Regional differences in supply of organic matter from kelp forests drive trophodynamics of temperate reef fish

Regional differences in trophic structure and availability of alternate sources of basal organic matter to food webs can affect the volume of organic matter converted into fish biomass. The present study combined stable isotope analyses (delta C-13 and delta N-15) with estimates of biomass density of 22 common reef fishes to compare supply of organic matter derived from macroalgae versus phytoplankton to reef fish communities among 30 sites distributed across Fiordland and the Marl-borough Sounds, 2 contrasting regions in terms of land-based stressors on the South Island, New Zealand. Fish communities in the Marlborough Sounds were supported by food webs that incorporated less organic matter derived from macroalgae compared to those in Fiordland. Contribution of organic matter derived from macroalgae to fish biomass decreased with trophic level in the Marl-borough Sounds, while fishes in Fiordland were supported by a more equal mixture of organic matter derived from phytoplankton and macroalgae among trophic levels. Total fish biomass density was 1.72 times higher in Fiordland, yet the fish community converted 2.91 times more organic matter to fish biomass, as a result of a higher proportion of biomass at high trophic levels. The observed patterns were consistent with limitation in supply of organic matter derived from macroalgae in the Marlborough Sounds, where extensive losses of kelp forest habitat linked to land-based stressors have been reported. The results highlight the importance of considering regional variability in basal organic matter source pools, particularly those produced from sensitive kelp forest habitats, when applying ecosystem-based approaches to managing coastal resources.