Stream ecosystem function in urbanizing landscapes

Ecologists have described an urban stream syndrome with attributes such as elevated nutrients and contaminants, increased hydrologic flashiness, and altered biotic assemblages. Ecosystem function probably also varies with extent of urbanization, although there are few stream networks in which this prediction has been studied. We examined functional characteristics of 6 tributaries of the Chattahoochee River near Atlanta, Georgia, USA, whose catchments differed in degree of urbanization. We conducted short-term NH4- and PO4-addition experiments to measure nutrient uptake velocity, which is the rate at which a nutrient moves through the water column toward the benthos. Both NH4 and soluble reactive P uptake velocities decreased as indicators of urbanization (i.e., % of catchment covered by high-intensity urban development) increased. The amount of fine benthic organic matter (FBOM) also decreased with increasing urbanization, and uptake velocities were directly related to FBOM. Uptake velocities were not related to ecosystem metabolism (gross primary production [GPP], community respiration [CR], or net ecosystem production) as measured with diel oxygen curves. However, NH4 uptake velocity increased as total stream metabolism (GPP + CR) increased in these streams as well as in other North American streams, suggesting that biotic demand drives NH4 uptake velocities across a wide range of stream ecosystems. Measures of ecosystem function responded differently to urbanization: ecosystem metabolism was not correlated with indicators of urbanization, although breakdown rate of Acer barbatum leaves was positively correlated and nutrient uptake velocities were negatively correlated with indicators of urbanization. Elevated nutrient concentrations associated with urbanization are usually attributed to increased inputs from point and non-point sources; our results indicate that concentrations also may be elevated because of reduced rates of nutrient removal. Altered ecosystem function is another symptom of an urban stream syndrome.