P-release from DOP by phosphatase activity in comparison to P excretion by zooplankton. Studies in hardwater lakes of different trophic level

Hydrolysis of natural dissolved organic phosphorus (DOP) in three hardwater lakes of different trophic level was calculated from kinetic data of phosphatase activity (PA) in different size fractions. DOP as well as kinetics of PA were determined every fortnight in depth profiles during the year 1990. 60% of DOP was assumed to be suitable substrate for phosphatases. The rate of hydrolysis increased markedly with higher trophic level. Average hydrolysis rate: of DOP in polytrophic lake Thaler See was 3.26 nM P min(-1) (6 mu g P-PO4 l(-1) h(-1)). In oligotrophic Lake Herrensee, dissolved phosphatases were responsible for more than half of the total hydrolysis. In the other two lakes, bacterial and algal surface PA dominated hydrolysis in changing parts depending on kinetics and DOP concentration. The regeneration rate of phosphate by PA was compared to phosphorus (P) excretion rate of zooplankton. Excretion was calculated from zooplankton data and excretion equations from the literature. In oligotrophic Lake Herrensee, excretion by zooplankton recycled in average 18% of the phosphate amount which was hydrolysed from DOP by PA. With higher trophic level, relevance of P excretion from zooplankton decreased drastically.