Role of sediment and internal loading of phosphorus in shallow lakes

The sediment plays an important role in the overall nutrient dynamics of shallow lakes. In lakes where the external loading has been reduced, internal phosphorus loading may prevent improvements in lake water quality. At high internal loading, particularly summer concentrations rise, and phosphorus retention can be negative during most of the summer. Internal P loading originates from a pool accumulated in the sediment at high external loading, and significant amounts of phosphorus in lake sediments may be bound to redox-sensitive iron compounds or fixed in more or less labile organic forms. These forms are potentially mobile and may eventually be released to the lake water. Many factors are involved in the release of phosphorus. Particularly the redox sensitive mobilization from the anoxic zone a few millimetres or centimetres below the sediment surface and microbial processes are considered important, but the phosphorus release mechanisms are to a certain extent lake specific. The importance of internal phosphorus loading is highly influenced by the biological structure in the pelagic, and lakes shifting from a turbid to a clearwater state as a result of, for example, biomanipulation may have improved retention considerably. However, internal loading may increase again if the turbid state returns. The recovery period following a phosphorus loading reduction depends on the loading history and the accumulation of phosphorus in the sediment, but in some lakes a negative phosphorus retention continues for decades. Phosphorus can be released from sediment depths as low as 20 cm. The internal loading can be reduced significantly by various restoration methods, such as removal of phosphorus-rich surface layers or by the addition of iron or alum to increase the sediment's sorption capacity.