Body size and trophic position in a temperate estuarine food web

We used stomach contents and stable isotope ratios of fishes and macroinvertebrates, collected bi-monthly over 18 months from Mad Island Marsh, a small tidal estuary on the northwestern Gulf of Mexico coast, to examine potential body size–trophic position relationships. Mean body size (length) of predator taxa yielding measurable prey items were significantly correlated with body size (length) of their prey and mean volume of their stomach contents, however, the strength of the correlation was greater when two larger detrivores (i.e., striped mullet and gizzard shad) were excluded from the analysis. Similarly, trophic positions estimated by volumetric stomach contents were also significantly related to predator body size but not related to mean volume of stomach contents, but again excluding those detrivores from the analyses increased the strength of the relationship. Trophic positions estimated from stable isotopes and δ15N as an index of trophic position were also unrelated to predator body length, but significantly related to predator body mass. Although estimates of trophic positions in this tidal estuary using both methods were largely concordant, there were some exceptional zooplanktivorous and detritivorous species that had higher trophic levels according to nitrogen isotope ratios. Excluding those species from the analyses increased the strength of relationships between size and trophic positions of predators. A significant relationship between body sizes of consumers and their prey supports the view that body size is a key variable influencing trophic interactions and the structure of aquatic food webs. Our results also suggest that body size (especially consumer mass) is a good predictor of trophic levels estimated by stable isotopes, whereas consumer length is an important trait predicting the trophic level estimated from stomach contents in this tidal estuarine system.