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A modified differential filtration was used to measure extracellular release of primary products by natural phytoplankton populations and subsequent incorporation of these products by bacteria. Water samples were incubated with I4C labelled bicarbonate for 6 h and 24 h , respectively. A time course experiment showed that 6 h are sufficient for attaining an approximate isotopic equilibrium between the fractions of inorganic carbon and released organic carbon. Results reveal a strong relationship between primary production and release of organic substances. The highest concentrations of exudates (239 pg C 1-') released by phytoplankton amounts to 27.6 % of the primary production. Percentage of released substances depends on the composition of the algal populations. Skeletonema costaturn released between 5.1 % and 12.5 % of the primary products; nanoflagellates, up to 40 %; Prorocentrurn rnicans, slightly over 20 %; Chaetoceros sp., about 25 %. Up to 90 % of available exudates were utilized by bacteria. Bacterial incorporation of exudates fluctuated during the year. The highest percentage of exudates was incorporated in the presence of dinoflagellates and nanoflagellates. Bacterial utilization of exudates is compared with that of glucose.

Bacterial Incorporation of Organic Substances Released by Natural Phytoplankton Populations