Scientists Start at the Base of the Food Chain to Understand Contaminant Affects on Energy Cycling in Streams

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Study examines the potential adverse effects of fungicides on leaf decomposition by microbes and aquatic invertebrates.

Stream food webs are complex and depend on the successful breakdown of energy sources at the base of the food web. Alterations in the energy flow at the base of the food web can result in changes for organisms at higher trophic levels, such as fish and ultimately humans.

Leaf litter is a primary source of carbon and energy for stream food webs. Leaves that fall into streams are colonized by fungi and bacteria. This microbial film attracts aquatic macroinvertebrates who graze the microbial film and shred the leaves. The breakdown of the leaves by fungi, bacteria, and macroinvertebrate shredders promotes energy flow through carbon and nutrient cycling.

USGS scientists recording information on water-quality samples

U.S. Geological Survey (USGS) scientist works with University of Maine researcher recording information on water-quality samples and field water-quality parameters. The scientists deployed bags of dried maple leaves (Acer rubrum) into Little Birch Stream for a 14 day period during which the leaves were colonized with stream bacteria and fungi.

(Credit: Adria Anne Elskus, USGS - LSC Conte Anadromous Fish Laboratory. Public domain.)

Previous studies have demonstrated that fungicides can affect microbial communities including aquatic fungi, but few have examined the effects on the microbes and macroinvertebrates at realistic concentrations observed in the environment. Fungicides have widespread use in urban and agricultural landscapes and could change carbon and nutrient cycling in stream food webs through adverse effects on microorganisms and macroinvertebrate leaf decomposers.

U.S. Geological Survey (USGS) scientists have taken a first step to understanding the potential adverse effects of fungicides at environmentally relevant concentrations on leaf decomposition in streams. They tested the effects of two fungicide formulations (azoxystrobin-propiconazole and boscalid-pyraclostrobin) in a laboratory setting on leaf decomposition by microbes and an amphipod shredder (Hyalella azteca).

Results show that there were differential effects of the two fungicide formulations on leaf decomposition and amphipod growth. Amphipod growth was reduced at the highest concentrations of the boscalid-pyraclostrobin formulation, whereas the azoxystrobin-propiconazole formulation tended to increase leaf decomposition. There was not a change in microbial metabolism by either formulation.

The present study adds to the increasing body of literature on the effects of realistic concentrations of current-use fungicide formulations on leaf decomposition by microbes and macroinvertebrate shredders. Questions still remain about how fungicides are affecting the amphipod growth and leaf-litter decomposition and how that may cause significant changes in energy flow that could affect organisms higher on the food chain.

The USGS Ecosystems Mission Area's Environmental Health Program (Toxic Substances Hydrology and Contaminants Biology) and Fisheries: Aquatic and Endangered Resources Program funded this study.