The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.
Bibliographical noteFunding Information:
We gratefully acknowledge the help of Julie Lietz in the preparation of this manuscript. This work was supported by the USEPA Great Lakes National Program Office as part of EPA Contract No. EP-C-15-012 , Scientific and Technical Support with CSRA, LLC, under the direction of Louis Blume, Project Manager, as well as by agreements with Cornell University, Department of Natural Resources and Buffalo State College under Prime Agreement Award GL-00E01184 from the U.S. EPA “Great Lakes Long Term Biological Monitoring of Zooplankton, Benthos, and Chlorophyll a” and with Regents of the University of Minnesota (to E. Reavie) from the U.S. Environmental Protection Agency under Prime Agreement Awards GL-00E23101-2 and GL-00E01980 “Great Lakes Biological Monitoring: Phytoplankton”. The views expressed in this paper are those of the authors and do not necessarily represent the views or policies of the USEPA.
© 2018 International Association for Great Lakes Research
- Lake Huron
- Lake Michigan