Phosphorus cycling in urban aquatic ecosystems: connecting biological processes and water chemistry to sediment P fractions in urban stormwater management ponds

Phosphorus (P) dynamics in urban landscapes may differ from that in natural landscapes due to different P sources and unique environmental conditions. However, many aspects of P cycles in urban areas, especially within engineered aquatic ecosystems, remain largely unknown. Through this work, we aim to contribute to better understanding of P cycling in urban aquatic ecosystems by investigating P fractions in sediment and their relationship with ambient chemistry in surface water from six urban stormwater management ponds located in Ontario, Canada. We found that organic P contributed up to 75% of total P in pond sediment, but this percentage decreased significantly between our two sampling events in June and September 2012. This decrease coincided with increased rates of extracellular enzyme (especially phosphatase) activities, which is indicative of fast mineralization processes in these ecosystems. Moreover, the decreased sediment organic P was matched by increased water column P concentration. This inverse relationship suggests that the large organic P pool in pond sediment, and its fast decomposition, contributed to internal release of P from sediment and increased water column P concentrations. The dominance of organic P in sediment and the putative role of relevant biological processes (i.e., decomposition and productivity) in urban ponds found in this study strongly contrast with classic water management expectations of physicochemically controlled P dynamics and long-term P storage in sediment of aquatic ecosystems. This difference suggests that urban stormwater ponds may perform poorly in terms of P retention and thereby contribute to poor water quality in terms of P pollution to downstream urban watersheds. Thus, stormwater pond design and future management strategies should consider these biogeochemical features of urban ponds, including internal P release, to help prevent eutrophication of downstream ecosystems.