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 δ 15 N and δ 13 C of the macroalgae and consumers among 5 study sites across this gradient. Analysis of δ 15 N and δ 13 C 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 (δ 15 N,-5 ‰ and δ 13 C,-37 ‰). Differences in δ 13 C 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.
Field surveys and laboratory experiments were used to investigate the influence of the physical environment on variability in δ13C and δ15N signatures of Ulva pertusa, an abundant macroalgae inhabiting the low salinity layer (LSL) of Doubtful Sound, a New Zealand fjord. Field surveys revealed significant spatial variability in δ13C (‐18% to ‐12%) and δ15N (0% to 6%). δ13C was enriched at high irradiance sites and depleted at the fjord’s wave‐exposed entrance. δ15N signatures increased from 0% at the fjord head where freshwater influence is greatest to an oceanic signature of 6% at the fjord entrance. δ15N also increased by up to 4% between 2‐m depth and the LSL‐seawater interface (4‐m depth); this pattern was less pronounced near the ocean. During laboratory experiments, δ13C of U. pertusa became significantly enriched under high levels of irradiance (>50 mmol quanta m−2 s−1). When exposed to high irradiance, increases in water motion rapidly depleted δ13C signatures by as much as 5%. Variability in δ13C of U. pertusa in Doubtful Sound is largely a function of the light regime, which influences rates of photosynthesis and in turn the algae’s dependence on HCO3−, an enriched source of carbon. However, increased water motion at the fjord entrance counteracts the influence of irradiance, leading to enhanced flux of CO2 and depleted δ13C signatures. Variation in δ15N of U. pertusa is less dependent on the physical environment and instead is driven by the source pool signature, which in turn varies between freshwater and marine sources of nitrogen.
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