Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zoo-plankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the 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 the 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 accounted for about half the observed variation in excretion rates. The mean excretion 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 between 22 and 239% of the P-demand of phytoplankton during the different months of the study period. In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, 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 daphnids 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 zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used. About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the 'real' rates. [KEYWORDS: phosphorus; nutrient; regeneration; Daphnia; herbivores; phytoplankton; P-demand; primary production; Lake Vechten ; limnetic zooplankton; phosphate excretion; nitrogen-excretion; loosdrecht lakes; food quality; phytoplankton; release; rates; ammonia]
Original languageEnglish
Journal publication date1995

ID: 66389