Plasticity in the diet of two echinoderm species across an ecotone: microbial recycling of forest litter and bottom-up forcing of population structure

Many coastal marine populations persist across gradients in benthic productivity. In the New Zealand fjords there is a sharp gradient in available biomass between the wave-washed outer coasts, dominated by kelps, and the quiescent inner fjords, where estuarine seaweeds and terrestrial inputs predominate. In Doubtful Sound we found significant variation in abundance of macroalgal groups, the grazing sea urchin Evechinus chloroticus and the detritivorous sea cucumber Stichopus mollis, and in delta N-15 and delta C-13 of the macroalgae and consumers among 5 study sites across this gradient. Analysis of delta N-15 and delta C-13 from tissue of the 2 consumers relative to the primary carbon source pools with a mass balance model indicated that diet was primarily influenced by composition and quality of macroalgal food, except at the innermost sites, where there was evidence for terrestrial inputs. These results demonstrate that it is important to resolve relative abundance of food sources and specific isotopic variation to resolve spatial patterns in diet from stable isotope analysis across environmental gradients. Isotopic analysis of E. chloroticus stomach contents from the innermost sites provided strong evidence that terrestrial detritus was being assimilated via microbial recycling (delta N-15, -5 %. and delta C-13, -37%.). Differences in delta C-13 of stomach contents versus those of tissues provided a basis to measure assimilation. There was a strong correlation between this proxy for assimilation with growth parameters among study sites across the ecotone. This analysis indicates a strong bottom-up influence on vital rates of E. chloroticus within the fjord, with links to the source-sink structure of the population.