Daphnia population growth but not moulting is a substantial phosphorus drain for phytoplankton

1. Negative effects of zooplankton on the availability of phosphorus (P) for phytoplankton as a result of the retention of nutrients in zooplankton biomass and the sedimentation of exoskeletal remains after moulting, have been recently proposed. 2. In a mesocosm study, the relative importance of these mechanisms was tested for the freshwater cladoceran Daphnia hyalina x galeata. A total of 13 mesocosm bags was suspended in a mesotrophic German lake during summer 2000 and fertilised with inorganic P in order to obtain a total nitrogen to total P ratio closer to the Redfield ratio. D. hyalina x galeata was then added at a logarithmically scaled density gradient of up to 40 ind. L-1. Zooplankton densities, dissolved inorganic, particulate organic (seston < 100 μm),as well as total nutrient concentrations were monitored. Additionally, nutrient concentrations of sediment water removed from the bottom of the mesocosm bags via a manual pump were determined. 3. Seston carbon (C), seston P and total P were significantly negatively correlated with Daphnia densities. The amount of particulate P (&SIM;5-6 μg P L-1) sequestered from the seston compartment by Daphnia corresponded roughly to the increase of zooplankton biomass ( population growth). Soluble reactive phosphorous (SRP) was at all times high (&SIM;25-35 μg P L) 1) and possibly unavailable to phytoplankton as a result of P adsorption to calcite during a calcite precipitation event ( whiting). P concentrations determined in sediment water were generally < 60 mug P m(-2) and thus never exceeded 1% of the total amount of P bound in particulate matter of the overlying water column. 4. Seston C : P ratios followed a polynomial second-order function: At Daphnia densities < 40 ind. L) 1 a positive linear relationship was evident, which is explained by the stronger reduction of P compared with C in seston, and transfer of seston P to zooplankton. Highest seston C : P ratios of &SIM; 300 : 1 were observed at Daphnia densities of &SIM;30-50 ind. L-1, which is in agreement with proposed threshold values limiting Daphnia reproductive growth. At Daphnia densities > 40-50 ind. L-1 C : P ratios were decreased because of the strong reduction of seston C at close to constantly low seston P-values of similar to3-4 mug P L-1. 5. At least for Daphnia, it may be concluded that - unlike population growth - the sedimentation of faecal pellets and carapaces after moulting seem negligible processes in pelagic phosphorus dynamics