The effects of Bythotrephes longimanus invasions increase with time since establishment in three lakes in northern Minnesota

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Marissa D. Cartwright, Biology and Environmental Science, Coe College, Cedar Rapids, IA and Carrie Kissman, Biology and Environmental Science, St. Norbert College, De Pere, WI

Background/Question/Methods
Invasive species such as the spiny water flea (Bythotrephes longimanus) have been known to disrupt ecosystems by adding predation pressure to prey species like Daphnia sp. and Bosmina sp. Here we compared the population densities of zooplankton species found in three northern Minnesota lakes, Burntside, Shagawa, and Fall, of which the former was identified as infested in 2010, and the latter two in 2014. We hypothesize that in lakes where B. longimanus had been introduced earlier, B. longimanus populations would be greater and the population densities of favored prey species would be smaller. Triplicate zooplankton tows were taken in three locations in each lake during one week in July of 2020. Zooplankton species were identified, counted, and densities were calculated. Zooplankton densities for each lake were compared and analyzed using the Kruskal-Wallis One-way Analysis of Variance and the Dwass-Steel-Chritchlow-Fligner Test in SYSTAT 13.
Results/Conclusions
Our results indicate that the earliest infested lake, Burntside, has significantly lower population densities for most prey zooplankton species, including Daphnia magna, D. lumholtzi, Diaphanosoma birgei, calanoid copepods, nauplii, and total zooplankton density, supporting the hypothesis that greater invasion duration of B. longimanus leads to reduced zooplankton densities. Holopedium gibberum densities were greater in Burntside, which is likely due to their gelatinous sheath providing protection against predation. Interestingly, B. longimanus densities in Burntside were not significantly different from Shagawa or Fall, failing to support our hypothesis, and likely reflecting their patchy distribution in the water column. In addition, Shagawa is located closest to the nearest town, Ely, leading to a greater chance of additional unintentional reintroductions by boat users since 2014. Lastly, Burntside has nearly three times the surface area of the other two lakes, and tows were unable to be taken from the deepest part of the lake due to logistical constraints. Although more studies need to be conducted to look at the interactions of other variables with B. longimanus on prey population density, the findings provide insight about the spread and management of aquatic invasive species near and within the Boundary Waters Canoe Area Wilderness.