Characterizing nitrogen attenuation by headwater slope wetlands across different land uses

Abstract

Rapidly increasing population pressure and land conversion from forests to urban and agricultural uses have been important stressors on coastal wetlands. Headwater wetlands in the southeast U.S. are particularly vulnerable because of their small size- they are easily filled or impacted through ditching, piping or impoundments. Since headwater areas have a disproportionately high influence on watershed flow and nutrient flux, alterations to their functionality can have impacts to downstream and coastal waters. Although headwater wetlands are generally understood to support water quality, the magnitude and mechanisms for this function are often uncertain. We compared flow, NO2−-N + NO3−-N (NO2 + NO3) concentrations, and NO2 + NO3 loads at the inlet and outlet of four headwater slope wetlands across common land covers/uses (i.e., commercial-urban, residential-suburban, agriculture, and forested) in coastal Alabama, USA. Watershed sizes draining to wetlands ranged from 0.5–1.8 km2 and each wetland showed distinct and shared trends. Measured NO2 + NO3 concentrations were typically low (<0.4 mg L−1) in all wetlands except for the agricultural watershed. The wetland in the residential watershed experienced high NO2 + NO3 loading rates into the wetland (21.6 kg ha−1 yr−1) however this was because of unusually high discharge to the wetland relative to its watershed size (overall mean: 0.13 m3 sec−1). Annual NO2 + NO3 loading rates from the forested and commercial-urban lands were much lower (<1.0 kg ha−1 yr−1). Despite drainage alterations at all sites, wetlands still provided annual NO2 + NO3 load reductions ranging from 13.1–62.0% although the commercial-urban wetland achieved reductions primarily through hydrologic recharge. Evidence from this study and others along the northern Gulf of Mexico coast indicate that low-order stream discharge can be relatively high and in some cases may drive high nutrient loading rates. Even though the natural drainage to most headwater wetlands has changed, they still can provide reliable attenuation of nitrate and other nutrients particularly where efforts are made to maintain a more natural wetland hydroperiod.