INTERANNUAL VARIABILITY OF ALGAL POPULATIONS AND THEIR INFLUENCE ON LAKE METABOLISM

1. An input-output phosphorus budget is given for Windermere and its t wo basins based on data available for the late 1980s. The annual areal total phosphorus loading for the whole lake was 1.04 g P m(-2) yr(-1) and for the North and South Basins were 1.08 and 1.70 g P m(-2) yr(-1 ), respectively. For the whole lake and its South Basin the values wer e similar to the upper range of critical loads calculated according to the equation of Vollenweider (1976) for the transition between oligot rophy and eutrophy while that for the North Basin (1.08 g P m(-2) yr(- 1)) was within this range of critical loadings but towards its lower e nd. 2. Changes in the quality of summer phytoplankton are described fo r Windermere, particularly its South Basin, between 1978 and 1989 in r elation to the utilization of nitrate-nitrogen (NO3-N) in the epilimni on, deoxygenation of the hypolimnion and the ratio of epilimnetic volu me to hypolimnetic volume, E(v)/H-v. The two basins of Windermere with values of E(v)/H-v of 0.79 (South Basin) and 0.50 (North Basin) have contrasting conditions of summer deoxygenation. The shallower South Ba sin shows marked interannual variability in the development of hypolim netic anoxia. Years with large hypolimnetic anoxia during autumn are c orrelated with the production during summer of large populations of th e poorly grazed blue-green alga Oscillatoria bourrellyi and exhaustion of NO3-N in the upper layers. During years when anoxia does not devel op the summer phytoplankton consists of small easily grazed algae or l arger ones subject to parasitic epidemics. The deeper North Basin neve r becomes anoxic even though it can contain similar sized populations of O. bourrellyi to the South Basin. 3. A possible explanation of the between basin and, for the South Basin, between year variation of util ization of NO3-N and level of hypolimnetic deoxygenation is that algal quality can determine lake metabolism dependent upon lake or basin mo rphology. Poorly grazed large forms such as O. bourrellyi act as sinks for NO3-N. On sedimentation such populations act as a 'short circuit' mechanism descending into deeper layers in sufficient quantities to c ause anoxia. Other species subject to crustacean or microbial grazing are mineralized in the epilimnion with little sedimentation to the dee per waters. Subsequent recycling of nitrogen as NH4-N takes place in t he upper layers or thermocline which is more readily taken up by subse quent production. The influence of such 'short circuit' mechanisms is reduced in deep lakes and exacerbated in shallow ones. 4. The success of species such as O. bourrellyi is dependent upon a sufficient inocul um, an adequate supply of nutrients and the depth of intermittent mixi ng. The importance of these factors in regulating presence and timing of summer populations is illustrated and discussed.