The Role of River Mouths and River Plumes in Southern Lake Michigan: Implications for Fish Production
River mouths and river plumes are heterogeneous habitats where riverine waters and materials mix with oceanic, estuarine, and lacustrine systems. In these confluences, bidirectional transfer of materials between a river and lake can create spatially complex food webs, where consumers in the receiving body are subsidized by resources from the tributary and vice versa. In southern Lake Michigan, tributaries deliver seasonally warmer, more productive water into the relatively cool, oligotrophic lake. These physically and biologically distinct areas may enhance growth and survival of young fish and thereby serve as an important nursery habitat. This research addresses the ecological effects of food web shifts caused by invasive species in Lake Michigan, which has recently transitioned to a more oligotrophic ecosystem, leading to declines in fish populations that support lucrative fisheries. Despite burgeoning interest in the nearshore Lake Michigan food web, the energy dynamics of river mouths and plumes require further investigation. To evaluate the relative importance of lake and river contributions for fishes in three southern Lake Michigan river mouths, during 2011 and 2012, we characterized their thermal environments, light availability, and lower trophic levels. We evaluated growth rates and feeding patterns of larval fishes to quantify habitat suitability for larval alewife and yellow perch. Furthermore, we collected juvenile and early-adult yellow perch, round goby, and alewife, which were analyzed for stomach contents and stable isotopes δ13C, δ15N, δ2H, and δ18O of tissue and δ13C and δ18O of otoliths. To further elucidate river mouth energy pathways, we quantified fatty acid signatures of fish tissue and prey. Our results demonstrate spatial and temporal heterogeneity in physical and lower trophic level patterns within river mouths whereby tributary habitats are warmer, more turbid, and provide a more favorable growth environment for first-feeding larval fishes than adjacent nearshore lake habitats. Moreover, characterization of river mouth food webs indicates that young fish are utilizing a variety of tributary and nearshore lake resources and that alewife production is subsidized by a mixture of lacustrine and riverine derived nutrient sources. We have also identified nursery habitat utilization and indicated that alewife may undergo seasonally migratory behavior in river mouths. This investigation advances our understanding of the early-life habitat utilization and resource use by these ecologically and economically important fishes throughout the Laurentian Great Lakes. By characterizing river mouth food webs, we have demonstrated the essential role of river mouths in fish growth and survival, which has broad implications for the population stability of these species and informs resource management in the protection of Great Lakes habitats.
Hook, Purdue University.
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