APPLIED ISSUES A simple method for approximating the supportive capacities and metabolic constraints in lakes and reservoirs C. S. REYNOLDS and S. C. MABERLY Centre for Ecology and Hydrology Windermere, Ambleside, Cumbria, U.K. SUMMARY 1. A method for calculating and comparing the stoichiometric yields of phytoplankton standing crop (as chlorophyll a) is presented. 2. Some worked examples show how the model distinguishes lakes in which the supportive capacity is constrained mainly by the available phosphorus or by the nitrogen supply or by the light availability. Sites are identified where control moves among the constraining factors. 3. An instance of lake management is given where the diagnosis, remedy and the remediation have conformed closely to the model predictions. Keywords: carrying capacities, chlorophyll a, nutrient limitation, phytoplankton, stoichiometry Introduction The management of lakes and reservoirs to secure better water quality, particularly to correct the symp- toms of eutrophication, is a common objective in many countries. However, the only generalised model available to guide the setting of targets or the measurement of success is still the Vollenweider/ Organisation for Economic Co-operation and Devel- opment (OECD) regression (Vollenweider & Kerekes, 1980). Useful and valuable though the equation is as a statement about the long-range behaviour of lakes, it is not a predictor of the behaviour of any individual site (Reynolds, 1992), neither was it ever intended to be used as a site-specific guide to the management of water quality (Vollenweider, 1989). Moreover, as it relates phytoplankton crops (as chlorophyll a) to total phosphorus, its utility does not extend to lakes and reservoirs where other factors, such as nitrogen or light, may be more likely to be limiting crop size. In general, managers seeking to apply measures to restore water quality (itself often a euphemism for reducing excessive crops of phytoplankton) need to be informed about (i) the present carrying capacity of the water body and its regulation, (ii) the magnitude of the load reduction required to secure a reduced capacity, and (iii) how sensitive the system will be to a load reduction and within what time scale. As contractors to the water industries of several nations, we have frequently been confronted with these challenges. Experience has led us to develop a simple stoichiometric approach to the calculation of the supportive capacity of each of the critical resources in a given waterbody. This we have used to isolate those factors that are most likely to be genu- inely limiting the standing crop of phytoplankton it can support and to predicting how sensitive the system might be in responding to the achievement of a practicable reduction in any one of them. In spite of the simplicity of its assumptions, the approach is founded upon published relationships (Reynolds, 1992, 1999) and its outputs have proved verifiable in the cases of several English lakes, including Winder- mere. Verifiable or plausible outputs have been Correspondence: Professor C. S. Reynolds, Centre for Ecology and Hydrology Windermere, The Ferry House, Ambleside, Cumbria LA22 0LP, U.K. E-mail: csr@ceh.ac.uk Freshwater Biology (2002) 47, 1183–1188 Ó 2002 Blackwell Science Ltd 1183