Phytoplankton community and hydrochemical characteristics of the Western Black Sea

Abstract

Since the early 1990s the persistent reports about the irreversibly degrading Black Sea have started to disappear. A large set of data, reflecting the temporal and spatial variability of microalgae and hydrochemical parameters in Western Black Sea waters (WBS), collected in 2001-2002 (EU 5th FP project "daNubs"), allows us to compare the present-day ecosystem functioning to previous ones and to certainly conclude on positive signs of relaxation. The years 2001 and 2002 were without ample, negative biological and hydrochemical events in the WBS. Nutrient general trend was a decreasing one since the late 1980s, and inorganic phosphorus and nitrogen concentrations were frequently below the method detection limits during summer autumn periods of 2001-2002. The shelf waters were properly oxygenated. The phytoplankton community was producing biomass in a decreasing tendency, especially obviously since 1995. Simultaneously, there was an increase in Si:P and Si:N molar ratios and concurrent increase in the diversity of mass algal species (mainly diatoms) but only a few of them generated exceptional blooms. Reduction of the ratios between phytoplankton and zooplankton biomasses became evident. The observed ecological status in 2001-2002 confirms that the Black Sea is no longer a reference point for progressive water quality deterioration.

A three‐dimensional coupled model of the western Black Sea plankton dynamics: Seasonal variability and comparison to SeaWiFS data

“…Therefore it seems that the simulated decrease of ammonium benthic fluxes, as compared to the 1995–1997 period, is consistent with a similar decrease of nitrogen net sedimentation (model simulated ∼3 mmol N/m 2 , compared to ∼5 mmol N/m 2 from Gregoire and Friedrich [2004]). Such a decrease of the net sedimentation rates and subsequently of the phosphate and ammonium fluxes can be attributed to the phosphate river load reduction (∼50%) since the 1995/1997 period [ Velikova et al , 2005; Cociacu et al , 1997].…”

“…This is a fair approximation for the open Black Sea area where productivity is mostly controlled by the subsurface nutrient pool which is characterized by low N/P ratios due to the removal of nitrogen through the denitrification process that takes place in the suboxic layer [ Murray et al , 1989; Murray et al , 2005]. However, the observed N/P ratios [ Cociacu et al , 1997; Ragueneau et al , 2002; Velikova et al , 2005] in the Danube influenced waters and river nutrient loads imply a P‐limitation and therefore the model was extended to include PO4 and the phosphorus part of detritus as two additional state variables. This was accomplished by adding a phosphorus limitation on the phytoplankton growth function while assuming a fixed N/P stoichiometry for phytoplankton and zooplankton biomass.…”

“…The strong eutrophication combined with the invasion of the ctenophore Mnemiopsis Leidyi and other synergetic factors such as overfishing [ Gucu , 2002], led to a significant deterioration of the Black Sea ecosystem and particularly that of the Northwestern shelf area. The recent decrease of nutrient river loads since the beginning of 1990s [ Velikova et al , 2005; Cociacu et al , 1997], along with the appearance of a predator of Mnemiopsis , the ctenophore Beroe [ Finenko et al , 2003], have contributed to the gradual recovery of the Black Sea ecosystem [ Kideys , 2002].…”

“…These were employed to improve and assess the model ability to reproduce the observed seasonal primary production variation over the western Black Sea. For model validation, we have also taken into account available in situ measurements (Chl‐a, nutrients, inorganic suspended matter, light attenuation) that were obtained during the 2002–2003 period in the Romanian and Bulgarian shelf areas [ Velikova et al , 2005; A. Cociasu, unpublished data].…”

Temporal variation of surface chlorophyll a in the Romanian near-shore waters

“…Despite the numerous studies on inter-and intraannual variations of both quantitative and qualitative composition of the phytoplankton community in the Romanian coastal waters, chlorophyll a (Chl a), the main indicator of phytoplankton biomass and primary productivity level, has been rather sporadically discussed (Skolka, 1968;Bologa, 1977Bologa, , 1978Bologa et al, 1980Bologa et al, , 1985Mihnea, 1988Mihnea, , 1997Velikova et al, 2005;Vasiliu et al, 2010). Relatively rare during 1960Relatively rare during -1970, the chlorophyll measurements have been intensified since the mid-1980s, once it was included in the list of biological parameters monitored within the Romanian Monitoring Program (Mihnea, 1988(Mihnea, , 1997, and later in the list of indicators of eutrophication within the Water Framework Directive and Marine Strategy Directive.…”