Re-oligotrophication by phosphorus reduction and effects on seston quality in lakes

We review published data of re-oligotrophication studies in different types of lakes and discuss whether re-oligotrophication by phosphorus reduction measure can affect seston quality in lakes due to an increase in C:P ratios of food for zooplankton. We analyze whether such an increase in elemental ratio may give rise to a stoichiometric constraint or bottleneck for regulation of phytoplankton by zooplankton and thus cause a decrease in trophic transfer efficiency. Because seston C:P ratios of seston tend to decrease with increasing P concentration in lakes, i.e. by increasing eutrophication, the reverse may be true, i.e. the C:P ratio of the seston will increase when lakes become reoligotrophicated by P reduction. This may imply lower food quality for P-demanding seston grazers, like Daphnia, which have both relatively low C:P ratios and are quite homeostatic with regard to C:P ratios. Consequently. the reduced grazer biomass may lead to lower grazing pressure on the phytoplankton and may result in lower trophic transfer efficiency. On the other hand, oligotrophication commonly leads to improved food quality in terms of composition of the phytoplankton community, e.g. increase of mixotrophs and reduced share of cyanobacteria. These two qualitative traits of seston (higher C:P ratios and change in phytoplankton composition) may thus work in opposite directions during re-oligotrophication. Most investigated lakes - both deep and shallow - responded by improvement in underwater light climate due to reduction of P loadings. Several compensatory mechanisms prevented the occurrence of a stoichiometric bottleneck for top-down control. From a few documented cases of both deep and shallow lakes, where seston stoichiometry, the presence of Daphnia and the trophic transfer efficiency were concurrently measured during the process of re-oligotrophication, we may conclude that the anticipated increase in C:P ratios of the seston is not straightforward. It depends on lake-specific properties such as morphometry, water residence time, water temperature and trophic structure. A stoichiometric bottleneck is only likely to occur during summer in (1) the epilimnion of small deep and stratified lakes with a long water residence time and low mixing regime, and (2) shallow lakes with high C:P ratios caused by a relatively high detritus content of seston and/or grazing resistant algae, like colony forming cyanobacteria. (C) 2008 Elsevier GmbH. All rights reserved.