Phosphorus and nitrogen dynamics in urban waters: examining nutrient inputs to watersheds and sediment transformations in diverse lentic ecosystems

Abstract

Urbanization substantially alters the distribution and cycling of phosphorus (P) and nitrogen (N), with widespread consequences for aquatic ecosystems and the communities that rely on them. Cities concentrate human activities that generate an excess of P and N, which are then transported via impervious surfaces and stormwater networks to local surface waters. Such excess nutrient loading degrades water quality and contributes to eutrophication in local and downstream ecosystems. This dissertation focuses on three key processes that drive nutrient availability in urban aquatic ecosystems: sediment P mineralization and denitrification in stormwater ponds and shallow lakes, and nutrient inputs to stormwater from urban tree litterfall in the Minneapolis-St Paul metropolitan region. First, I use sediment incubations to demonstrate that the mineralization of organic P, an under-examined process in aquatic research, can be an important mechanism mobilizing bioavailable P from organic-rich sediments common in urban lentic systems. Furthermore, increasing temperatures exacerbated sediment P release, suggesting that warming may further destabilize sediment P. Second, I explored denitrification across diverse urban lentic sites and found reduced denitrification rates in sites with exceptionally high road salt inputs and high concentrations of total P. Road salt salinization drastically alters water column mixing, while high total P is associated with the loss of macrophytes. Both of these factors may reduce N removal capabilities by limiting the availability of reactants for denitrifiers. These findings underscore the differing drivers of urban P and N cycling, which challenge our ability to mitigate the effects of both. To that end, the final component of my research focuses on a possible shared point of management for P and N: litterfall from street trees. Using datasets to estimate nutrient inputs from street tree litterfall at the watershed scale derived from intensive street sweeping, I found that litterfall inputs frequently comprise a substantial fraction of total watershed nutrient exports, especially during the fall and spring and in watersheds with high street canopy cover. Efforts aimed at removing street tree litterfall may therefore meaningfully reduce nutrient loading to urban waters. Taken together, this body of work demonstrates that cities pose unique challenges but also opportunities for maintaining the integrity of surface waters. In an increasingly urban world, continuing to develop our understanding of the dynamic relationships between urbanized landscapes and their waters is essential for maintaining the integrity of surface waters at local and global scales.