Extracellular release of photosynthetic products by freshwater phytoplankton populations, with special reference to the algal species involved

SUMMARY. Over three successive years, depth profiles of C‐fixation and excretion, chlorophyll‐a concentrations, phytoplankton species composition and bacterial numbers were determined in Lake Vechten, a slightly eutrophic lake in The Netherlands. Special attention was given to the method used to measure extracellular release. Excretion of dissolved organic 14C depended largely upon the photo‐synthetic activity of the phytoplankton, ranging from 0–2.5 mg m‐1 h‐1, representing a percentage extracellular release (PER) of 0–25%. During a period in August, however, a subsurface chlorophyll‐a maximum at 5–7 m depth coincided with high excretion rates of up to 10 mg Cm‐3 h‐1 (PER = 55%). Phytoplankton analysis revealed a stratification in numbers of Mallomonas caudata with a maximum at 5–7 m depth. The results suggest that in these water layers bacterial populations grew at the expense of the dissolved organic carbon compounds excreted by Mallomonas caudata. This means that extracellular release can temporarily function as an important nutrient source for the heterotrophie community in addition to the more or less constant dissolved organic carbon pool.