Zooplankton Community Changes in Mille Lacs Lake, Minnesota after Zebra Mussel (Dreissena polymorpha) and Spiny Waterflea (Bythotrephes longimanus) Infestations.

Untitled Document
Aquatic Invasive Species (AIS): Spiny water flea (Bythotrephes longimanus)
Zebra mussel (Dreissena polymorpha)
Project Location: Mille Lacs Lake, Minnesota
Project Start Date: 06/2006
Expected Timeframe/Duration: ongoing
Sponsoring Organization: Minnesota Department of Natural Resources-(Division of Ecological and Water Resources, Division of Fish and Wildlife)
Web Links:
Primary Contact: Jodie Hirsch, MN DNR-Ecological and Water Resources
Minnesota Department of Natural Resources

Secondary Contact:
Project Summary:

Annual monitoring of zooplankton in Mille Lacs Lake since 2006 has revealed significant changes in the zooplankton community that coincide with the recent infestations of zebra mussels and spiny waterfleas.   Zebra mussels were first detected in Mille Lacs Lake in 2005, with an abundant population by 2011. Spiny waterfleas were first detected in 2009.   The monitoring record (2006-2013) details significant declines in zooplankton densities, biomass and species richness that became apparent 2011.  Post-infestation years reveal an estimated 40-60% loss in total zooplankton biomass compared to pre-infestation years. Small cladoceran species have been most impacted and their decline is most pronounced during mid- to late-summer months when spiny waterflea abundance peaks, suggesting size-selective, direct predation as a cause.  Densities and biomass of cyclopoid copepods also decreased in post-infestation years, suggesting competition interactions with spiny waterfleas.  In contrast, little change in larger species such as Daphnia galeata mendotae and calanoid copepods was observed over the same timeframe.  Zebra mussels may potentially impact these species by direct competition for food.  However, impacts from food competition may be slower to develop than those from direct predation, which emphasizes the critical need for long-term monitoring. 

The observed changes in zooplankton density, biomass and species richness, which may not yet have stabilized, are particularly noteworthy because these zooplankton likely represent an important pelagic energy source in Mille Lacs Lake.  Zooplankton monitoring will continue in Mille Lacs Lake in the coming years to help further understand potential impacts these two aquatic invasive species may be having on the trophic dynamics of this lake.