Gross Excretion Rates of Natural Marine Phytoplankton and Heterotrophic Uptake of Excreted Products in the Southern North Sea, as Determined by Short-Term Kinetics

Phytoplankton extracellular production has been determined kinetically during the spring phytoplankton bloom in three marine biotopes representative of the Southern North Sea (Scheldt estuary, Belgian coast and English Channel), differing greatly in their biological activities. Short-term kinetics of phytoplankton extracellular compounds produced during 14C incubations have been shown to increase our present capability for estimating gross phytoplankton extracellular production, if compared with a single measure after 4-h incubation. Underestimates of up to 60 O/O were found when a fixed incubation of 4 h was used. This underestimate can be attributed either to heterotrophic uptake of excreted products or to the presence of a lagphase in 14C extracellular compounds prod.uction, related to physiological properties of phytoplankton. The data indicate that the phytoplankton extracellular release percentage, expressed with respect to the total photo-assimilated carbon, ranges from 0 to 62 O/O with lower values in the estuarine area. Seasonal variations show, in Belgian waters and in the English Channel, an increase in phytoplankton excretion percentage during the bloom from about 20 to 60 "/o. Heterotrophic uptake of phytoplankton extracellular products has been calculated from the evolution curve of I4C phytoplankton excretion products by means of a simple mathematical model. Results indicate that heterotrophs can occasionally remove up to 39 V" of the total amount of excreted matter per hour indicating that phytoplankton excretion can sometimes produce substances wlth high turnover