m-2 , respectively (Kırkım et al., unpublished data). In this study, Low dissolved oxygen values of lower layer and soft substratum of sediment resulted in wide distribution of the opportunistic polychaet M. palmata (the mean abundance of 450 ind. m-2) that were adapted to such conditions. Molluscs were among the second abundant taxa, accounting for 32% of the total number of macrofaunal species. The most common bivalve, C. gallina (69%) had a highest frequency value of the 39 stations, followed by the bivalve P. rudis (64%), the gastropod Cyclope neritea (Linne, 1758) (59%) (Kirkim et al., unpublished data).
The Black Sea
Turkish Black Sea coastal zone
|1980-1990s||2000-2007||For all time
Harvesting of the bivalve Rudipates decussatus by dredging the mediolittoral zone damaged the benthic community and destroyed fish habitats, particularly Solea and Scophthalmus. Some important molluscs (e.g Donax sp., Turitella sp., Mactra sp.) were under the threat due to coastal degragation and destruction. The dredging of sand from the sea also destroyed the benthic habitats along the Turkish coast (?zt?rk, 1998). Illegal bottom trawling for Rapana venosa harvesting has raised ecological concerns with respect to the benthic communities and especially the mussel beds. The population decline of the habitat-structuring species Mytilus galloprovincialis in the impacted areas was accompanied by degradation of the associated benthic community from "mussel bed" type to "silt bottom" type dominated by opportunistic polychaetes and oligochaetes. The mollusc species M. arenaria replaced the dominant species Lentidium mediterraneum in the coastal sandy strips and thus affected negatively biodiversity of the Black Sea ecosystem. On the other hand, the high biomass of M. arenaria provided food for the benthic fish and coastal birds.
higher abundance under
sensitive species to hypoxia;
indicators of clean waters
an indicator of organic pollution
an indicator of clean waters
higher abundance in organic rich environments
an indicator of clean waters
an indicator of clean waters
Decapod cructaceans Crangon crangon and Paleamon spp. biomass and denstiy also decreased in last 10 years. An exception is Mercierella enigmatica (= Ficopomatus enigmaticus) (Polychaeta), whose density has increased; however, this species grows on coastal substrates and is inaccessible for the benthos consuming fish. Presently, domestic and chemical pollution is the main factor controlling the state of macrophytobenthos along the southern Black Sea coastal waters.
Available observations appear to indicate that eutrophication and different survival ability of benthic species in hypoxic conditions played an important role in the development and formation of macrobenthic communities. It appears that the invasion of Beroe ovata in 1999 did not play any major role for either the recovery of benthic communities or the development of a new stable structure. On the contrary, disturbing quasi-stability of the system, the community started experiencing more pronounced fluctuations in both abundance, biomass and species structure. On the other hand, the Mediterranization process or invasion of of the system by new species continued.
Marine Ecology and Fisheries Research Institute (MEFRI) and Georgian Fisheries Trust data focused on monitoring the distribution of invasive species starting by 1949. These data sets suggested that Rapana invasion caused sharp decline in the oyster Ostrea edulis stock due to the presence of roughly 30 Rapanas per 1 live oyster. The data in 1950 further showed considerable spreading of Rapana along the entire Georgian coastal waters. This was followed by the reduction of other commercial mollusks as the abundance of Rapana continued increasing.
In 1978-1979, the new opportunistic species filtrating mussel Cunearca cornea was found initially with sizes 1.0-2.5 cm, and 6-8 cm individuals in the vicinity of the Chorokhi River mouth. This bivalve was especially abundant on the Anaklia bank where mussel collectors were installed in 1978-80. Presently, Cunearca cornea is widely distributed in Georgian waters (Gogmachadze & Mickashavidze, 2005).
The last study of benthic communities was conducted in 2003-2004 on a seasonal basis by monitoring 16 stations along the Georgian coast (Table 8.7). In these studies, new exotic species Anadara inaequuivalvis and Mnemiopsis leidyi were found together with significant changes in zoobenthos biodiversity in comparison with previous data (Gogmachadze & Mickashavidze, 2005; Mickashavidze, 2005). Out of 65 macrozoobenthos species recorded, 27 were Molluscs (41%), 18 Crustacean (28%), 20 Polychaeta (31%). Both the zoobenthos species diversity and total abundance were highly variable regionally and seasonally (Fig. 8.32). The species diversity increased as compared to 1990 for all these groups (Fig. 8.33).
Table 8.7. Quantitative characteristics of the benthic communities in Georgian Black Sea waters in 2003-05.
|Nephthys longicornis, Chamelea gallina, Lentidium
mediterraneum, Mytilaster lineatus,Balanus improvisus
|Nephthys longicornis, Melinna palmate; Chamelea gallina, Ciclope donivani, Lentidium mediterraneum; Cumela pugmata, Ampelisca diadema|
?In autumn 2003 strong underwater current was registered, the benthic samples were absolutely void of any species.
2 The main part of biomass is formed by molluscs Rapana thomasiana and Crustacea Callianassa truncate, 50.6 and 34.4 g/m2 respectively.
The data presented for the Russian coastal waters of the Black Sea are based on the materials collected during seasonal surveys of the R/V ?Akvanavt? in 2001-2007 (Table 8.8). During every survey 22-54 stations were visited and five grab samplings were collected at each station at depth range from 10 to 45 m (Fig. 8.34). The previous studies (Chikina and Kucheruk, 2004; 2005) indicated that the northeast Black Sea coastal waters were classified in two different regions according to the state of benthic communities: the first one extends from Kerch Strait to Anapa in the northern sector, and the second one from Gelendjik to Adler in the southern sector that encompasses almost 90% of the Caucasian coastal ecosystem (Fig. 8.34). Most of the changes in zoobenthic communities during the last 10 years took place noted in the southern (Gelendjik-Adler) region, whereas the Anapa?Kerch Strait region remained fairly stable.
|Year||Month||Number of stations|
According to the studies in 1957, 1963, 1968 (Kiseleva, 1967, 1981; Kiseleva and Slavina, 1965, 1966) and in 1980 (Nikolaenko and Povchun, 1993), the species composition and quantitative characteristics of macrozoobenthos possessed a stable structure until the 1950s. Shallow waters with sandy bottom (< 30 m) were inhabited predominantly by bivalve Chamelea gallina. This community was taken over by Mytilus galloprovincialis community at depths of 35-40 m, and Modiolus phaseolinus community at depths deeper than 60 m. This benthic community structure has then been altered when the carnivorous gastropod Rapana venosa invaded the region in 1947. Its first impact was to eliminate oyster banks, bivalves Ostrea edulis, Chlamys glabra and Mytilus galloprovincialis. The niche has then been filled by small bivalves Gouldia minima at intermediate depths. This bivalvia having better reproduction and growth capabilities provided sufficient food resource and thus provided Rapana to settle into the regional biocoenosis permanently and to expand into shallower depths where Chamelea gallina inhabited (Kiseleva, 1967, 1981; Kiseleva & Slavina, 1965, 1966). Later on, a new alien opportunistic bivalve species Anadara inaequivalvis invaded the system. But, neither Rapana nor Anadara imposed critical predation pressures on the regional benthic ecosystem structure. In the mean time, the Chamelea gallina biocenosis was able to promote higher production in response to moderate level eutrophication and its biomass increased from ~80 g m-2 in the 1950s to ~250 g m-2 in the 1980s prior to the population outburst of Mnemiopsis (Fig. 8.35). The bivalves Pitar rudis and Anadara inaequivalvis constituted subpopulations of this biocenosis with lower biomass and abundances. The predator Rapana also revealed low biomass less than 50 g m-2 at 10-30 m depth range.
The outburst of Mnemiopsis after 1988 affected the food web structure by reducing thickness of the euphotic zone and increasing organic material sedimentation rate and reinforcing oxygen deficiency of subsurface levels and thus bringing the lower boundary of phytal zone to shallower depths (Alekseev & Sinegub, 1992). The belt of Cystoseria associations was shifted simultaneously to 10-12 m depths, Chamelea gallina and its successor Gouldia minima at the depth range of 20-30 m and Mytilus galloprovincialis at the depth range of 30-50m completely disappeared.? Chamelea gallina dominance was then confined to the narrow coastal belt shallower than 11m. Heavy Mnemiopsis predation on bivalve larvae limited settlement of young bivalves whereas adult bivalves were consumed by the predator gastropod species Rapana. Consequently, macrozoobethic communities within 5-30 m coastal zone have been degraded seriously during the 1990s.? In 1999, the mean Rapana biomass and abundance reached at 100 g m-2 and 50 ind. m-2, respectively. But its population was aggregated at shallower sandy bottoms (5-15 m) and no Rapana settlement was observed at depths deeper than 15m.
The collapse of Mnemiopsis in 1998-1999 triggered substantial changes in the macrozoobethic community structure. Reduction of its predation strength on bivalve larvae in 1999 allowed for mass settlements (on the order of thousands) of Chamelea gallina larvae and juvenile at 10-18m depth range and of Anadara inaequivalvis at 20-25 m in 2000, whereas such settlements was less than 100 ind.m-2 prior to the Mnemiopsis collapse. A consequence of such highly dense young bivalve community was their very slow growth rate. They attained 5 mm length at most in two years instead of 8-15 mm under normal conditions. Therefore, the sudden jump in bivalve biomass to 200 g m-2 in 2000 was followed by their slower biomass increase in the subsequent two years up to 350 g m-2 for Anadara, 100 g m-2 for Pitar rudis and 60 g m-2 for Chamelea in 2003. Higher Anadara biomass was due to their opportunistic character for space and food consumption (Van Hoey et al., 2007). As expected, such slowly growing abundant bivalve population was attack by the opportunistic predator Rapana. As 1 ind. per 10 m2 was a typically observed Rapana population, the population density of young Rapana increased to 8 ind.m-2 in 2001 and 100 ind. m-2 in 2002. Their massive grazing pressure on bivalve (Chamelea, Anadara, Pitar rudis) populations caused an abrupt decrease in bivalve biomass and abundance from 470 g m-2 and 1292 ind.m-2 in 2002 to 35-45 g m-2 and 29-61 ind.m-2 in 2003-2004 (Fig. 8.36).? This was accompanied by biomass increase of Rapana from 3 g m-2 in 2001 to ~35-45 g m-2 in 2002-2003 as well.
The abrupt loss of bivalves further shifted the macrozoobethos community structure to a Polychaeta-dominated system with an increase of Polychaeta species from 10 to 16, abundance from 300 ind.m-2 to 1494 ind.m-2 and biomass from 2.5 to 7.5 g m-2 in 2003-2004. At the same time, the lack of sufficient food for high Rapana population caused decline of their population to a background level (< 5 g m-2) in 2004-2005. Thus, the Beroe invasion in 1999 introduced interesting prey-predator interactions with strong year-to-year fluctuations in the macrozoobethic community structure during 2000-2004.
Following significant changes in the qualitative and quantitative characteristics of zoobenthos community along the entire Black Sea in the 1970-1980s in response to intensifying eutrophication and other complementary factors, some increase in benthic species diversity and relative recovery of hypoxia sensitive groups during the post-eutrophication period suggested an adjustment process of benthic communities towards a new quasi-stable balance. On the basis of autumn 2003 observations, the Bulgarian shelf benthic macrofauna was identified as in ?good? ecological state except some hotspots subject to local anthropogenic impacts. The northern sector of Romanian shelf (from Sulina to Constanta) had ?moderate? state of zoobenthic community structure that however improved towards the south with increasing distance from the Danube discharge zone. Coastal zone between the Danube-Dniester River outflows was in the ?poor-to-moderate? state, but the zoobenthos community structure in Odessa coastal area was heavily disturbed. The recovery of shallow (15-30 m) and medium (30-50 m) depth benthic communities is encouraging and signals for a rehabilitation trend. Albeit to such slow recovery, the general state of zoobenthos community structure over large areas of the Ukrainian and Romanian shelves is still fragile and suffers from active role of opportunistic and invasive species that continue to exert undesirable disturbances into the system. High capacity for regeneration and food consumption of these opportunistic species (e.g. bivalves Mya arenaria, Anadara inequivalvis, Rapana venosa) still allow them to expand and destroy benthic food web. The conditions appear to gradually progress to the south and east away from the source region of the pollution and eutrophication.
Resuspension and redistribution of fine sediments and silting of large coastal areas due to bottom trawling remains to be an ecological concern that alter sediment type, destroy mussel beds, degrade the associated benthic community from "mussel bed" type to "silt bottom" type dominated by opportunistic polychaetes and oligochaetes. But the link between bottom trawling and its effects on macrozoobenthos has not been studied in sufficient detail yet. Determining the cumulative direct and indirect effects and ecological consequences of hypoxia, high organic load, invasive and opportunistic species, trawling is often complicated and largely unknown. Their quantification is necessary in order to improve our understanding the recovery process.
The present assessment study demonstrated many information gaps in our present state of knowledge of zoobenthos structure of the Black Sea due to lack of systematic observations. The observations are mostly based on scientific cruises, designed for some other purposes interests, which may not very be compatible with monitoring strategy. The present level of knowledge does not allow for a more solid assessment beyond making rather trivial statements such as ?recovery but still fragile structure?, ?prone to undesirable disturbances?, etc.? Answering questions like ?where the present benthic system stand in terms of its stability?, ?how it is close to its former background state?, ?whether it is approaching to it or going to be stabilized at an alternative state? require implementation of a comprehensive and systematic monitoring strategy that should resolve regional heterogeneities in benthic structure and their pronounced interannual changes.
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