Invertebrate Study

2010 Invertebrate Study Complete

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Introduction

As a base level in the food chain, the aquatic macroinvertebrate community affects an entire lake system.  Maintaining a stable condition in the lake littoral zone, the most diverse area of a lake system (Wetzel, 2001), is therefore crucial to maintaining an ecologically sound body of water.  There is evidence that suggests water level fluctuations have a profound effect on lake littoral zone macroinvertebrate communities (e.g., Aroviita, 2008; Haxton, 2008; White, 2008).  Late-winter drawdowns on impoundments lead to poor quality, in terms of abundance and diversity, of the benthic invertebrate community.  This affects the productivity of the fish and wildlife community as potential food sources are diminished. 

Lake Wissota in Chippewa County was created in 1917 with the construction of a hydroelectric dam on the Chippewa River.  Lake Wissota was historically drawndown 10-15 feet every winter (with some drawdowns beginning as early as mid December and some lasting as late into the spring as the middle of April) in anticipation of the spring thaw and to sustain power-production.  This drawdown was maintained for several weeks annually (Chippewa River Settlement Team, 2001).  The Lower Chippewa River Settlement Agreement was established in 2001 as part of the Federal Energy Regulatory Commission re-licensing procedure. One issue raised during re-licensing was concern over the effects the winter drawdowns were having on the Lake Wissota ecosystem. As required in this agreement, Lake Wissota drawdowns were reduced from the 10-15 ft norm to a 3 ft maximum maintained for a maximum of seven days with a final extensive drawdown occurring in 1999 (Appendix A).

A study conducted in 1993/94 on Lake Wissota demonstrated that the extended period of winter drawdown had negative effects on the macroinvertebrates inhabiting the littoral zone impacted by the drawdown (Delong and Mundahl, 1995). Benthic invertebrate samples were taken at 2 ft, 5 ft, 10 ft and 15 ft depths on transects around the main body of Lake Wissota, Moon Bay and Little Lake Wissota (see map Appendix D).  Sampling periods occurred in November 1993 (pre-drawdown), immediately following the ice out in April 1994 (post-drawdown) and throughout the spring and summer 1994 to determine re-colonization of the littoral zone and post emergence populations.  This study revealed significant effects of an extended late winter drawdown on the benthic macroinvertebrate community.  These effects were manifested in significant declines in the total numbers of individuals and densities of invertebrates in the post-drawdown period compared to pre-drawdown period.  There were significantly higher declines in densities observed in the 2 ft and 5 ft depths (depth affected by drawdown) than at the 15 ft depth (depth that remained inundated).  Given the documented effects of extended periods of drawdown it is expected that a reduction in the drawdown (from 10-15 ft to 3 ft) will have resulted in a stabilization of the benthic invertebrate community with less fluctuation in community structure, which would lead to the overall improved health of the littoral zone ecosystem.  The goals of this study were to document: the response of the benthic invertebrate community since the drawdown regime was changed in 2000, the current composition of the benthic invertebrate community and the value of critical habitat areas as fish and wildlife food sources.

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