Trophic structure and potential carbon and nitrogen flow of a rhodolith bed at Santa Catalina Island inferred from stable isotopes

Abstract

In many coastal marine systems with low productivity, cross-habitat exchange of subsidies has been shown to have significant bottom-up effects. California rhodolith beds (Lithothamnion australe Foslie) support invertebrate communities whose biomass doesn’t appear to be supported by the limited productivity of rhodoliths. Detrital subsidies from the water column and adjacent giant kelp Macrocystis pyrifera forests may supplement the base of the food web in these beds. Stable isotope analyses were conducted using seawater organic matter, sediment organic matter, and macroalgae as endmembers to determine their relative importance to consumers and create trophic structure of a rhodolith bed off Santa Catalina Island. Using cluster analysis on carbon δ13C and nitrogen δ15N values of 13 invertebrate consumer taxa, five trophic groups were identified: planktivore, zooplanktivore, detritivore, herbivore, and carnivore. The isotope ratios of sediment organic matter from within rhodoliths were similar to benthic and drifting kelp M. pyrifera tissue, suggesting neighboring kelp habitats, or other unmeasured sources, may contribute to the organic matter within rhodoliths. Detritivores, herbivores, and carnivores appeared to consume particulate organic matter from the water column directly or indirectly through prey. Follow-up experiments indicated that increasing surface area of giant kelp pieces increased drift rates while smaller kelp material moved less and may have greater potential to be retained within rhodolith beds during periods of increased water motion. Overall, temporal fluctuations in the supply and export of suspended particulate organic matter from the water column and drift macroalgal subsidies from adjacent kelp forests may have considerable effects on secondary production and community structure of rhodolith beds.