Impact of epilimnetic phosphorus supply and food web structure on phosphorus binding forms in settling material and sediments in a thermally stratified lake

Knowledge about the contribution of food web structure and nutrient concentration in lakes to phosphorus (P) sedimentation and remobilisation at the sediment surface is still poor. Using four large enclosures placed in a eutrophic, thermally stratifying lake, we studied the effects of the structure of the planktonic food web (with and without planktivorous fish, ±F treatments) and nutrient concentration (with and without fertilisation, ±N treatments) on P sedimentation. We investigated the total P content and P binding forms in settling material (TPSM) and of the uppermost 1 cm sediment layer (TPSed) during three consecutive stratification periods (2005-2007). Additionally, epilimnetic P (SRPEpi, TPEpi), chlorophyll a and biomass of total crustacean and Daphnia were measured. On a seasonal scale, Daphnia biomass tended to negatively influence chlorophyll a, sedimentation rate of total particulate matter and of P, but the latter two criteria did not differ significantly between treatments due to large fluctuations within each enclosure. The contents of TPSM and loosely adsorbed P in settling material decreased in the following order: −F/+N > +F/+N > −F/−N > +F/−N, indicating greater effects of nutrient addition than of food web structure. In sediments, organically bound P was 9-23% higher in −F variants compared to the corresponding +F treatments, thus indicating an effect of food web structure. Furthermore, positive correlations between SRPEpi, TPEpi, TPSM, TPSed, sediment reductant-soluble P and calcite bound P revealed an effect of the epilimnetic P concentration on P sedimentation and specific P binding forms. Compared to the composition of different P binding forms in the settling material, a considerable decrease of loosely adsorbed P (12-26%) and reductant-soluble P (14-21%), as well as an increase of organic P (14-26%) were observed in the uppermost 1 cm-layer of the sediments in all treatments. We conclude that both nutrient enrichment (+N) and food web structure (−F) enhance the P sedimentation and P content at the sediment surface. However, in addition to food web effects on organic P content in settling matter and sediments, factors like iron concentration and calcite precipitation might be of importance for P sedimentation and storage in sediments in complex systems such as lakes.