Impacts of agricultural phosphorus use in catchments on shallow lake water quality: About buffers, time delays and equilibria

Phosphorus (P) losses caused by intensive agriculture are known to have potentially large negative effects on the water quality of lakes. However, due to the buffering capacity of soils and lake ecosystems, such effects may appear long after intensive agriculture started. Here we present the study of a coupled shallow lake catchment model, which allows a glimpse of the magnitude of these buffer-related time delays. Results show that the buffering capacity of the lake water was negligible whereas buffering in the lake sediment postponed the final lake equilibrium for several decades. The surface soil layer in contact with runoff water was accountable for a delay of 5– 50 years. The most important buffer, however, was the percolation soil layer that may cause a delay of 150–1700 years depending on agricultural P surplus levels. Although the buffers could postpone final lake equilibria for a considerable time, current and target agricultural surplus levels eventually led to very turbid conditions with total P concentrations of 2.0 and 0.6 mg L−1 respectively. To secure permanent clear water states the current agricultural P surplus of 15 kg P ha−1 yr−1 should drop to 0.7 kg P ha−1 yr−1. We present several simple equations that can be used to estimate the sustainable P surplus levels, buffer related time delays and equilibrium P concentrations in other catchment–lake systems