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As part of an assessment of the ecological quality of surface waters, recent European water legislation, the water framework directive (WFD), requires the setting of environmental objectives for particular chemicals. As part of this, many European countries are choosing to develop a quality classification scheme for total phosphorus (TP) concentration. The assessment of ecological quality and its component parts, such as TP, must be based on the degree of divergence of a water body from an appropriate baseline, or 'reference condition'. For this reason, it is important to determine TP reference conditions for all lake types, or alternatively, models for predicting reference TP concentrations on a site-specific basis. With this purpose in mind, we have assembled a large dataset of European lakes considered to be in reference condition; 567 lakes in total. Data has been collated on TP concentration, mean depth, alkalinity, humic type, altitude, area, and geographical region. Reference TP concentrations have been derived from this dataset using two empirical approaches. Firstly, lake-type specific TP reference conditions were derived simply from descriptive statistics (median and percentiles) of TP concentrations by lake type. Secondly, empirical models were developed for estimating sitespecific reference TP concentrations from a set of potential predictor variables. TP concentrations were found to vary with lake type and by geographical region. TP increased with colour and alkalinity, and decreased with lake depth and altitude. There was no clear relationship between TP and lake area. Altitude, mean depth and alkalinity were selected as independent explanatory variables for prediction of site-specific reference TP concentrations, with different models developed for humic and non-humic lakes. A simpler site-specific predictive model based solely on the morphoedaphic index (MEI) (a combination of a site's alkalinity and mean depth) was also shown to be comparably effective.

The impact of various zooplankton groups on the cycling and retention of phosphorus in a mesotrophic lake in late summer was estimated in a 1 O-d mesocosm experiment with addition of 33P. Specific assimilation rates for various dissolved and particulate fractions and individual zooplankton species were estimated by application of tracer models. By means of mathematical inverse methods, these estimates were used to construct a balanced P budget that included the different zooplankton groups.Altogether, the consumers (including the mixotrophic Dinobryon) were responsible for >60% of the estimated P sedimentation (-1% d-l). The microconsumers had the large$t specific P assimilation (0.28 d-l) and regenerated 45% of assimilated P; Daphnia had an intermediate P assimilation (0.17 d-l), but regenerated <20%, whereas Cyclops assimilated P quite slowly (0.07 d-l) and regenerated 230% of its assimilation of algal P ~20 pm, due to its feeding on microconsumers.The microconsumers, and the daphnids in particular, thus act mainly as sinks for P, whereas the cyclopoids act mainly as sources of P by assimilating and regenerating P otherwise trapped in the microconsumers. The mechanisms responsible for these different roles seem related to growth rate and P content of the consumers relative to that of their diet.Phosphorus controls algal growth in many lakes (Einsele 194 1;Dillon and Rigler 1974;Schindler 1977). Zooplankton and other phagotrophic consumers modify the P supply to phytoplankton in two ways: they act as P sinks by incorporating P and as P sources by regenerating Acknowledgments

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