The spatial distribution of nutrients in the South Basin of Windermere

SUMMARY. The vertical and horizontal distribution of electrical conductivity, soluble reactive phosphorus (SRP), total phosphorus, ammonia-nitrogen and nitrate-nitrogen in the South Basin of Windermere is described and related, where possible, to spatial variations in phytoplankton and zooplankton. For most variables, the maximum range of horizontal variation on a single day was greater than the maximum range of seasonal variation measured at a single station over a full year. Horizontal variations in SRP and ammonia were particularly high, with coefficients of variation often exceeding 100%. The errors associated with sampling at a single station were most pronounced when local accumulations of zooplankton or phytoplankton occurred in the lake. Horizontal variations in conductivity were primarily influenced by the discharge of treated sewage effluent into the central region and the mass transport of water from the more oligotrophic North Basin. Occasionally, more widespread variations in conductivity appeared to be related to spatial variations in photosynthetic activity. The major factor influencing the horizontal distribution of SRP was the discharge of treated effluent into the central region. Despite the intensive recycling of phosphorus, SRP concentrations were nearly always 10–30% higher near the sewage works than elsewhere in the basin. On a few occasions, significant horizontal differences in SRP concentration were also detected within downwind accumulations of crustacean zooplankton. Horizontal variations in total phosphorus were produced by spatial differences in SRP concentration or by local accumulations of phytoplankton or zooplankton. Horizontal differences in ammonia concentration appeared to be produced, both by the effluent discharge and by the turbulent transfer of nutrients from sediments in shallow water. Observations of effluent movement and dispersion demonstrate that wind-induced water movements tend to recirculate nutrient-rich water in the central region and limit mixing along the axis of the basin. The statistical implications of nutrient heterogeneity are discussed in relation to sampling strategy and the possible effects of persistent nutrient concentration gradients on phytoplankton patch formation are assessed.