Date of Award

August 2023

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Harvey A Bootsma

Committee Members

Brenda Moraska-Lafrancois, Mike Pauers, Jerry Kaster

Keywords

Benthic Invertebrates, Cladophora, Dreissenid Mussels, Food Web, Lake Michigan, Nuisance Algae

Abstract

Dreissenid mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) have changed the fundamental community structure and biogeochemical processes of the Lake Michigan nearshore zone. There is evidence that dreissenids promote the nuisance growth of benthic algae, especially Cladophora sp. In addition, benthic macroinvertebrates have benefitted from the presence of dreissenid mussels due to the increased structural complexity along with greater nutrient availability in the benthos. These effects of the dreissenid mussel invasion are well documented; however, little is known about how these systems respond to the removal of dreissenid mussels from a once-populated area. Therefore, it is important to further our understanding of the role dreissenid mussels play in the benthic ecosystem as we contemplate future management strategies in Lake Michigan.There have been few attempts to remove large areas of dreissenid mussels in the Great Lakes: however, mussel removal attempts have been successful in both initially removing the populations from an area and preventing the re-establishment of those populations. This may be due to heavy predation of juvenile dreissenid mussels by the round goby (Neogobius melanostomus) which may inhibit the repopulation of dreissenids once they are removed. In 2021, we removed a 140 m2 area of dreissenid mussels by depriving them of oxygen and nutrients using a benthic barrier membrane. This was followed by monitoring the response of the benthic community over a one-year period and comparing community composition to that on a control site where mussels were still present in large numbers (5471 ± 1709 individuals m-2). The goal of this project was to further our understanding of the relationship between dreissenids, benthic algae, and benthic macroinvertebrates in the Lake Michigan nearshore zone in Good Harbor Bay near Sleeping Bear Dunes National Lakeshore. Benthic invertebrate abundance and algae biomass were measured by gathering in situ samples. Additionally, in situ incubations and oxygen measurements along with laboratory experiments were used to determine the production of the algae. The internal phosphorus in the algal tissue was measured and compared to the algae production to determine how the concentration of internal phosphorus affects algae production. These experiments showed an increase in algae production when in the presence of mussels compared to algae grown in the absence of mussels. The results of in situ and lab experiments were comparable and revealed that Lake Michigan benthic algae has a minimum internal cell P quota (the minimum required to support net growth) that is lower than previously reported values of Cladophora and corresponds with the very low molar carbon: phosphorus ratios frequently measured in Lake Michigan Cladophora, while the measured maximum growth rate was similar to previously reported values. We used the results of in situ and laboratory experiments in a Cladophora growth model to highlight the sensitivity of benthic algal community response to small internal P changes in extremely low P environments and explore the response of the benthic algal community to light and nutrient conditions. Throughout the summer, we observed depressed non-dreissenid invertebrate abundances, lower algae productivity, and major differences in algal community composition on the mussel-free site compared to the control site. We show that this may be attributed to a lack of nutrient cycling and availability, in the absence of dreissenid mussels, accompanied by reduced habitat structural complexity. The algae on the mussel-free site had lower particulate phosphorus concentrations than that on the control site. Lower 13C-enrichment of algae in the presence of mussels is interpreted as reflecting the photosynthetic fixation of isotopically light CO2 respired by mussels, highlight the dependence of benthic algae on both carbon and nutrients released by dreissenids. These findings can be used to explore the effects of future large scale dreissenid removal efforts on nuisance benthic algae and nearshore food web structure.

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