The importance of considering biological processes when setting total maximum daily loads (TMDL) for phosphorus in shallow lakes and reservoirs

Total maximum daily loads (TMDL) are required by the US Environmental Prote ction Agency for pollutants that have impaired the designated uses of surfa ce waters in the nation. Setting an appropriate TMDL requires quantitative information on both the external pollutant inputs and the processes affecti ng pollutant dynamics within the ecosystem. Here we focus on phosphorus (P) , a globally important pollutant of freshwater lakes. We consider how biolo gical processes (including those related to algae, plants, invertebrates an d fish) can influence the ability of lakes to assimilate P, and in turn the ability; of managers to select appropriate TMDLs. The primary focus is on shallow eutrophic lakes, with Lake Okeechobee (Florida, USA) serving as a c ase study. The paper deals only with in-lake processes as they relate to se tting the TMDL and not the subsequent issue of load allocation among pollut ion sources. The results indicate that the ability of a shallow lake to ass imilate P is substantially reduced when surplus levels of P occur in the wa ter column, the phytoplankton becomes dominated by cyanobacteria. the benth ic invertebrate community becomes dominated by oligochaetes, and submerged plant biomass is low. If some of these biological changes can be reversed i n a rehabilitation program then the lake may be able to support a higher TM DL. (C) 2001 Elsevier Science Ltd. All rights reserved.