Dissolved oxygen is a well-known control over P cycling in lakes. Stormwater ponds are designed to mix often to overcome problems associated with low DO. Our studies revealed that ponds mix less frequently than expected due to effects of wind sheltering and road salt. As a consequence, a majority of ponds remain thermally and chemically stratified for much of the summer. Stratification and high sediment oxygen demand produced anoxic conditions throughout much of the water columns of many (> 40%) ponds during summer in the Twin Cities area. While the causes of unexpectedly widespread prevalence of low oxygen conditions in many ponds are not completely known, they appear to arise from combined effects of high organic matter inputs in stormwater, heavy tree cover near ponds, high aquatic primary productivity, and the presence of free floating plants.
Anoxic conditions in extensive areas of ponds promoted P release from pond sediments. Anoxic conditions were also consistently associated with elevated stormwater pond TP concentrations. As for lakes, internal loading of P occurred at high rates under anoxic conditions and was strongly related to the redox-sensitive P in the sediments. Despite high temporal and spatial variation, mean DO and anoxic fraction measurement both showed strong relationships with surface water TP. Other parameters, such as pond age, size, and depth, were weakly related to TP levels. Major findings of this project related to pond water P, emphasizing dissolved oxygen controls on TP, are integrated into a simplified depiction of stormwater pond phosphorus dynamics in Figure 5-1.
Vegetation near and in ponds played key roles in pond P (Figure 5-1). The presence of shoreline trees reduced wind-driven mixing. Trees also added organic matter to stormwater, and directly to ponds, further increasing oxygen consumption via microbial respiration. Vegetation in ponds, and especially free-floating plants (i.e., duckweed) strongly reduced oxygen availability in ponds by lowering oxygen transfer. These dynamics may create a positive feedback loop where high duckweed cover drives down DO, further increasing P availability, leading to faster duckweed growth. The role of vegetation, near and in ponds, including submerged macrophytes which were not studied in this project, clearly deserves more research attention given their central role in pond P cycles.