Our conceptual understanding of the role of heterotrophic bacteria in pelaglc ecosystems and in ocean biogeochemical cycles is closely linked to our understanding of how their growth rate, abundance, and diversity is controlled. Here we discuss consequences of the simplifying assumption that there are only 5 potentially important interactions between heterotrophic bacteria and their biological and chemical environment. We consider 3 possible types of growth rate limitation: (1) organic carbon, (2) inorganic phosphate, and (3) organic and inorganic nitrogen; and 2 types of cell losses: (1) predation by heterotrophic flagellates, or (2) lysis by infectious viruses. Incorporating this into sirnple food web structures, we discuss 4 classes of models, 2 based on carbon limitation and 2 based on mineral nutrient limitation of bacterial growth rate. Bacterial abundance is assumed to be controlled by protozoan predation in all cases. For each class, we derive expressions describing bacterial carbon demand, and dscuss the control of bacterial carbon demand, growth rate and diversity. It is shown how models predicting an ecosystem production of dissolved organic carbon (DOC) exceeding bacterial carbon demand may be constructed assuming either a low degradability of the DOC, or mineral nutrient h i t a t i o n of bacterial growth rate. For 2 classes of models, infectious viruses are shown to affect neither growth rate nor abundance of the steady state bacterial community. For all 4 classes of models, viruses are suggested to control diversity of the steady state bacterial community.

Nutrient limitationand grazing control of the planktonic community were studied in the northern Baltic Sea off the SW coast of Finland during the phytoplankton growth season of 1985. In situ experiments based on a 23 factorial design were performed in mesocosm enclosures on 10 occasions. The manipulations used included phosphorus (PO,? ) and nitrogen (NH,+ ) additions and the removal of metazooplankton by 1 OO+m prefiltration.In each experiment, the responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, and protozooplankton were followed for 2 d. Orthogonal multiple regression analysis was used to reveal which manipulations had statistically significant effects. Nitrogen was found to be the basic limiting nutrient for phytoplankton throughout the productive season. During early summer, only the combined addition of P and N evoked a clear increase in the growth of phytoplankton.In general, bacterial productivity was not highly affected by the manipulations. In summer the removal of metazooplankton caused a rapid increase in the amount of protozooplankton in the units with loo-pm prefiltration or prefiltration combined with N addition. In the absence of metazooplankton, the nutrient-induced increase in primary productivity was channeled to protozooplankton, whose growth in the units where metazooplankton was present was severely limited by food competition or by direct metazooplankton grazing.

In 1985, algal biomass, primary productivity (incorporation of 14C, acidified water sample), excretion of organic matter (exudation), and bacterial secondary productivity were followed off the SW coast of Finland, in the nothern Baltic Sea. Molecular size fractionation of the dissolved (net) excreted organic carbon pool (EOC,) was carried out by gel filtration. During the phytoplankton growth season the mean algal standing stock was 0.54 g C m-', and picoalgae (< 2 pm) represented on average 17 % of the total algal biomass. The primary production was 84.2 g C m-' yr-l. Annual EOC, and total exudation (EOC, plus bacterial uptake of exudates) values amounted to 4.6 and 7.1 % of primary production; at the same time, the net and total exudation averaged 7.5 and 10.8 % of the current phytoplankton carbon biomass per day. Net bacterial production in the trophogenic layer was 12.1 g C m-2 yr-l (3~-thymidine method) or 38.4 g C m-' yr-' (dark 14C02 uptake); hence, assuming an assimilation efficiency of 50 %, bacteria were able to satisfy 25 or 8 % of their carbon demand via exudate uptake. Throughout the productive period, EOC, consisted mainly of compounds of 300 to 600 daltons; larger compounds (1500 and > 10 000 daltons) were also observed. On the basis of the size of the excreted compounds, exudation probably took place via mediated transport across the algal cell membrane, rather than via passive leakage from the cell.

During the spring bloom in 1988, the dynamics of planktonic carbon flow were studied weekly in the euphotic layer in the northern Baltic. The spring bloom developed after the formation of a s l~g h t vertical salinity gradient near the surface at the end of April, and a peak in phytoplankton primary productivity and biomass (dominated by the dinoflagellate Peridiniella catenata) was reached about 1 wk later. The biomass of all heterotrophic compartments, especially that of bactena and copepods, increased strongly durlng the peak and declining phases of the algal bloom, showing that their success was closely linked with the bloom. During the whole bloonl period, the integral pnmary production (I4C incorporation) was 45.5 g C m-', and 'new' (NO<-N-based) production contributed about 80% of this value. The rotifers-copepods grazing chain and the bacteria-heterotrophic nanoflagellates-ciliates 'microbial loop' consumed directly about the same amount (3.5 g C m-') of phytoplankton carbon. Algae accounted for 64% of the total carbon consumption of zooplankton. Sedimentation corresponded to 72% of the primary production. The sum of algal biomass increase and loss factors (exudation, grazing and sedimentation) was 94% of the integral primary production, which supports our conclusion that there is a strong imbalance between primary and secondary production in the vernal planktonic food web off the SW coast of Finland.

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