Zooplankton community differences among four main pools of Lake St. Croix (Minnesota–Wisconsin) 2010–2011

Lafrancois, Toben; VanderMeulen, David; Karns, Byron

Lake St. Croix is a natural impoundment of the St. Croix National Scenic Riverway (Minnesota and Wisconsin) under imminent threat of silver and bighead carp (Hypophthalmichthys molitrix and H. nobilis) establishment, opportunistic omnivores that have dramatically altered zooplankton communities in similar systems. The purpose of this project was to characterize zooplankton at four long term water quality monitoring sites established by the National Park Service–Great Lakes Inventory and Monitoring Network. Three of the sites are located in Lake St. Croix at, in descending order, Stillwater, Minnesota, Hudson, Wisconsin, and Prescott, Wisconsin. The fourth site is in a reservoir above a hydroelectric dam in St. Croix Falls, Wisconsin, a barrier to migration of carp into upstream areas of the Riverway. Quantitative zooplankton samples were collected monthly from May to September of 2010 and 2011. PERMANOVA analysis of zooplankton community composition identified significant differences among sites (p = 0.005), over sampling month (p = 0.002), and between years (p = 0.002). There were no significant interactions between site, month, and year. Ordinations show that the upstream reservoir was uniformly dominated by Rotifera and total zooplankton biomass was three to five orders of magnitude lower than that found at the three pool sites. The upper and lower Lake St. Croix pool sites had similar total zooplankton biomass but were characterized by different dominant crustacean taxa in productive months. In 2010 two sites in Lake St. Croix were dominated by rotifers, correlating to periods of low retention, while the central site near Hudson, Wisconsin, was dominated by rotifers all season long. However, in 2011 this site was crustacean-dominated and supported the highest biomass over all other sites, due primarily to a large bloom of Mesocyclops spp. Comparison of communities and environmental conditions between 2010 and 2011 suggests crustacean dominance of the zooplankton community was most likely related to water retention time. We conclude 1) Lake St. Croix is a mosaic of connected but distinct zooplankton communities, with taxa differing among pools in ecologically significant ways; 2) Lake St. Croix supports preferred zooplankton prey species for both silver and bighead carp; and 3) the reservoir above the dam will not act as a recolonization source of preferred zooplankton prey species for downstream areas of the river or Lake St. Croix pools.

Zooplankton community differences among four main pools of Lake St. Croix (Minnesota–Wisconsin) 2010–2011

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