Shell-bearing plankton fluxes in the central Black sea, 1989-1991

Seasonal and vertical flux variations of coccolithophorids, discrete coccoliths, silicoflagellates, ebridians, and diatoms were investigated using time-series sediment traps deployed in the central Black Sea for 2 years from September 1989 through August 1990 (Year 1) and from October 1990 through September 1991 (Year 2). The following marine plankton groups were not observed during the enumeration: dinoflagellates, radiolarians, planktonic and benthic foraminifers, and pteropods. High fluxes of coccolithophorids, discrete coccoliths, and silicoflagellates occurred from September to December in Year 1 and October to December in Year 2. High ebridian fluxes occurred during short periods from October to November in Year 1 and from February to April in Year 2. High diatom fluxes occurred from September to January in Year 1 and from October to April in Year 2. The seasonal variations and magnitudes of most plankton assemblages were similar between the Shallow trap (at a depth of 400 m) and the Bottom trap (at a depth of 2090 m) suggesting that the plankton shells settled at a rate of 60 m d−1 or greater without significant loss or lateral input. The exception was the diatom flux in Year 2, which was offset by approximately 2 months between the Shallow and Bottom traps, suggesting a sinking speed of approximately 38±18 m d−1. The slower setting speed is attributed to Pseudo-nitzschia spp., which was the dominant diatom taxon in Year 2. Maximum flux magnitudes of each plankton assemblage were on average two times greater in Year 2 than in Year 1. Emiliania huxleyi, Distephanus pulchrus, and Pseudo-nitzschia spp. dominated the fluxes of coccolithophorids, silicoflagellates, and diatoms, respectively. Significant changes in taxonomic composition of the silicoflagellate and ebridian fluxes were observed between Year 1 and Year 2. The higher fluxes in Year 2 and the observed taxonomic changes imply that the upper water column in Year 2 was warmer and more eutrophic compared to Year 1, possibly linked to greater advection of nutrient-rich coastal waters to the center of the Black Sea through meso-scale eddies.