ABSTRACT: Spatial distribution, cell division and cellular chlorophyll content were studied in populations of the diatom Nitzschia delicatula Hasle during 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 10⁹ cells m^-3 and biomass 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 biomass and the percentage of dividing cells in the population. High values 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 eastern cyclonic gyres. On average, for phytoplankton communities predominantly made up of N. delicatula, estimated production was 2.3 times as high as primary production measured by the ¹⁴C technique. Vertical changes of chlorophyll cell content were used as an index of the mixing in phytoplankton assemblages. While phytoplankton biomass was inversely related to the mixed layer depth, population growth rate depended upon mixing activity in the upper water column.

KEY WORDS: Phytoplankton . Black Sea . Winter bloom . Cell division . Photoadaptation

Published in MEPS Vol. 129 (1995) on December 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995