Seasonal dynamics in the concentrations and retention of phosphorus in shallow Danish lakes after reduced loading

Abstract

To investigate how phosphorus concentrations and retention were affected during recovery after reduced external phosphorus loading, phosphorus concentrations and loading were seasonally monitored for 11 years in 15 shallow eutrophic Danish lakes. Hydraulic loading, inlets, and lake water were sampled monthly during the winter (October–April) and bimonthly during the summer (May–September) to estimate monthly mass balances. Total phosphorus concentrations in the lakes were two to three times higher during summer than during winter; internal loading seems to be of great significance. The retention of phosphorus was negative from April until October, the most negative values generally occurring in May and July (–30% of external loading). In June, retention was less negative (−12%), probably influenced by clearwater phases which often appear in early summer. During the 11 years, the period with negative phosphorus retention decreased from five to four months and, as a median, the maximum phosphorus concentration declined from ca. 0.4 to 0.2 mg phosphorus l^−l. In Lake Engelsholm, which was biomanipulated by a 66% reduction of the zooplanktivorous fish stock (mainly bream, Abramis brama), marked changes were observed in biological structure. Also affected were phosphorus concentrations and retention. Simultaneously, with an increase in Secchi depth from 0.7 to 2.0 m, summer concentrations of total phosphorus decreased to less than 50% of the pre-biomanipulation level. The period exhibiting negative phosphorus retention declined from six months before the intervention to four months afterwards. It is concluded that the recovery period after reduced phosphorus loading may last long in shallow eutrophic lakes, but that the recovery is significantly influenced by biological structure. During recovery, both the duration and net phosphorus release rates from the sediment declined progressively.