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BULGARIAN ACADEMY OF SCIENCES Institute of Oceanology — Varna, Bulgaria. LONG – TERM SHIFTS IN THE BLACK SEA PLANKTON COMMUNITY (BULGARIAN COAST). K. Stefanova, S. Moncheva, V. Doncheva, L. Kamburska. Problem.
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LONG – TERM SHIFTS IN THE BLACK SEA PLANKTON COMMUNITY (BULGARIAN COAST)
K. Stefanova, S. Moncheva, V. Doncheva, L. Kamburska
The Black Sea is one of the world’s semi-enclosed seas particularly sensitive to anthropogenic impacts due to its isolation and large river inflow. The identified key ecological problems are the anthropogenic eutrophication, the unsustainable exploitation of the biological resources and the invasion and expansion of exotic species. Three periods are differentiated in the Black Sea ecosystem evolution in terms of the anthropogenic pressure - reference near natural state period (up to the early 70-ties) – pre-eutrophication , period of early and intensive eutrophication (from the 70-ties to the early 90-ties) and the recent period manifesting signs of ecosystem recovery (last 10 years). The 1970s are recognized as a period of extremely high nutrient loading into the basin; the 1980s are renowned for gelatinous and aliens outbreaks, and overfishing, while the 1990s are known as predisposed to the climatic forcing (Moncheva et al., 2001; Oguz, 2005-a). These ecological issues have provoked remarkable ecosystem alterations which have been well documented.
Area of long term monitoring
The map represents the monitoring area along the Bulgarian Black Sea coast during the last ten years. The long term shifts are assessed mainly on data collected from 3 miles in front of c. Galata, which is under the indirect impact of various influence .
A long term data array of physical, chemical and biotic parameters presented in the table are discussed in the study.
The diversity and community structure of Copepods and Cladoceras – typical for the Black Sea ecosystem decreased substantially from pre-eutrophication period to nowadays. Many of the dominant mesozooplankton species, supporting fish populations were replaced by smaller, less-valuable species.The average abundance of both key groups demonstrate similar changes among the years (Konsulov et. al., 1998; Konsulov & Kamburska, 1997, 1998).
O. nana, C. ponticus, inhabiting the upper mixed layer, and reproducing predominantly in the warm season (Gubanova et al, 2000) have disappeared during 1980s and 1990s . The species density and biomass decline could most probably be related to a combination of two factors, gelatinous predatory pressure and pollution.
Figure The average biomass of O. nana and C. ponticus (syn. C. kroyeri) in front of Bulgarian Black Seacoast
Figure Long-term dynamic of fodder summer zooplankton biomass (without N. scintillans) along the Bulgarian Black Sea coast in the period 1967-2001 and total fish catches (Prodanov, Konsulov et. al. 2001)
The eutrophic Black Sea ecosystem has produced more zooplankton biomass than it used to in its pre-eutrophication phase. The combination of eutrophication-induced bottom- up control and top-down control by gelatinous carnivores and small pelagic fishes lead to strong variations in the mesozooplankton (Oguz, 2005-b). Critically low zooplankton biomass of the Bulgarian Black Sea coast is registered in the middle of 1980-ies when the highest fishing efforts were registered and after M. leidyi introduction (Prodanov, Konsulov et. al. 2001).
The regime shifts of summer biomass prove the significant alteration of zooplankton qualitative and quantitative structure. The large fluctuations of copepods and cladoceras contributed mostly to the overall mesozooplankton biomass deviation. Cladocera in the early 70-ies overdominated the mesozooplankton composition,whereas Copepoda maintained high density in pre- and early eutrophication phases. In general key groups gradually diminished during eutrophication.
Regime-shift of summer mesozooplankton biomass (by Radionov, 2005)
Overfishing caused the reduction of medium and large pelagic fish catches and their removal from the system made the smaller and less-valuable planktivorous fishes (anchovy, sprat) the dominant predators in the ecosystem (Oguz, 2005-b). This change doubled the exploited stock of anchovy and sprats, and subsequently their total catch at the end of the 1970s to middle of the 1980s (Prodanov et. al. 1997). As a result, a different type of top-down control started operating on the lower levels of the food web lead to two-fold decline in mesozooplankton biomass and a comparable increase in phytoplankton biomass (Daskalov 2002). This was not a result of eutriphication only, but a process that also included overfishing (i.e. top-down effect). The catches of small fishes started decreasing during the post eutropication phase. The niche vacated by these fish groups was gradually replaced by gelatinous zooplankton (the jellyfish Aurelia aurita followed by M. leidyi) and other opportunistic species as N. scintillans (Mutlu, 1999Oguz, 2005-b).
N. scintillans an indicator species of eutrophication become dominant with frequent and massive blooms in the two phases of eutriphication.
The increase in density and biomass of moon jelly in the 1970s might have been associated with overfishing and removal of mackerel, which was a main predator of Aurelia in Black Sea (Arai, 2001).Becauseof their competitive advantage for foodas compared to small pelagics, and theirpredation on eggs and larvae of smallpelagics, the total gelatinous biomass,mostly of the jellyfish Aurelia aurita,reached its peak in the beginning of 1980s,and finally attained the low value when the populationof the ctenophore Mnemiopsisleidyi exploded.
Regime-shifts of A. aurita (by Radionov,2005)
In pre-eutrophication period N. scintillans average abundance was low, while in the next periods (1978-1988) it increased about 7 fold (9087 ind/m3) with a higher range of summer oscillations. Two extremely high maxima are evident in 1977 and in 1989 (Kamburska et. al. 2003-a), the decrease of fodder zooplankton biomass during the 80-ies being concurrent with increased fish catches.
The comparison between the different periods reveal a high variability of plankton community characteristics (abundance of dominant groups and N. scintillans, phytoplankton blooms and catches of pelagic fishes) resulting in dissimilarity between the years suggested by the PCA analysis.
The PCA analysis extracts 2 components to which variability could be attached (PC1) and (PC2).
Phytoplankton communities similar to zooplankton manifest parallel structural changes in terms of major taxonomic groups (Bacillariophyceae, Dinophyceae) during the different phases of the Black Sea ecosystem evolution.
The share of dinoflagellates in the blooms increased from 15% prior to1970 to 60% during the 80s and 90s on the account of diatoms relative decrease. After 70s summer blooms occurred regularly in addition to the typical spring blooms, dominated mainly by dinоflagellates, coccolitophores and euglenophytes.
The Bac:Din biomass ratio is considered normally an indicator of the taxonomic structure of phytoplankton communities, the classical spring-summer value for undisturbed system reported to be 10:1 (Petrova-Karadjova,1984, Moncheva et. al., 1997). Thus the inversion of this ratio during the 80-ies reflects the higher dominance of opportunistic Dinophyceae species.
In contrast to the 80ies in the recent period the ratio Bacillariophyceae: Dinophyceae increased in favour of the diatoms, (about 3 times in spring and twice in summer) suggesting a trend of diatom dominance recoveryas typical for the reference period of the evolution of the Black Sea coastal ecosystem (Moncheva et. al., 2001).
The PCA performed on data array including additional parameters – Danube discharge, sun spot activity, temperature, zooplankton biomass, Dinophyceae blooms the same clusters were discriminated.
Cluster A- (1960-70s), corresponding to the reference period of the ecosystem, is characteristic with low blooms densities and frequency, high grazing and low N. sci., relatively high temperature and low level of eutrophication component.
Cluster B - The split of the 1980s (B) into two subgroups - B (1980-1985) and B1 (1986-1989) is more apparent along the PC1 axis (blooms/grazing). It is parallel to the different amplitude of Din blooms increase (maximum values recorded during 1986-1989) and the contrasting trend of ZB dynamic (reduction of ZB, more explicit since 1986). Both subgroups are projected at coordinates corresponding to the highest level of eutrophication component.
Cluster C - The 1990-1993 (C1 subgroup) of the recent period marks the transition between the reference (the 70s) and the intensive eutrophication (the 80s) episodes. The association of 1994-1996 (C2 subgroup) to the reference A, reflects more the similarity of the interaction fashion between the selected determinants despite the differences of the real values. A decreasing trend of nutrients and phytoplankton blooms and an increasing trend of ZB was observed but sustained at a higher level (respectively lower for ZB), in comparison to the reference period, at higher T and similar SSA
The C3 subgroup corresponds to a blooms/grazing mode characteristic for the transition episode (C1) at higher T and maintained decreasing trend of eutrophication componen (Moncheva et. al., 2001)
Recently (1997), a new ctenophore species, Beroe ovata (Mayer, 1912) has been introduced into Black Sea basin. Тhis species is the only one predator of M.leidyi. Possible recovery of the Black Sea zooplankton diversity, community structure and dynamic could be expected as a result of efficient predation of Beroe on Mnemiopsis
Figure Long-term dynamic of dominant mesozooplankton groups and exotic ctenophores abundance in summer at 3 miles -transect Galata (by Kamburska et al. 2003-b).
The long-term summer dynamic of the dominant mesozooplankton groups, the exotic ctenophores M.leidyi and B.ovata manifest significant inter-annual variability. Despite of the existing oscillations before the invasion of M. leidyi, during the 80ies-90ies, the average abundance of the two zooplankton groups sustain a lower level compared to the previous period (1967 ‑ 1982). Although the dominant groups maintained a long-term decreasingtrend more explicit since 80ies (R= 0.74), most likely the occurrence of both exotic ctenophores contributed substantially to the recent pattern of mesozooplankton variability (Kamburska et. al. 2003-b)
The coupling copepods /cladocera versus M. leidyi/B. ovata interacting in a strong trophic relationship (typical prey-predator modes) is more significant and effective in summer-autumn period.Before the introduction of Beroe, the M. leidyi abundance was higher, which led to a decrease of mesozooplankton abundance. The ratios mesozooplankton / M.leidyi were higher in 1991-1992, and varied in a huge range (from 3 to 1100). The lowest ratio was less than classical one of 10:1 (at least 3), recorded in 1995. The occurrence of Beroe in the late 90ies resulted in a sharp decline of its prey. The prey-predator coupling M.leidy‑ B.ovata modified the mesozooplankton growth as well. When the ratio Mnemiopsis/ Beroe abundance was higher, as a consequence Mnemiopsis grazing pressure on copepods and cladocera abundance was low and an increasing of dominant groups was evident. However, that prey-predator ratio did not reach the classical one (maximum was 5), which could suggest an effective trophic utilization of secondary produced organic matter (Kamburska et. al. 2003-b).
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