Sediments as a source of nutrients to hypereutrophic marshes of Point Pelee, Ontario, Canada

Elevated concentrations of nutrients, resulting in prolific algal growth, were reported in some open-water ponds in one of the finest surviving wetlands on the Lower Great Lakes, Point Pelee National Park, Ontario, Canada. A study was undertaken to identify the sources of nutrients to the most impacted pond in the complex, Sanctuary Pond. Sediment chemistry and chemistry of interstitial water were examined to determine the role of benthic sediments in nutrient dynamics. The results indicate high concentrations of dissolved nutrients (about 4 mg/l of P and over 20 mg/l of N) in sediment porewater, a consequence of organic matter decomposition. No spatial differences were observed between porewater profiles, nor in the levels of dissolved constituents. Likewise, sediment P concentrations were similar at both investigated sites. Thermodynamic calculations, carried out to determine mineral equilibria of phosphate minerals potentially controlling the concentrations of dissolved P in sediment porewater, suggest that P generated from organic matter decomposition should be reacting with Fe, Ca and Al to form authigenic mineral phases vivianite, hydroxyapatite and variscite. The high levels of nutrients in porewater, the lack of spatial differences in sediment and porewater nutrient concentrations and the absence of known external nutrients sources suggest that at the Sanctuary Pond internal regeneration of nutrients from sediments is responsible for the hypereutrophic conditions in the pond.