Internal phosphorus loading in shallow Danish lakes

High phosphorus concentrations due to internal loading from the sediment wi th a strongly negative impact on lake water quality, is often seen in shall ow lakes after a reduction of external loading. To analyse the nature of in ternal loading we studied 1. the seasonal phosphorus concentrations of 265 Danish shallow, mainly eutrophic lakes; 2. seasonal phosphorus mass balance s and retention for eight years in 16 eutrophic lakes, and 3. phosphorus ma ss balances and changing sediment phosphorus profiles for 15 years in one h ypertrophic lake. Lake water, inlets and outlets were routinely sampled 10- 26 times annually. Total phosphorus (TP) concentrations during summer were two-four times higher than winter values in lakes with a mean summer total phosphorus concentration (TPsum ) above 0.2 mg P l(-1). Annual phosphorus r etention decreased with increasing TPsum and was lower than predicted from the Vollenweider model, particularly in lakes with TPsum above 0.2 mg P l(- 1). The seasonal phosphorus retention in lakes with TPsum below 0.1 mg P l( -1) was positive during the whole season, except July and August when mean retention ranged from -10 to -30% of inlet loading. In lakes with TPsum abo ve 0.1 mg P l(-1), the retention was positive during winter, but negative f rom April to September. The negative retention was most pronounced in lakes with the highest TPsum, particularly in May and July when mean retention r anged from -50 to -68% in lakes with TPsum above 0.2 mg P l(-1). The retent ion was generally less negative in June, when a clearwater phase typically occurs and close to 0 also in lakes with a high TPsum. Mass balances from t he hypertrophic lake have now shown a 15-yr net annual negative retention f ollowing reduced external loading. Sediment profiles suggest phosphorus rel ease from depths down to 25 cm and that net internal phosphorus loading may persist for another 15 yrs. It is concluded that internal loading of shall ow eutrophic lakes may have a considerable and persistent impact on summer TP after reduced external loading.