The importance of catchment and lake processes in the phosphorus budget ofa large lake

Total phosphorus (P) river inputs and lake concentrations for the Neagh sys tem in Northern Ireland are compared from the period 1974-1997. The main ro utes of P transfer between the lake water and the sediment are settlement o f abiotic particles and planktonic diatoms, summer sediment release and re- sedimentation in the last months of the year. The annual river loading to t he lake varied between 0.7 and 1.8 g P m(-2), and sediment release can be a s much as 1.4 g P m(-2). A simple model evaluated the effect of sediment-wa ter exchanges on the phosphorus available for spring phytoplankton growth. It showed that re-sedimentation of released P and washout over the winter g reatly mitigated its effect. Correlation analysis demonstrated that the ver y large summer releases of sediment P were not related directly to the spri ng inputs from diatom settlement. No long-term trends in P release were see n. (C) 2000 Elsevier Science Ltd. All rights reserved.