Date of Award

August 2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Freshwater Sciences

First Advisor

Harvey A Bootsma

Committee Members

John Janssen, Frederick W Goetz, Dong-Fang Deng

Abstract

Lake Michigan is the second largest Laurentian Great Lake and is characterized by broad spatiotemporal variation in ecosystem processes such as nutrient cycling and energy flow. Variation in ecosystem processes—such as those induced by invasive dreissenid mussels—and subsequent changes in the spatiotemporal distribution of resources (i.e. prey or habitat) has resulted in major changes in Lake Michigan’s fish community. The ability of fish species to cope with spatiotemporal resource dynamics is at least partially dependent on their ability to shift apparent trophic position along various resource gradients. The objectives of this research were to quantify trophic positions of Lake Michigan fishes following major ecosystem changes and identify potential mechanisms responsible for spatiotemporal variation.

To address these objectives, stable C and N isotopes were used to quantify the trophic position of many common Lake Michigan fish species. Consumer stable C and N isotopes vary as a function of resource reliance and primary energetic pathways (e.g. pelagic, profundal, and nearshore benthic). Therefore, isotopic composition was quantified to determine the relative importance of various energetic pathways and prey resources to Lake Michigan fish species. In some instances, isotopic variation could be evaluated relative to environmental resource gradients and facilitated assessment of species and community level trophic responses over those gradients.

The trophic positions of common nearshore fish species were evaluated across the Lake Michigan basin. Mixing models were used to evaluate the relative importance of various prey sources and energetic pathways to each species across the basin. Differences in prey sources and primary energetic pathway varied as a function of water clarity for some nearshore fishes. It is likely that regional water clarity, derived from remote sensed Kd(490), affects the relative importance of pelagic versus nearshore benthic primary and secondary production with resultant impacts on diets and primary energy sources for some nearshore species. For other nearshore species, there was nearly ubiquitous reliance on pelagic energy pathways regardless of water clarity or relative importance of nearshore benthic energy pathways.

Among the Lake Michigan salmonine community, trophic position was assessed during a period when preferred alewife (Alosa pseudoharengus) prey abundance and size structure was changing. Isotopic niche area and mixing models were used to quantify reliance on alewife and alternate prey sources. When the alewife population was small and dominated by larger individuals, some salmonines relied more heavily on alternate prey sources and energy pathways. Conversely, at higher alewife abundance and smaller size composition, most salmonines shifted their diet to greater reliance on this abundant prey source. Differential trophic responses of the salmonine community to alewife population dynamics highlighted variable capacity for reliance on alternate energy pathways and potential for niche partitioning among this predator community.

Unlike many fish species, abundance of cisco (Coregonus artedi) is apparently increasing in Lake Michigan and condition remains high. Much is unknown about the trophic ecology of this population and the mechanisms for its recent expansion. The trophic position and diet of this species were assessed relative to other common Lake Michigan fishes using measures of isotopic niche area and diet mixing models. It appears that cisco currently occupy the trophic role of a pelagic predator similar to Pacific salmonines (Oncorhynchus spp.) and brown trout (Salmo trutta). Despite apparently stronger reliance on pelagic prey, cisco exhibited considerable reliance on alternate nearshore benthic energetic pathways as well. Ability to use multiple energetic pathways may have aided their ongoing population expansion.

Nearshore fishes, salmonines, and cisco exhibited varying degrees of reliance on alternate prey and energetic pathways. In the case of certain nearshore fishes and salmonines, reliance on alternate prey sources was apparent over spatial and temporal resource gradients, respectively. Reliance on alternate energetic pathways likely conveys stability to these fish populations resulting from the asynchrony of production across dominant energetic pathways. Species reliant on a single energetic pathway are likely subject to stronger fluctuations in available prey resources. Sustaining important fisheries and maintaining ecosystem function in midst of ongoing ecological change may benefit from consideration of the ability of fish species to rely on alternate energetic pathways.

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