Description
Elevated nutrient loads can impair ecological functioning in receiving water bodies, including those along urbanized coastlines. Riparian corridors provide ecosystems services including retention of anthropogenic nutrient loads and can therefore protect downstream ecosystems, but the retention varies with discharge and is not well documented in southern California. This study examined longitudinal patterns of stream discharge (Q), nutrient concentrations and nutrient loads in Escondido Creek, which discharges to an ecologically sensitive coastal wetland in Southern California. The upper portion of the watershed is urbanized, the middle subwatershed is undeveloped, and the lower subwatershed has lowdensity residential land use. Q and concentrations of dissolved inorganic nitrogen (N) and soluble reactive phosphorus (SRP) were measured at six sample locations on 9 different sampling dates covering a range of Q. A mass balance was calculated from observed data to measure the fraction of nutrient retention or release (R) in five reaches of Escondido Creek downstream from the highly urbanized reach. Stream discharge decreased with distance downstream from the most upstream site to the most downstream site during this investigation, indicating significant channel infiltration and loss to evapotranspiration. Nitrate concentrations and loads decreased with distance from the urban reach for every sampling event, including those with high discharge. By contrast, SRP concentrations increased with distance downstream, but load decreased because Q decreased. Overall, 80%, 90%, and 50% of the water, nitrate, and ammonium discharged from the urban upper watershed were retained in the mid and lower subwatershed, and 40% orthophosphate was added to the mid lower subwatershed. Overall retention in the mid- and lower subwatershed increased with discharge. Retention increased with the percent of the subwatershed undeveloped, except for the least developed watershed, which showed downstream increases in load due mainly to increases in discharge. Retention rates for nitrogen were high compared with other streams of comparable size, suggesting that riparian systems downstream of urban areas may play a large role in protecting coastal ecosystems from high nitrogen loads in southern California.
