Date Approved

12-20-2024

Graduate Degree Type

Thesis

Degree Name

Biology (M.S.)

Degree Program

Biology

First Advisor

Eric Snyder

Second Advisor

Joseph Rathbun

Third Advisor

Bill Flanagan

Fourth Advisor

Mark Luttenton

Academic Year

2024/2025

Abstract

Freshwater mussels (Unionidae) are ecosystem engineers and keystone species vital to stream ecosystems yet represent the most imperiled group of freshwater fauna. Conservation of unionids necessitates further understanding how assemblages interact with watershed features. This study investigates mussel occupancy, distribution, and assemblage structure upstream and downstream of tributary confluences in the lower Grand River, Michigan, alongside updated surveys of the unionid community with comparisons to previous assessments conducted by Michigan Natural Features Inventory (MNFI) in 2002–2003. Unionid density, richness, and diversity were greater upstream of confluences. No significant distance-gradient effects were observed. Assemblage structure differed between upstream and downstream reaches with downstream assemblage structure being more predictable, though overall species composition was similar. Environmental modeling found water depth, benthic velocity, woody debris coverage, algae/macrophyte coverage, percent sand composition, temperature, dissolved oxygen, C. fluminea denssity, and soluble reactive phosphorus as important predictor variables of unionid distribution at confluences. Tributary confluences represent complex and dynamic areas that are influenced by a variety of watershed and local level influences yet may support robust mussel assemblages where suitable conditions are present. Unionid distribution within tributary confluence effects is patchy and determined by localized habitat stability rather than longitudinal patterns. Semi-quantitative surveys detected 26 unionid species, including 5 species state-listed as threatened or endangered, and 7 species of special concern. Four species were newly detected compared to previous MNFI surveys: white heelsplitter (Lasmigona complanata), pink heelsplitter (Potamilis alatus), paper pondshell (Utterbackia imbecillis), and lilliput (Toxolasma parvum). Our surveys show increased site-level detection and density of freshwater mussels at 5 the site-level, but no significant difference in richness or abundance compared to previous surveys. NMDS and post-hoc analysis show no significant shifts in site-level assemblage structure between current surveys and those conducted by MNFI. These results suggest overall stability in the mussel community, with positive trends potentially reflective of improving water quality and conditions. Overall findings underscore the importance of stable microhabitats, local and watershed level influences, and long-term monitoring in the conservation of unionid communities. Conservation strategies in the watershed should prioritize habitat enhancement, sedimentation mitigation, pollution abatement, and continued monitoring to understand long-term trends

Available for download on Friday, January 23, 2026

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