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Baltic Sea phytoplankton responses during N 2 -fixing cyanobacterial blooms, dominated by Nodularia spp., Aphanizomenon sp., and Anabaena spp., were studied using nutrient enrichment experiments. Nitrogen fixation, phytoplankton growth, primary productivity, and phytoplankton species' responses were investigated in areas representing open Baltic Sea water and Gulf of Finland water. Responses to additions of N, P, Fe, and the organic chelator EDTA were studied. Phytoplankton biomass and primary productivity were N, P, or N+P-limited. Phosphorus limitation was more prominent at the Gulf of Finland sites, where the dissolved inorganic-nitrogen to phosphorus (DIN:DIP) ratio was >16:1 during the study period. At the open sea sites, with DIN:DIP ratio

Denitrification in the sediments of the open northern Baltic Sea was measured using the isotope pairing method. The highest denitrification activity was found in the central Gulf of Finland, where denitrification varied between 150 and 650 μmol N m-2 d-1. The bulk of the denitrification was coupled to the NO3- production by nitrification. The mass of benthic fauna was found to be the most significant factor affecting the rate of denitrification. Denitrification rate was highest in late summer and early autumn. Calculated as an average for the Gulf of Finland, the denitrification rate obtained in the present study is 45 kt N yr-1. This gives a release of ca 30% of the external N input by denitrification.

The main compartments of the nucrobial food web were studied in a hydrodynamically complicated area to determine the response of bacterioplankton to spatio-temporal discontinuities in the water column structure. The samples were divided according to water masses into those repre- senting frontal areas and those representing low-saline areas. In the upper mixed water layer (UML) bacterial production was higher in the frontal water (average 4.5 mg C m3 dl) than in the low-saline water (3.7 mg C irr3 d-I). However, the proportion of bacterial production of the primary production was about the same in the frontal water (15 5%) and in the low-saline (16 %) water. The data implied that the recorded frontal upwelling event did not drastically change the mode of production from regenerated to new production. Furthermore, the data indicated that heterotrophic flagellates did not respond to increased bacterial abundance during the intervals between hydrodynamic events. Below the thermochne, the turnover time of bacterial numbers was less than in the UML, as was thymidine incorporation (TdR) per cell, but leucine incorporation (Leu) per cell was highest in the UML. The average molar ratio of Leu to TdR was 7.7 in the UML and 3 below the thermocline. The molar ratio showed an increase in the growth rate during a storm event. Leu and TdR methods did not give equivalent rates of bacterial production over the daily timescale, although they gave quite similar estimates when averaged over the whole study penod (11 d). Our data indicated that one should be very cautious in using conversion factors, which are derived from surface water, to calculate bactenal production throughout the water column, and that sometimes even higher conversion factors should be used below the thermocline than in the UML.

Experimental enclosures were used to follow responses of the plankton~c microbial food web to varying short-tern~ (5 d) perturbations induced by adding inorganic nutrients (N and P) and a top predator (fish) during a 21 d period in late summer, on the coastal area of the Baltic Sea. Biomass, production, growth and grazing of picoand nanoplankton assemblages were estimated, and a carbon budget for the microbial loop during the experiment was constructed. The microbial food web was a highly dynamic system. Varying perturbations due to nutrient loading and the top predator provoked eutrophication in the enclosures, but they affected the microbial loop only slightly. The amplitudes of oscillation in abundance of coupled communities were amplified, but the frequencies of oscillations in the microbial loop were not affected by the perturbations. Changes in the route of carbon flow through the microbial food web occurred In relatively short time scales. These changes seemed to be dependent on the phasing of the coupled oscillations between the communities, and the structure within the different communities. Ciliates were only loosely connected to the microbial loop: although ciliates and heterotrophic nanoflagellates (HNF) showed predator-prey-like coupled oscillations, the ciliates gained most of their carbon from other sources, and most of the HNF carbon loss was due to factors other than ciliates. HNF were the most important consumers of picoplankton during the HNF maximum, but they were also dependent on other sources of nutrition.

Several conversion factors are r e q w e d for the eshmation of bacterial biomass, net production and carbon demand from epifluorescence microscopy and measurements of [3 H]-thymidlne and [3 H]-leucine incorporation rates. These conversion factors were evaluated simultaneously in mixed cultures of bacterial assemblages from the Weddell/Scoha Confluence of the Southern Ocean. The cultures were grown in the dark at + l 'C. Conversion factors were calculated for each culture by regression analyses of cumulative parameters. Average conversion factors were: 1.1 X 1018 cells per m01 thymidme incorporated into cold TCA precipitate, 7.5 X 1015 pm3 of biovolume per m01 leucine incorporated into cold TCA precipitate, 0 4 pgC biomass per wm3 of biovolume, and 40% carbon growth yield.