Intraspecific Variation in Elemental Composition of the Least Killifish Tracks Spatial Variation in Periphyton Composition

Many animals, especially freshwater fish, display significant population variation in elemental composition. How often that variation reflects patterns of interspecific variation remains under-explored. We examined the elemental composition and trophic niche of Heterandria formosa, an omnivore whose trophic position varies among populations. We studied eight populations, four from freshwater springs and four from shallow lakes. We analyzed water chemistry along with elemental composition and stable isotopes of carbon and nitrogen in periphyton and fish to determine the extent of population variation in elemental composition and any associations it might have with variation in the composition of the basal resources or the trophic level of the fish. Water chemistry varied along a gradient from lakes with lower values of pH, higher concentrations of chlorophyll-a, and lower nitrate concentrations to springs with higher values of pH, lower chlorophyll-a concentrations, and higher nitrate concentrations. The population variation in elemental composition of the fish tracked the variation observed in the periphyton samples. There was a narrow range of C:N ratios among populations, compared with studies of other species, but differences among them were statistically significant. There was little variation in C:P and N:P ratios as well. Mantel analyses revealed that differences between pairs of populations in elemental compositions of fish were not correlated with differences in the water chemistry. The differences between pairs of populations in the percentages of carbon and nitrogen in fish were strongly correlated with the pairwise differences in those variables in the periphyton, indicating that the elemental composition of the fish tracks that of its basal resource. The average values of δ¹³C in the fish populations were highly correlated with the average values of δ¹³C in the periphyton samples at those locations. Fish populations in springs, which experience less predation pressure and display higher population densities, occupied lower trophic positions than populations in lakes, which experience more predation pressure and exhibit lower population densities. The association between the elemental composition of fish and periphyton could be driven by either bottom-up or top-down effects. The constancy of elemental composition despite variation in trophic position contrasts with well-described patterns of interspecific variation. This might reflect the limited range of variation in trophic position among populations compared to the ranges observed in interspecific analyses.