Pre-restoration baseflow dissolved nutrient dynamics in an urban forested headwater system, Charlotte NC
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Abstract
Headwaters provide ecosystem services for humans such as clean water, recreational opportunities, nutrient removal, and biodiversity. Baseline stream nutrient and TSS concentrations and loadings are valuable water quality characteristics to quantify changes following landuse or land management activities. I quantified baseline concentrations for a study period of 18 months in the Reedy Creek headwaters, located in the Piedmont of North Carolina. Surface water samples were collected in the nested landuse subwatersheds (agricultural, developed, and forested control) to be used as a pre-restoration dataset. 2016-2017 monthly surface water concentrations were used to calculate annual loading for the subwatersheds for ammonium, nitrate, phosphate, total suspended solids (TSS) and total phosphorus (TP). Concurrent flow and area ratio discharge approximations were used to estimate stream discharge at ungaged tributaries from the USGS gage located at the outlet of the total watershed USGS gage (0212427947 Reedy Creek at SR 2803 NR Charlotte, NC).At the monthly time scale, dissolved nutrient concentrations varied with landuse, however, the results were not statistically significant. There were significant seasonal differences in concentrations for ammonium, TSS and TP concentrations. The average concentration of these three constituents vary significantly with growing season having a greater average concentration than the average concentration for the dormant seasonNitrate loading was highest from the agricultural watershed (0.57 kg/ha) compared to the whole watershed (0.54 kg/ha), the developed subwatershed (0.35 kg/ha) and the control subwatershed (0.18 kg/ha). The phosphate annual load from developed subwatershed was 0.024 kg/ha which is 2.2 times greater than the control subwatershed. The mainstem had a phosphate annual load of 0.026 kg/ha which was 2.4 times greater than the control (C2) subwatershed. These results suggest that the development does influence the annual nutrient loads for nitrate and phosphorus at baseflow when compared to the control (C2) subwatershed, although there was not a statistically significant difference. There was shown to be a significant difference in the growing season and dormant season means for daily loading in g/hectare for all constituents analyzed, suggesting that nutrient and TSS loading during the growing season is greater than loading during the dormant season. The mainstem sites (R2 and R1) had the greatest monthly loading for all constituents. The agricultural and developed subwatersheds had greater peak loading at the monthly scale compared to the control subwatershed. The large spikes in loading are most likely due to increased concentrations resulting from landuse disturbance and increased discharge due to channel incision and scour in all impacted tributaries. The results demonstrate that landuse affects nutrient loading through changing the discharge regime at baseflow and by altering nutrient concentrations in a primarily forested watershed.