Oligochaetes as a possible entry route for terrigenous organic carbon into estuarine benthic food webs

Estuaries receive large quantities of terrestrially derived organic material, yet the current view is that such terrestrial carbon is unimportant for temperate estuarine benthic trophic dynamics beyond microbial processing. This consensus, however, may derive from a lack of consideration in estuarine food web studies of taxa with evolutionary affinities to freshwater systems where these taxa process terrestrial detritus. Here, we used a multiple stable isotope approach (carbon, nitrogen and sulphur) in 3 similar estuarine systems to test whether taxa with high (oligochaetes), medium (amphipods) and low (marine polychaetes and other crustacean taxa) evolutionary associations with freshwater systems differed in their assimilation of carbon derived from contrasting detrital sources. Oligochaetes had isotopic signatures significantly different to those of other organisms, yet not significantly different from tree and ground plant signatures, demonstrating that they assimilate terrestrial carbon. In contrast, amphipods and marine taxa had isotope signatures that indicated a reliance on marine algal carbon, independent of where in the estuary they were sampled, suggesting that, unlike oligochaetes, these taxa do not have an inherent physiological ability to successfully assimilate terrestrial material. These findings indicate that terrestrial carbon can play a significant role in estuarine systems, with oligochaetes providing the metazoan entry route for this carbon source into food webs, and that evolutionary detritivore-detritus associations may influence present-day trophic dynamics within estuaries.