WINTER BLOOM OF THE DIATOM NITZSCHIA-DELICATULA IN THE OPEN WATERS OFTHE BLACK-SEA

Spatial distribution, cell division and cellular chlorophyll content w ere studied in populations of the diatom Nitzschia delicatula Hasle du ring the winter phytoplankton bloom in open waters of the Black Sea in 1991. This diatom contributed 39 to 87 % (mean 72 %) to phytoplankton biomass. Cell numbers were as high as 6 x 10(9) cells m(-3) and bioma ss was as high as 2.7 g m(-3). The frequency of dividing cells in the population varied from 0 to 37%. A diurnal rhythm in cell division was not observed. Production of N. delicatula was estimated from carbon b iomass and the percentage of dividing cells in the population. High va lues of carbon biomass (3 to 5 g C m(-2)), production (1 to 3 g C m(-2 ) d(-1)) and daily P/B coefficients (0.5 to 0.8) indicated the active growth of the population in the central parts of the western and easte rn cyclonic gyres. On average, for phytoplankton communities predomina ntly made up of N. delicatula, estimated production was 2.3 times as h igh as primary production measured by the C-14 technique. Vertical cha nges of chlorophyll cell content were used as an index of the mixing i n phytoplankton assemblages. While phytoplankton biomass was inversely related to the mixed layer depth, population growth rate depended upo n mixing activity in the upper water column.