How does ecological function correspond to morphology in Great Lakes coregonines?

David Bunnell (USGS, dbunnell@usgs.gov), Jake Vander Zanden (University of Wisconsin-Madison), Ben Martin (Idaho Department of Fish and Game)

Human activities have caused widespread biodiversity loss, particularly in freshwater systems. For example, historical fish assemblages of the deepwater habitats of the Laurentian Great Lakes were dominated by up to eight closely related taxa from the genus Coregonus. Human activities decimated this diversity during the 20th century. Given the importance of ‘portfolio effects’ in which biodiversity stabilizes ecosystem processes and increases resilience, restoration efforts can be enhanced when we understand how biodiversity losses affected ecosystem productivity, stability, and resilience. This project has two major findings. First, we conducted controlled tissue preservation experiments to determine how amino acid-specific nitrogen isotope values are influenced or potentially biased. We combined this with the available literature data – and concluded that tissue preservation does not bias or increase variance in amino acid-specific nitrogen isotopes values. Second, we expanded on previous work using amino acid-specific stable isotopes to describe trophic niche partitioning among coregonines from historical (1917-1928) and modern populations (2014-2022) in Lakes Superior and Michigan. In Lake Superior, five deepwater cisco species from the 1920s exhibited exceptionally clear trophic niche partitioning. In contrast, contemporary Lake Superior deepwater ciscoes showed a high degree of trophic overlap and average trophic position shifted downwards by ~0.5 trophic level. Interestingly, shallow-water Cisco and Lake Whitefish have not undergone similar downward trophic shifts between the two time periods. Likewise, the one deepwater cisco still present in Lake Michigan underwent a downward trophic shift similar to the deepwater ciscoes in Lake Superior. Similar to trends observed in Lake Superior, shallow-water Cisco and Lake Whitefish in Lake Michigan either maintained or slightly increased their trophic positions between the time periods. Finally, there was a surprisingly high degree of coherence in trophic position among the coregonine species in the two lakes, for both historic and modern times despite large environmental and food-web differences between the two lakes. No morphological characters were strongly correlated with coregonine trophic position. Our results indicate that coregonines in the Upper Great Lakes once exhibited trophic niche partitioning – indicating that morphological diversity was accompanied by different ecological roles. Why trophic partitioning has eroded, even in Lake Superior where anthropogenic stressors have not been as intensive as Lake Michigan, is not known. These results suggest that current coregonine biodiversity is not providing the productivity, stability, and resilience that it once conferred more than a century ago.