THE IMPACT OF ARTIFICIAL PONDS ON THE BIOGEOCHEMISTRY OF URBAN STREAMS

Many studies have been conducted on large lakes and their ability to retain and transform nutrients within river systems. In contrast, few studies have examined the potential for artificial urban ponds to retain and transform nutrients from inflowing streams. This study examined how processes such as photosynthesis, respiration, and denitrification transform dissolved carbon and nitrogen in small urban impoundments. Sixty-five water samples were collected from five different urban ponds in or near Greenville, South Carolina during June-July 2012. The water collected from each pond and its inflowing and outflowing streams were analyzed for major ions, alkalinity, ammonium, dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) concentrations. Specific ultraviolet absorption at 254 nm (SUVA₂₅₄) and fluorescence index (FI) also were measured to determine the origin of DOC. Concentrations of dissolved oxygen, bicarbonate, and DOC, as well as pH and FI, were significantly higher within the ponds compared to inflowing streams. By contrast, concentrations of carbon dioxide, nitrate, silicon, and sodium were significantly lower in the ponds compared to the inflowing streams. Also, sodium concentrations were lower in the outflowing streams compared to the inflowing streams. FI, but not SUVA₂₅₄ differed significantly among sample locations. A low FI for inflow and outflow samples suggest a terrestrial origin of DOC. In contrast, the higher values in the ponds suggest a microbial origin of DOC. DOC results indicate a significant terrestrial component that is modified in the pond by primary production (release of DOC from algae and aquatic plants). The changes in chemical composition between inflowing streams and the pond waters suggest influences of photosynthesis and denitrification, especially in regard to the carbon dioxide, DOC, and nitrate data. Also, the decline in silicon concentrations within the ponds suggests the influence of diatom production. Overall, our comparisons of the composition of stream and pond waters suggest that the ponds studied are biogeochemical hotspots within urban stream systems during summer months and that the ponds function as sinks for nitrogen, carbon, and silicon.