INFLUENCE OF RIPARIAN VEGETATION ON COUPLED BIOGEOCHEMICAL CYCLES IN RESTORED URBAN STREAMS (Invited Presentation)

Altered hydrology and increased pollutant loads in urban areas have led to degraded stream ecosystems and increased pollutant transport. Enhanced biogeochemical processes in riparian zones are often bypassed by hydraulically efficient drainage systems that route stormwater directly to the channel. Stream restoration projects across the country are being implemented that reconnect the stream with its floodplain and stabilize the channel with riparian vegetation and instream grade control structures. The cumulative effects of restoration on instream nutrient (nitrogen, phosphorus) processing were investigated in six restored streams in North Carolina. Based on multiple linear regression analysis, retention of nitrate and phosphate at the reach scale was positively correlated with water column concentrations and sediment carbon and negatively correlated with stream width, depth and canopy cover. The type and extent of riparian vegetation controlled the amount of sunlight reaching the stream surface and influenced both the quantity and quality of organic matter inputs to the stream. Loss of canopy cover in newly restored streams led to an overall increase in nitrate uptake as autotrophic production contributed to higher levels of fine benthic organic matter. Additionally, experimental microcosms highlighted the importance of nitrate supply and secondarily carbon quantity and quality on denitrification rates. Newly restored streams with herbaceous riparian vegetation and minimal shading exhibited high standing stocks of algal biomass which corresponded to higher rates of denitrification likely fueled by bioavailable carbon. These shifts in carbon supply have the potential to greatly influence biogeochemical processes and thereby overall water quality.