In this article, principles of phosphorus retention and phosphorus release at the sediment-water interface in lakes are reviewed. New results and hypotheses are discussed in relation to older models of phosphorus exchange between sediments and water. The fractional composition of sedimentary phosphorus is discussed as a tool for interpretation of different retention mechanisms. Special emphasis is given to the impact of biological, particularly microbial, processes on phosphorus exchange across the sediment-water interface and to the significance of biologically induced CaCO3 precipitation to phosphorus retention in calcareous lakes.

Exchange of phosphorus across the sediment-water interface

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

SUMMARY. 1. This review considers the factors which determine the recovery of eutrophic lakes following a reduction in the external phosphorus loading.



2 The mean phosphorus content of a lake should decrease roughly in proportion to the reduction in phosphorus input. Where the lake phosphorus concentration does not decrease as predicted, then the release of phosphorus from the sediment is implicated.



3. The current understanding of the processes by which sediment phosphorus is mobilized and transported into the photic zone is described. The extent to which phosphorus release can maintain lake phosphorus concentrations following the reduction in external loading is influenced by: lake morphometry, flushing rate, sediment type, trophic state and history of enrichment.



4. A reduction in the phytoplankton biomass of a lake is dependent upon the size of the decrease in lake phosphorus concentration and the degree to which phosphorus limits primary production. The importance of phosphorus in limiting phytoplankton production tends to decrease with increasing lake trophic status.



5. Improvements in the condition of highly eutrophic lakes require very large reductions in external phosphorus loading, whereas in mildly enriched lakes moderate changes in the supply of phosphorus have noticeable effects on phytoplankton biomass.

Lake restoration by reducing external phosphorus loading: the influence of sediment phosphorus release

Enhancement of dissolved phosphorus release from sediment to lake water by Microcystis blooms--an enclosure experiment in a hyper-eutrophic, subtropical Chinese lake.

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 1−1. Annual phosphorus retention decreased with increasing TPsum and was lower than predicted from the Vollenweider model, particularly in lakes with TPsum above 0.2 mg P1−1. The seasonal phosphorus retention in lakes with TPsum below 0.1 mg P 1−1 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 above 0.1 mg P 1−1, 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 TPsum above 0.2 mg P 1−1. 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.

Jens Moller Andersen

Papers

Effect of nitrate concentration in lake water on phosphate release from the sediment