Patterns of nutrient loading in forested and urbanized coastal streams

Abstract

As part of a larger investigation of the effects of coastal urbanization on estuaries, stream nutrient loading was examined over a range of hydrologic and seasonal conditions for an urbanized and a forested watershed (11 ha versus 37 ha). Despite the smaller size, the urbanized stream produced 72% greater annual streamflow volume (162 versus 94 × 10 3m 3 · yr −1), and 66% greater annual sediment load than the forested stream (1796 versus 1082 kg · yr −1 ). This was due to the longer period of groundwater interception at the urbanized site (increased drainage), and the elevated sediment production resulting from deep excavation (2.1 versus 0.4 m), bank instability, and resuspension of sediment. Mean annual DOC concentration in the urbanized stream (13 mg C · l −1 ) was only half as concentrated as the forested stream (26 mg C · l −1 · yr −1). However, the annual DOC load between streams was within 10% (urban 2.25 · 10 3 versus forest 2.5 × 10 3kg · C · yr −1 ) due to the greater runoff volume at the urbanized stream. More than twice the amount of dissolved inorganic nitrogen (NO x N and NH 4N) flowed out of the urbanized watershed than from the forested watershed (34 vs. 14 kg · N · yr −1 ). An even bigger difference between sites was observed with respect to the NO x N load. Greater runoff volume at the urbanized stream combined with higher mean annual concentration of NO x N ( 130 versus 43 μg N · l −1 · yr −1 ) resulted in 11 × more NO x N loading at the urbanized stream than the forested stream (18.0 versus 1.6 kg · N · yr −1). Near channel soil aeration brought on by deep excavation may have promoted more oxidized (and mobile) forms of mineral N. Transport patterns of the two forms of mineral nitrogen varied substantially between streams, with the urbanized site exhibiting a steady release of the NH 4N and NO x N the effects of an extensive unsaturated soil. In contrast, nitrogen loading of NH 4N and NO x N at forested Oyster Creek was more episodic, with spring pulses generating much of the load of NO x N (47%), and summer periods of high concentration resulting in most of the annual load of NH 4N (32%).