The Spatiotemporal Evolution of Storm Pulse Particulate Organic Carbon in a Low Gradient, Agriculturally Dominated Watershed

Streams and rivers integrate and transport particulate organic carbon (POC) from an array of aquatic and terrestrial sources. Storm events greatly accelerate the transport of POC. The sequences by which individual POC inputs are mobilized and transported are not well-documented but are predicted to be temporally transient and spatially dependent because of changes in forcing functions, such as precipitation, discharge, and watershed morphology. In this study, the 3rd−4th order agricultural stream network, Clear Creek in Iowa, U.S.A., was sampled at a nested series of stations through storm events to determine how suspended POC changes over time and with distance downstream. Carbon and nitrogen stable isotope ratios were used to identify changes in POC. A temporal sequence of inputs was identified: in-channel algal production prior to heavy precipitation, row crop surface soils mobilized during peak precipitation, and material associated with the peak hydrograph that is hypothesized to be an integrated product from upstream. Tile drains delivered relatively 13C- and 15N-depleted particulate organic carbon that is a small contribution to the total POC inventory in the return to baseflow. The storm POC signal evolved with passage downstream, the principal transformation being the diminution of the early flush surface soil peak in response to a loss of connectivity between the hillslope and channel. Bank erosion is hypothesized to become increasingly important as the signal propagates downstream. The longitudinal evolution of the POC signal has implications for C-budgets associated with soil erosion and for interpreting the organic geochemical sedimentary record.

Mild impacts do not subsidize aquatic insect communities in an Atlantic Forest stream

Odum’s perturbation theory hypothesizes that toxic pollutants cause damage to ecosystems early in the course of contamination. In contrast, organic pollutants enrich the ecosystem until it exceeds their carrying capacity, an effect known as the subsidy-stress gradient. Understanding this dynamic can improve the efficiency of river restoration programs and bring significant benefits to society by providing ecosystem services that were lost. However, the initial effects of the most common human-induced disturbances in Atlantic Forest streams are not well known, indicating the necessity to evaluate the subsidy-stress gradient in these vulnerable and diverse ecosystems.AimWe evaluated the composition and abundance of the community of aquatic insects from leaf litter of headwater streams in three conditions: a fully forested area (reference stream), a low-intensity urban settlement (urban stream), and a region with small farms dedicated to the cultivation of fruits and vegetables (agricultural stream).MethodsWe used alpha and beta diversity metrics and a specific biotic index to test the subsidy-stress gradient prediction.ResultsThe agricultural stream showed the most degraded ecological condition. The urban stream and the reference stream showed similarity in alpha diversity metrics. According to the biotic index, the streams showed a gradient of environmental quality, with the reference stream showing the best quality and the agricultural stream the worst quality.ConclusionsThe agricultural stream showed a decrease in the environmental quality consistent with the effect predicted by the subsidy-stress gradient due to toxic pollutants’ contribution. However, the low-intensity enrichment of organic matter from the urban settlement causes a disorder in the ecosystem that reduces its environmental quality, contrary to the predicted by the subsidy-stress gradient.