Rapid stable isotope turnover of larval fish in a Lake Superior coastal wetland: Implications for diet and life history studies

Trophic linkages of larval fish in Lake Superior coastal habitats can be identified using naturally occurring differences in the stable isotope ratios of nitrogen (¹⁵N:¹⁴N, δ¹⁵N) and carbon (¹³C:¹²C, δ¹³C). We measured ¹³C and ¹⁵N values in common fish larvae weekly during spring run-off (late-April to mid-July) in the hydrologically complex drowned river mouth of the St. Louis River, the second largest tributary to Lake Superior. For all species, δ¹³C was increasingly negative with increasing weight as the fish developed from the yolk-sac stage, during which they possess a maternally-derived isotopic signature, to an exogenously feeding larvae. Trends in δ¹⁵N with increasing weight varied among species; an increase, decrease, and no change in δ¹⁵N were observed. A weight-based stable isotope turnover function modeled well the observed changes in δ¹³C and δ¹⁵N. In general, fish obtained a constant signature after a 10-fold gain in body mass, implying their tissue was at isotopic equilibrium with their diet. Difference between yolk-sac and larvae δ¹³C and δ¹⁵N revealed distinct patterns in larval origin and settlement. Based on the species analyzed, we identify two specific Lake Superior coastal wetland-dependent fish early life histories that incorporate habitat use, movement, and trophic dynamics. This study thus provides a methodological approach that can potentially help resolve interactions between watershed character, coastal productivity, and Lake Superior that are of significance to the lake's fisheries.