Nutrient Uptake in Urban Wetlands of Valdivia, Chile (Smith et al., 2022)

Wetlands are key transitional areas between terrestrial and aquatic ecosystems that serve as critical carbon and nutrient sinks, but our understanding of wetland biogeochemistry in urban settings is limited. Urban development alters the timing and magnitude of hydrologic and geomorphic processes of wetlands, thereby influencing the source and composition of dissolved organic matter (DOM) and concentration of dissolved organic carbon (DOC) and nutrients [nitrogen (N) and phosphorus (P)]. Our objective was to quantify the influence of ambient DOC and nutrient concentration, landscape physiography, and dissolved organic matter (DOM) composition on water-column and whole-ecosystem nutrient uptake capacity in urban freshwater wetlands of Valdivia, Chile. We added dissolved inorganic nitrogen (NO3- + NH4+) and soluble reactive phosphorus (SRP; PO4-3) to open-water areas in wetlands (<em>n</em> = 9) along a gradient of watershed impervious cover, used as an indicator of urban land use. As added nutrients were removed from the water column, we quantified NO3-, NH4+, and SRP first-order loss rates (<em>k</em>), uptake velocity (<em>v</em>f), and total daily maximum uptake rate (<em>U</em>vol). We also used structural equation modeling (SEM) to identify significant predictors of nutrient uptake, including variables such as landscape physiography (e.g., % impervious cover, vegetation density, and water residence time), ambient nutrient and carbon concentration, and fluorescence spectroscopy indices (i.e., indicators of DOM quality). Watercolumn uptake of NO3- and SRP was a significant proportion of whole-ecosystem N and P uptake, particularly in smaller, more urban wetlands with high availability of labile DOM and short water residence time. N:P molar ratios were variable across wetlands indicating either N or P limitation, however, uptake was not explained by stoichiometry alone. Indicators of microbial DOM (e.g., BIX, FI) were positively correlated with NO3- and SRP uptake in the water-column, suggesting labile carbon may prime DOM and nutrient turnover rates. Overall, we show that increases in carbon lability and N:P stoichiometry interact to influence N and P uptake capacity in nutrient-polluted or nutrient-limited urban wetlands. Our study identifies the need to incorporate hydrologic and landscape variables–in addition to relative nutrient limitation–to better understand and manage nutrient removal capacity in urban wetland ecosystems.