Internal phosphorus loading in shallow Danish lakes

High phosphorus concentrations due to internal loading from the sediment with a strongly negative impact on lake water quality, is often seen in shallow lakes after a reduction of external loading. To analyse the nature of internal loading we studied 1. the seasonal phosphorus concentrations of 265 Danish shallow, mainly eutrophic lakes; 2. seasonal phosphorus mass balances and retention for eight years in 16 eutrophic lakes, and 3. phosphorus mass balances and changing sediment phosphorus profiles for 15 years in one hypertrophic 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 retention decreased with increasing TPsumand was lower than predicted from the Vollenweider model, particularly in lakes with TPsumabove 0.2 mg P l-1. The seasonal phosphorus retention in lakes with TPsumbelow 0.1 mg P l-1was positive during the whole season, except July and August when mean retention ranged from −10 to −30% of inlet loading. In lakes with TPsumabove 0.1 mg P l-1, the retention was positive during winter, but negative from April to September. The negative retention was most pronounced in lakes with the highest TPsum, particularly in May and July when mean retention ranged from −50 to −68% in lakes with TPsumabove 0.2 mg P l-1. The retention 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 the hypertrophic lake have now shown a 15-yr net annual negative retention following reduced external loading. Sediment profiles suggest phosphorus release 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 shallow eutrophic lakes may have a considerable and persistent impact on summer TP after reduced external loading.