Sediments have a variable but complex role in nutrient cycling in most aquatic systems, and are a potential 'internal' source of pollutants. Release of phosphorus from lake sediment is a complex function of physical, biological and chemical processes and is not easy to predict for different systems. Nitrogen is not stored and released from sediments in the same way, so its turnover time within aquatic systems is quite rapid. Nitrogen concentrations tend to fall off relatively quickly following a reduction in external nitrogen loading, whilst this is not true for phosphorus because the sediments can hold such a large reservoir of this nutrient that input and output rates may become decoupled.
In some shallow coastal areas, tidal mixing is the dominant nutrient regeneration process, as the sediments are regularly disturbed and redistributed by changing water currents, making nutrient exchange with the water much more rapid.
Why is a lake in a catchment dominated by arable agriculture much more prone to eutrophication than one in a forested catchment?
First the arable catchment is likely to be receiving much more nutrient input in the form of fertilizers. Second and equally importantly, the soil structure is much less stable under arable systems and therefore more likely to erode and carry nutrients to the lake as suspended sediment.