Excretion of organic carbon by phytoplankton:its relation to algal biomass, primary productivity and bacterial secondary productivity in the Baltic Sea

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.