Variability in the water chemistry of shallow ponds in southeast England, with special reference to the seasonality of nutrients and implications formodelling trophic status

The variability in water chemistry of samples taken on a monthly basis (Oct ober 1990-December 1991) from 31 shallow, artificial ponds in southeast Eng land was examined. The survey revealed great within-year variation in the c oncentrations of nutrients. Total phosphorus (TP) concentrations displayed no overall marked seasonal pattern, although many sites experienced summer peaks with a simultaneous increase in soluble reactive phosphorus (SRP) con centrations, indicating that sediment P release occurred. SRP and nitrate ( NO3-N) concentrations displayed a marked seasonal pattern similar to that o bserved in deep, stratifying lakes, with 55% and 94% of the sites surveyed, respectively, experiencing a decline in concentrations in the spring, main taining low levels throughout summer and the highest levels occurring in wi nter. Dissolved silica (SiO2-(Si) also displayed a marked seasonal distribu tion with a spring decline in concentrations associated with diatom growth, followed by an increase in the summer. The study demonstrated that intra-annual variability in nutrient concentrat ions is high and tends to be greatest in the most enriched waters. Thus, a high sampling frequency is required to provide representative annual mean d ata. Furthermore, annual means rather than winter-spring means provide more appropriate estimates of TP and SRP in these waters, owing to the importan ce of internal cycling of nutrients in summer. The findings are especially relevant to sampling strategy design and the averaging of seasonal water ch emistry data for use in predictive models of lake trophic status.