Experimental reductions in stream flow alter litter processing and consumer subsidies in headwater streams
| Authors: | Robert M. Northington, Jackson R. Webster |
| Year: | 2017 |
| Type: | Scientific Journal |
| Station: | Southern Research Station |
| DOI: | https://doi.org/10.1111/fwb.12898 |
| Source: | Freshwater Biology |
Abstract
Summary
- Forested headwater streams are connected to their surrounding catchments by a reliance on terrestrial subsidies. Changes in precipitation patterns and stream flow represent a potential disruption in stream ecosystem function, as the delivery of terrestrial detritus to aquatic consumers and downstream transport are compromised. The potential effect of drying on decomposition is critical to understanding ecosystem processes in these detritus-driven ecosystems.
- In this study, we experimentally altered stream connectivity to examine how altered water availability would influence litter decomposition. We used three catchments in the Coweeta Hydrologic Lab in North Carolina, U.S.A., establishing sites that were always wet, intermediately wet and completely terrestrial, while also experimentally diverting significant surface flow from downstream locations for several months.
- The flow manipulation significantly reduced the amount of water available to the three study reaches, leading to drastic changes in organic matter accumulation. Stream temperature and dissolved oxygen changed during the manipulation but only in response to season. Nutrient concentrations remained low or below detection during the experiment, demonstrating no response to the manipulation.
- Red maple and white oak demonstrated significantly different mass loss during the first few days of the experiment, but by the completion of the experiment, location was a more significant determinant of breakdown rate than leaf identity. Leaves placed in sites that were consistently wet decomposed at a higher rate than those in bank or terrestrial sites.
- Initially, leaf decomposition varied significantly based on site type and presence of water, which led to disruptions in microbial and macroinvertebrate colonisation and processing. High flows during large winter storms reconnected dried reaches and stimulated rates of decomposition due to colonisation by microbes and macroinvertebrates.
- Macroinvertebrate densities tracked microbial biomass accumulation on leaves, but were dependent upon location. Leaves in the manipulated reaches had significantly higher macroinvertebrate densities compared to the upstream control in the winter and spring in response to the delayed wetting of large standing stocks of leaves and subsequent colonisation by microbes.
- Our study demonstrated that even minor disruptions in stream connectivity at key times of the year have community- and ecosystem-level influences that alter decomposition. The resilience of these ecosystems will depend on the frequency of disturbances and the ability of organisms to adapt to changing resource conditions.