Nutrient biogeochemistry of urban ponds: Water chemistry in a highly developed landscape

Background/Question/Methods  Urban ponds are being built into residential and commercial development across North America to reduce peak storm flow and to store water. These ecosystems are also often intended to collect and store nutrients and contaminants from stormwater runoff. However, their ability to retain biogenic nutrients appears to vary considerably among ponds and with the nutrient examined. This variability is likely caused by internal pond chemical and biological processes, which should be controlled by pond morphology, external nutrient inputs, and/or water residence time. We studied the nutrient biogeochemistry of 45 urban ponds in 4 Ontario, Canada municipalities with two samplings (June and August) in summer of 2009. We sampled surface and bottom waters of ponds that varied in depth and surface area.

Results/Conclusions  We found >75% of ponds to have more than 2ºC difference between surface and bottom waters. Many of these ponds also were supersaturated with dissolved oxygen (up to 200% saturation) in surface waters but had low dissolved oxygen concentrations in bottom waters. Total dissolved phosphorus was less than 40 ug/L in surface and bottom waters during both June and August. However, there was generally less dissolved phosphorus during August than in June. This was accompanied by high algal biomass and typically low seston C:N and C:P ratios. Indices of less dynamic elements, such as conductivity, showed less depth variability within ponds and were not different between sampling periods. Our results indicate that internal chemical and biological processes likely control internal nutrient cycling in these pond ecosystems and may account for variable nutrient retention rates previously reported.