Seasonal fluctuations of the phytoplankton biomass in the central Adriatic coastal area
Abstract
Investigations of seasonal fluctuations in the biomass and species composition of phytoplankton from the Kaštela Bay over the February 1974 - January 1975 period showed that all the year round phytoplankton was confined to the surface layer where fluctuations were also most significant.
Several successive phytoplankton blooms were recorded during the year, in the spring, summer, and autumn. Ranges of biomass fluctuations were rather small. The peak of phytoplankton density (about 1200 000 cells per liter) occurred during the summer bloom. Maximum chlorophyll a concentration (34.30 mg m-2) was recorded in the autumn. The fluctuation patterns of chlorophyll c and carotenoids followed that of chlorophyll a. Phaeopigment concentrations showed small annual variations. As a rule, their maxima coincided with those of chlorophyll a.
The Kaštela Bay phytoplankton community is characterized by the predominance of diatoms in all the seasons. They ranged from 65.5 to 95.8 percent. They ranged from 65.5 to 95.8 percent. Diatom dominance was chiefly due to Skeletonema costatum, particularly in the summer.
Chlorophyll a and phytoplankton density showed a general similarity with respect to their curve courses during the year. Increase and decrease in the total algal cell counts affected chlorophyll a content. Nevertheless, there was no simple relation between chlorophyll a concentrations and the number of phytoplankton cells since considerable variations in cell size and their pigment content were observed. Reduction in chlorophyll a was observed during the development of spring cells to the summer ones, whereas the cells of the autumn and winter communities were rich in chlorophyll a.
It was also established that nannoplankton (< 50 µ) made a principal contribution to the total biomass, whereas microplankton (> 50 µ) contribution was important only at certain seasons.
Micro- and nannoplankton phaeopigment contents indicated the selective grazing by zooplankton. Lorenzen’s grazing pressure index was poor indicator of zooplankton grazing pressure in the area of investigations with the exception for the summer months.
