Zooplankton and regime dynamics in shallow Minnesota lakes: Are they related?

Background/Question/Methods

Shallow lakes are known to alternate between clear-water and turbid-water regimes. Clear-water regimes are characterized by abundant submerged aquatic vegetation (SAV) and low density of phytoplankton. Alternatively, in turbid-water regimes, abundance of SAV is low and phytoplankton density is high.  Shallow lakes may switch between regimes, in part due to mechanisms driven by trophic cascades that affect predator-prey interactions. The role of zooplankton in triggering regime shifts or in maintaining regime status through phytoplankton grazing is unclear.  We studied zooplankton communities in shallow Minnesota lakes during 2010 and 2011, with lakes in clear and turbid regimes each well-represented in both years.  Using constrained ordination, we tested whether zooplankton communities were related to water-clarity regimes, water chemistry, fish abundance, SAV biomass, and phytoplankton (chlorophyll-a). We also used unconstrained ordination and analysis of dissimilarities to determine whether zooplankton composition in 2010 predicted a regime shift in 2011.

Results/Conclusions

Planktivore-benthivore fish biomass, SAV biomass, and regime were significant correlates of zooplankton community structure in 2010 and 2011. Zooplankton communities also differed among regions, especially during 2011, with northern lakes showing a larger proportion of predatory taxa such as Holopedium and Polyphemus.  There was no discernible relationship between 2010 zooplankton composition and regime stability or change in 2011.  These results are consistent with a role for top-down trophic cascades involving zooplankton in lake regimes, but also indicate that other mechanisms are likely to be important determinants of regime status in shallow Minnesota lakes.