AB 14:129-134 (2012)  -  DOI: https://doi.org/10.3354/ab00392

ABSTRACT: We examined the assimilation of terrestrial organic matter (TOM) by the ovoviviparous isopod Excirolana armata inhabiting an exposed sandy beach of Uruguay. The source of this terrestrial input was an artificial freshwater discharge (the Andreoni Canal). We analyzed carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios in the TOM, in seawater particulate organic matter (POM) and in E. armata at 3 beach sites: Andreoni, at the canal mouth, Coronilla, at 1 km from the mouth and Barra, at 13 km from the mouth. Our analysis revealed clear effects of the canal discharge on the beach ecosystem. TOM was observed in the beach-face detrital mixture only at Andreoni; it also showed the lowest isotopic values there. POM δ¹³C values were also lowest near the canal mouth (increasing with distance from the mouth), indicating the dominance of terrestrial material in the carbon pools of sites closest to the freshwater discharge (viz. Andreoni and Coronilla). The same trend was seen in E. armata. Mixing-model (IsoSource) results for E. armata at Andreoni and Coronilla suggested that TOM was the main contributor to diet at these sites, while POM was the main contributor at Barra. We concluded that E. armata is a useful indicator of TOM assimilation in sandy-beach ecosystems. Our results point up the need, when studying sandy beach food webs, to consider potential modifications by terrestrial allochthonous inputs.

KEY WORDS: Trophic subsidy · Excirolana armata · Freshwater discharge · Sandy beaches · Stable isotopes · Uruguay