ZOOPLANKTON AS A COMPOUND MINERALIZING AND SYNTHESIZING SYSTEM - PHOSPHORUS EXCRETION

Data on phosphate excretion rates of zooplankton are based on measurem ents using the pelagic crustacean zoo-plankton of Lake Vechten and lab oratory-cultured Daphnia galeata. In case of Daphnia sp we measured th e effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20 degrees C. The excretion rates of the natural zooplankton community, irrespective of the influence of t he factors mentioned, varied by an order of magnitude: 0.025-0.275 mu g PO4-P mg(-1) C in zooplankton (C-zp) h(-1). The temperature accounte d for about half the observed variation in excretion rates. The mean e xcretion rates in the lake, computed for 20 degrees C, varied between 0.141 and 0.260 mu g P mg(-1) C-zp h(-1). Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered b etween 22 and 239% of the P-demand of phytoplankton during the differe nt months of the study period. In D. galeata, whereas the C/P ratios o f the Scenedesmus used as food differed by a factor 5 in the experimen ts, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258 +/- 0.022 mu g PO4-P mg(-1) C h(-1)) of the d aphnids fed with P-rich food than those fed with P-poor food (0.105 +/ - 0.047 mu g PO4-P mg(-1) C h(-1)), both the categories of the animals were apparently conserving P. ii survey of the literature on zooplank ton excretion shows that in Daphnia the excretion rates vary by a fact or 30, irrespective of the species and size of animals and method of e stimation and temperature used. About two-thirds of this variation can be explained by size and temperature. A major problem of comparabilit y of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the an imals several times above the in situ concentration (crowding) and pro longed experimental duration (starving), both of which manifest in mar ked changes that probably lead to underestimation of the 'real' rates.