Use of allochthonous resources by zooplankton in reservoirs
Aquatic food webs are supported by primary production from within the system (autochthony) as well as organic matter produced outside of and transported into the system (allochthony). Zooplankton use allochthonous resources, especially in systems with high terrestrial loading and moderate to low internal primary production. We hypothesized that due to high terrestrial loads and remnant submerged terrestrial material, allochthonous resource use by zooplankton would be significant in all reservoirs and would decline along an increasing reservoir age gradient. Using hydrogen stable isotopes and a Bayesian mixing model, we estimated the contribution of allochthonous sources to organic matter pools and crustaceous zooplankton biomass for ten reservoirs. Dissolved organic matter (DOM) in all systems was dominated by allochthonous sources (posterior distribution median >92% allochthonous), while particulate organic matter (POM) composition varied (2–68% allochthonous) and had a lower allochthonous fraction in older reservoirs. There was no relationship between zooplankton allochthony and reservoir age. Crustaceous zooplankton allochthony varied among systems from 26 to 94%, and Chaoborus allochthony, measured in four reservoirs, was similarly variable (33–94%). Consumer allochthony was higher than POM allochthony in some reservoirs, potentially due to terrestrial DOM pathways being important and/or algal resources being inedible (e.g., cyanobacteria). As with many lakes, in the reservoirs we studied, allochthonous inputs account for a significant fraction of the organic matter of basal consumers.
KeywordsReservoirs Zooplankton Chaoborus Deuterium Allochthony Bayesian mixing model
We thank Jonathan Torre for assistance with collecting and processing samples and James N. Galloway for providing a canoe. David Tungate of the Rivanna Water and Sewer Authority graciously provided access to the reservoirs and Konrad Zeller and Andrea Terry provided assistance in the field. Some reservoir morphometric data were provided by Robert VanLier of the Virginia Department of Conservation and Recreation. This paper was improved by the comments from two anonymous reviewers and Jonathan Cole. Funding was provided by the National Science Foundation (Division of Environmental Biology—1144624), the Virginia Coastal Reserve Long Term Ecological Research Grant 1237733, University of Virginia College Council Research Grant, University of Virginia Small Research Grant, and the University of Virginia, Department of Environmental Sciences.
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