Date of Award

August 2024

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences

First Advisor

Harvey A Bootsma

Committee Members

Brenda Moraska Lafrancois, Michael J Pauers

Abstract

Invasive dreissenid mussels (the zebra mussel, Dreissena polymorpha, and quagga mussel, Dreissena rostriformis bugensis) have altered nutrient cycling, light availability, and trophic dynamics in Lake Michigan since their introduction. Benthic resources have become more important as pelagic nutrients have been consumed due to dreissenid mussel productivity. Light and nutrient availability are two factors that influence benthic algal growth and are known to vary spatially, particularly with depth. Variation in benthic algal quantity and nutritional quality with depth has implications for the entire benthic community, and a better understanding of the effects of depth on the nutritional quality of benthic algae provides a better understanding of trophic structure throughout the nearshore region. Thus, it is important to understand how the composition of the benthic community varies at different depths, particularly in terms of primary producers, primary consumers, and secondary consumers.

The availability and quality of benthic algae as a food source are important because benthic algae are a basal energy source, and high-quality algae are beneficial for growth, reproduction, and energy transfer efficiency for primary consumers. Additionally, primary consumers that have access to high quality resources become a high-quality food source for higher trophic levels. Benthic algal biomass, nutrient content, fatty acid content, and diatom abundance were measured and compared across four depths on three sampling dates in the Lake Michigan nearshore zone at Good Harbor Reef near Sleeping Bear Dunes National Lakeshore to determine how the quantity and nutritional quality of benthic algae vary with depth and date. Benthic algal biomass, community composition, nutrient content, and fatty acid content were significantly influenced by depth and date. Benthic algal biomass decreased with increasing depth and from July to October. Benthic algal highly unsaturated fatty acid content was strongly correlated to diatom abundance, but depth trends in nutritional quality were weak compared to seasonal trends. The deeper depths of 10, 15, and 20 meters generally had slightly higher nutritional quality than the shallow depth of 5 meters, while nutritional quality was higher in October, intermediate in July, and relatively low in August.

I hypothesized that differences in benthic algal food quality would be reflected at higher trophic levels. Benthic invertebrate, round goby, and dreissenid mussel abundances were measured and compared across depths to determine if quality and quantity of algae influenced biomass at higher trophic levels. δ13C and δ15N values were measured for benthic algae, benthic invertebrates, dreissenid mussels, and round gobies at each depth to determine depth trends in food web structure. Benthic invertebrate abundance and round goby abundance both varied with depth but peaked at different depths. Round goby density peaked at 10 meters and benthic invertebrate density peaked at 15 meters. Large (>5 cm) round goby density was highest at 5 meters, while total round goby density peaked at 10 meters. It is possible that goby densities were highest at the two shallower depths due to a preference for warm water. Stable isotope data indicated that benthic invertebrates may selectively consume diatoms rather than filamentous green algae, suggesting that quantity and quality of the whole benthic algal matrix is not as important as diatom quantity and quality. δ13C values for round gobies exhibited distinct depth trends, which indicated high depth fidelity and differences in round goby diet at each depth.

These findings can be used to provide context to the large base of knowledge regarding the benthic community at Good Harbor Reef. Depth trends in benthic algal nutritional quality as indicated by phosphorus content and fatty acid content were weak compared to temporal trends, meaning that seasonal effects on benthic algal quality may be more important to benthic invertebrates than spatial variation. The observation that benthic invertebrates may preferentially consume diatoms over filamentous green algae suggests that benthic diatoms may be a key energy source that should be investigated more closely, independent from Cladophora. Round goby diet exhibited distinct depth trends, indicating high depth fidelity and site-specific feeding patterns. This raises the question of how differences in round goby density and round goby reliance on mussels will affect mussel removal projects at different sites.

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