An empirical model describing the seasonal dynamics of phosphorus in 16 shallow eutrophic lakes after external loading reduction

Based on monthly mass balances on 7–8 yr of data from 16 shallow (mean depth: 1–10 m), eutrophic, unstratified, or only temporarily stratified Danish lakes, we developed a simple empirical model relating the seasonal variation in lake total phosphorus (TP) concentrations to external loading, accumulated phosphorus in the sediment, hydraulic retention time, and water temperature. The aim was to describe the early recovery phase following an external loading reduction, i.e., when internal phosphorus loading is high, and to include seasonal dynamics. We calibrated a common set of model parameters for all 16 lakes and lake-specific estimates of the exchangeable phosphorus pool in the sediment (Ps). Estimated annual mean TP deviated on average 12% from observed values in the 16 lakes. Moreover, the estimated seasonal dynamics and trend following the external loading reduction closely mimicked the observed pattern. The model was successfully tested on nine of the lakes for which data were available for an additional 7-yr period. The results suggest that TP in the sediment does not provide an adequate description of the exchangeable P pool. In Lake Arreskov, which has shifted from a turbid to a clear-water state following fish kill and biomanipulation, the model significantly overestimated TP, indicating that the model is inadequate for describing seasonal dynamics during the shift from a turbid to a clear-water state. Although simple, the empirical model predicts reasonably well the seasonal dynamics of TP following a P-loading reduction in a variety of shallow turbid lakes. Cultural eutrophication has considerably impaired lake ecosystems worldwide (Hutchinson 1973; Sas 1989). In northern temperate lakes, total phosphorus is regarded as the key factor of eutrophication (Schindler 1975). To improve water quality, many countries have during the last two decades reduced the external nutrient loading of lakes by improving wastewater treatment, including P removal; by increasing catchment retention capacity; and by reducing the phosphorus content of fertilizers and detergents (Phillips et al. 1999; Van der Molen and Boers 1999). However, following the external loading reduction, many lakes suffer from high internal loading, which delays recovery (Marsden 1989; Sas 1989; Jeppesen et al. 2005b). To help managers define acceptable external phosphorus loading levels and predict the effects of various measures to reduce the loading, numerous simple empirical and dynamic models have been developed. The most simple of the models relate lake water total phosphorus (TP) to external loading and require that the lakes be in a steady state (Dillon and 1 Corresponding authors ([email protected], [email protected]).