“Dead Zone” dynamics in Lake Erie: the importance of weather and sampling intensity for calculated hypolimnetic oxygen depletion rates
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Calculated hypolimnetic oxygen depletion (HOD) rates depend not only on environmental factors but also logistical ones. In particular, lack of understanding of the effects of weather in addition to how sampling effort determines calculated HOD rates complicates ecological understanding and environmental management of lake ecosystems. To better determine the roles of weather and sampling effort, we combined (1) weekly measurements of temperature and dissolved oxygen (DO) concentrations from seven stations in the Sandusky subbasin of Lake Erie’s central basin during 2005, (2) contemporaneous measures of storm activity and tributary discharge, and (3) a two-dimensional coupled hydrodynamic, chemical, and biological model of Lake Erie to investigate (1) how increased storm activity and tributary discharge affected short- (daily) and long-term (seasonal) dynamics of hypolimnetic hypoxia, and (2) how spatial (number of sites sampled) and temporal (sampling frequency) sampling effort affected calculated HOD rates. Our model closely replicated field-observed DO dynamics. When comparing baseline modeled dynamics to those in a second simulation with twice the number of days with high winds, however, we found that with more storm activity (1) periods of entrainment became more frequent, (2) the hypolimnion was warmer, (3) thermal stratification occurred 1 month later, whereas autumnal turnover occurred at least 1 week earlier shortening the duration of stratification by 1–2 months, and (4) HOD rates increased 12%. Further, spatial and temporal sampling intensity also affected calculated HOD rates. Consequently, adequately quantifying actual HOD rates requires sufficient sampling effort and the particular role of weather should be assessed with rigorous field and simulation studies, especially if HOD rates are used to indicate management success.
KeywordsStorm-induced mixing Hypoxia Anoxia CE-QUAL-W2
Collection of field data was supported by the Ohio Lake Erie Protection Fund (project 04-16), Franz Theodore Stone Laboratory, and the Ohio Sea Grant College Program; initial adaptation of CE-QUAL-W2 to Lake Erie was supported by the Ohio Sea Grant College Program (project R/EM 20); and, the addition of more detailed phytoplankton groups, zooplankton, and zebra mussels was supported by the Ohio Lake Erie Protection Fund (project 98-17). Ohio Lake Erie Protection Fund projects were administered by the Ohio Lake Erie Commission. Further field data collection for model parameterization and a portion of the time of J.D.C. was supported by the Ohio Division of Wildlife, Department of Natural Resources as part of the Federal Aid in Sport Fish Restoration Program (F-69-P, Sport Fish Management in Ohio) administered jointly by the United States Fish and Wildlife Service and the Division of Wildlife. Ram Yerubandi, Environment Canada, provided the thermistor chain data. M.A. Thomas, D.D. Kane, E.L. Quinlan, M. Beam, M. DuFour, and S. Percival were instrumental in assisting with field data collection. Comments provided by Doran Mason, Libby Jewett, Josef Ackerman and seven additional reviewers improved the quality of our manuscript.
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