Foraging dynamics of juvenile spot (Leiostomus xanthurus) in a river-dominated estuary, Apalachicola Bay, Florida: A diet and stable isotope analysis

The value of estuaries as nursery habitat for juvenile fishes depends on their connection with coastal watersheds that provide nutrients, freshwater, and other materials essential to the productivity of these ecosystems. This connectivity is of particular importance for river-dominated estuaries that experience considerable spatial and temporal variation in hydrography and flow of alluvial materials. Apalachicola Bay is one of the most productive estuaries in the northern hemisphere, largely due to the intensity and duration of freshwater output. Variation in nutrient and fresh water input to the Bay may alter habitat quality with regard to foraging success for juvenile estuarine-dependent fishes. Using juvenile spot (Leiostomus xanthurus), I am testing the hypothesis that spatial and temporal variation in diet composition, foraging success, and organic matter source are related to seasonal variation in river flow and associated environmental conditions. The diets of 868 juvenile spot were sampled monthly in 2009 and 2010 from 12 locations throughout Apalachicola Bay, Florida. Stomach fullness was quantified and prey was identified to the lowest possible taxonomic level, counted, and a volume of each prey category was estimated. Juvenile spot prey included harpacticoid and calanoid copepods, ostracods, small bivalves, nematodes, amphipods, chironomid larvae, and polychaetes. A stable isotope analysis of delta13C, delta 15N, and delta34S was conducted on 230 of the samples. Isotope values provide insight on the organic contributions over a longer time scale than the diet analysis and provide a better understanding of how alluvial materials contribute to juvenile fish growth during their residency in estuarine nursery habitats. Results from this study suggest that riverine input does affect juvenile fish foraging, but that this influence is likely mediated through the benthic prey community. Higher river flow in the first year of sampling coincided with increased stomach fullness and the demersal foraging behavior of this study species suggests that increased foraging is a result of a more abundant prey community. Diet composition did not differ greatly along a spatial or temporal gradient in relation to river discharge, but significant changes in isotopic signatures of juvenile spot were observed. A strong terrestrial signature in the isotope values in the primary nursery habitat, despite little variation in diet composition, suggests that riverine effects on estuarine consumers are mediated through the source of organic matter supporting the benthic prey community rather than effecting the abundance or composition of potential prey. It is important to understand how alluvial materials contribute to juvenile fish foraging in order to better inform policy makers on decisions regarding water diversions and upstream river modifications.