Evaluation of stream metabolism in small nested urban watersheds to assess effects of scale and stream restoration

Abstract:

Aquatic ecosystem functions including nitrate retention are significantly impacted by urbanization and stream restoration. Stream metabolism, the interaction between gross primary productivity (GPP), the conversion of sunlight into an organic energy source through photosynthesis, and ecosystem respiration (ER), the sum of all organic carbon converted to carbon dioxide, is an important control on nitrate retention. While previous work has shown spatial patterns of metabolism across restored and unrestored urban streams, this work examines finer spatial patterns within a small urban watershed by monitoring upstream and downstream of a restored reach. We hypothesize that nitrate concentrations can be attenuated by stream metabolism and higher autotrophy in a downstream restored stream reach. GPP and ER were modeled with the U.S. Geological Survey’s (USGS) streamMetabolizer package that incorporated dissolved oxygen (DO), electrical conductivity (EC), photosynthetically active radiation (PAR), and water level data from four nested locations across a 1.63 km2 watershed in the headwaters of Dead Run, a tributary of Gwynns Falls in Baltimore, Maryland. We identify and examine the drivers of in-stream metabolism at nested spatial scales to determine the influence on nitrate concentrations. Contrary to the River Continuum Concept, preliminary data from the nested watersheds show strong autotrophic trends in the headwaters and a switch to heterotrophy downstream. This work suggests the location of stream restoration in relation to nitrate sources could be an important consideration for successful watershed management strategies.