FACTORS CONTROLLING PHYTOPLANKTON AND BACTERIOPLANKTON IN LATE SPRING ON A SALINITY GRADIENT IN THE NORTHERN BALTIC

Effects of nutrient enrichment (NH4+, PO43-), sucrose addition and < 100-mu-m prefractionation (removal of large zooplankton) on phyto- and bacterioplankton were studied from 1985 to 1991 after the peaks of the vernal bloom off the SW coast of Finland. Three day experiments were performed in 100 l transparent plastic enclosures and followed a complete 2(3) factorial design. Phytoplankton productivity (C-14 incorporation) and biomass (chl a) were P-limited in a fjord-like inlet with low salinity, whereas in the open sea, significant responses were obtained only with the addition of both N and P. The dominant algal species accounted for the major part of the community response to nutrient enrichment, which suggests that these results can be extrapolated to the prevailing natural conditions. In late spring, a substantial increase in bacterial productivity (H-3-thymidine method) took place only with the addition of both sucrose and the nutrients that limited algal growth. No clear responses were observed in the bacterial biomass, which indicates strong control by grazers or viruses.