Watershed-scale Land Use Change Increases Ecosystem Metabolism in an Agricultural Stream

Stream metabolism, in the form of gross primary production (GPP) and ecosystem respiration (ER), is an important metric of stream ecosystem function, given GPP and ER are integrative measurements of basal ecosystem activity that are highly sensitive to environmental change. In agricultural streams of temperate North America GPP can be mediated by water column turbidity associated with soil erosion during periods when the terrestrial landscape is bare (that is, typically late fall through spring; Oct–May in N America). We estimated a 10-year time series of stream metabolism using continuous dissolved oxygen measurements in an agricultural watershed (Shatto Ditch, IN), comparing metabolism metrics before and after vegetative cover was added to fields during normally fallow periods when they would otherwise be bare. Adding vegetative cover reduced water column turbidity by 54% during days with high precipitation (upper 25th percentile). We also found that GPP varied seasonally with light and temperature (range = 0.1–17.2 g m⁻² d⁻¹) and increased significantly in spring with landscape vegetative cover addition. Finally, we used a subset of storms to show that turbidity was lower and GPP was higher during storms after adding watershed vegetative cover, suggesting that increased GPP could be attributed to increased light availability with less turbid water. We found that ER also increased after adding vegetative cover, which we attribute, in part, to increased autotrophic respiration. These results suggest that water turbidity is a mediating driver of stream metabolism, particularly when other primary drivers are not limiting GPP. Likewise, stream turbidity can be mediated by land cover on the surrounding watershed, demonstrating a clear linkage between land use and stream metabolic signatures.