We examined horizontal spatial patterns of fish densities and zooplankton biomass at a fine spatial scale of 50 m across seasons before, during, and after an experimental lake destratification to determine how interacting trophic levels may respond to alteration of thermal stratification. We used semivariogram analysis to calculate maximum distances of autocorrelation for fish and zooplankton separately, and cross variograms to determine whether relationships between fish and zooplankton are positive or negative. Fish became more dispersed during the manipulation, likely due to a flight response with the loss of preferred cold water habitat. There were no changes in zooplankton horizontal distributions with mixing, but we detected seasonal trends in distribution and biomass. We detected positive relationships between fish densities and zooplankton biomass for portions of the year, but did not detect any negative relationships. There was no effect of lake mixing on spatial interactions between fish and zooplankton. Our results indicate that external factors, such as seasonal wind patterns, may drive whole-lake zooplankton distributions, and that fish respond horizontally to change in vertically structured processes, especially when reliant on depth-dependent variables such as cold water.
Bibliographical noteFunding Information:
We thank all those who have contributed to this large-scale project throughout years of its development and implementation. We thank the entire staff of UW-Trout Lake Field Station for housing researchers during field seasons and for providing sampling equipment and insightful advice. We especially thank Tim Kratz for his key role in logistics. Without his work this project would not be possible. Several field assistants also played key role in data collection: Colin Smith, Jamie Dobosenski, Page Mieritz, Paul Schramm, Tom Thalhuber, Ian Harding, Matt Pawlowski, Avery Koblings, Weston Matthews, Jon Celmer, Ellen Heyn, and Sarah Jones-Witthuhn. Finally, we thank the anonymous reviewers who took the time to provide thorough comments and suggestions, which vastly improved our manuscript. This study took place at the University of Wisconsin-Madison Trout Lake station. Funding for this project was provided by a private donation to the University of Wisconsin-Madison, an A.W. Mellon Foundation award to S.R. Carpenter, the North Temperate Lakes LTER program (NSF grant DEB 0822700), and a Wisconsin Department of Natural Resources AIS grants (ACE1-084-10, ACE1-121-12, ACE1-141-13) to M.J. Vander Zanden.
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