Date of Award

Spring 2018

Degree Type

Honors Project

School

College of Liberal Arts

First Advisor

Leif Hembre

Abstract

Stocking fish for sport may induce food web cascades that impact the trophic state of lakes. Here, the practice of stocking rainbow trout into a Twin Cities metropolitan area lake (Square Lake) was examined to see if changes have occurred in the lake’s zooplankton community that could explain its declining water quality. Rainbow trout are size-selective predators that consume large-bodied herbivorous zooplankton (i.e., Daphnia pulicaria) and therefore may cause decreased phytoplankton grazing pressure and more eutrophic conditions. A paleoecological study was performed that compared the sediment record of Square Lake to that of a reference lake (Big Carnelian Lake) that has never been stocked with rainbow trout. The primary focus of this study was to use exoskeletal remains (Bosmina headshields and Daphnia post-abdominal claws) and resting eggs (Daphnia ephippia) to determine if changes in the abundances of multiple zooplankton species and the body size of Daphnia species have changed from pre-European settlement dates to 1981 onwards, due to the initiation of the trout stocking program. We found that for both proxies (post-abdominal claws and ephippia), D. pulicaria mean body size decreased significantly in Square Lake during the post-stocking years (after 1981), but the body size of D. mendotae did not decrease. No significant changes in D. pulicaria body size were seen in either lake before 1981. A shift in species composition from larger-bodied D. pulicaria to smaller-bodied D. mendotae after 1981 was not seen until late (~ 2010) in the sediment record. In addition to these findings, data on human population size and land development in the lakes’ watersheds were used to understand changes in the sediment and zooplankton community composition. For both lakes, it was found that since pre-European settlement to 1981 the sediment composition has become increasingly dominated by clastic and organic materials, and that the zooplankton populations have shifted in dominance from Bosmina to Daphnia. Differences in fish stocking practices of each lake appeared to have caused opposite changes in the mean body size of their D.pulicaria populations. The rainbow trout stocking in Square Lake has caused the mean body size to decrease, while the stocking of walleye into Big Carnelian Lake seems to have caused it to increase.

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