Although there has been reduction in nutrient loadings from external sources, high nutrient levels and a prolific algal growth remain major stresses, affecting the water quality in Cootes Paradise, a coastal wetland at the western tip of Lake Ontario. It has been documented that internal loading, resulting from sediment P release, may be a significant contributor of the total P input to some lakes. To assess the importance of internal loadings from sediments to Cootes Paradise, nutrient fluxes from sediments at three locations were estimated. The investigated sites were representative of different sedimentary environments, including sites receiving effluents from a sewage treatment plant and combined sewer overflows. An unaffected site in the main body of the marsh was included for comparison. The results derived from the Fickian diffusion model indicate that sites receiving outfalls from the sewage treatment plant and combined sewer overflow had the highest nutrient fluxes. These two sites had also the highest ammonia-nitrogen fluxes. The lowest flux was estimated at the unimpacted site located in the main body of Cootes Paradise. The differences in nutrient fluxes appear to be attributable to spatial heterogeneity of bottom sediments. Sites which had the highest sediment phosphorus concentrations had the steepest porewater nutrient gradients and the highest fluxes. The results suggest that sediment phosphorus geochemistry is important in regulating the phosphorus concentrations in porewater, which consequently, governs the phosphorus fluxes from sediments. The data suggest that sediment may be an important source of nutrients in areas containing nutrient contaminated sediments and release of nutrients from these sediments may delay the recovery of the marsh even after the reduction of the external phosphorus loading.

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