Abstract
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 urchinEvechinus chloroticusand the detritivorous sea cucumberStichopus mollis, and in δ15N and δ13C of the macroalgae and consumers among 5 study sites across this gradient. Analysis of δ15N and δ13C 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 ofE. chloroticusstomach contents from the innermost sites provided strong evidence that terrestrial detritus was being assimilated via microbial recycling (δ15N, –5‰ and δ13C, –37‰). Differences in δ13C 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 ofE. chloroticuswithin the fjord, with links to the source–sink structure of the population.