Prediction of annual and seasonal phosphorus concentrations in stratified and polymictic lakes

Phosphorus (P) concentrations in lakes arc summarized and reported in different ways. Annual whole lake P (P,,,,), epilimnetic summer P (P,,), and mixed water column P during fall turnover after summer stratification (P,,,) were investigated with data from up to 150 lakes. Differences between P averages are caused by temporal variation in thermal stratification and the size of internal P sources. To predict various P averages in specific lake types, I combined a P retention model that predicts annual retention due to sedimentation (R,,,,) with external and internal load estimates (L,,, L,,,) in a steady-state, P mass balance approach. Retention of internal and external loads in stratified and polymictic lakes can be predicted by the same model, R pred = E/(18 + q,,), where q,V is annual water load (m yr’). However, lakes with chemically enhanced P copre-cipitation (e.g., with calcium) are an exception. P averages can be predicted in stratified lakes, as P B”ll = We,, + gross L,,)k(l- R,,,) f’,,, = LxJq,U- RpwA although PcpL is overestimated in lakes with oxic hypolimnia. P concentrations can be approximately predicted in polymictic lakes as CL,,, + in-situ L,,,)lq,(l- R,,,