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


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.

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Data availability

The datasets analyzed during the current study are summarized in Table 1.


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I would like to thank D. Steller, P. Tompkins, S. Hamilton, R. Mehta, J. Redwine, M. Marraffini, M. Fox, A. Muth, K. Meagher Robinson, E. Robinson, B. Higgins, D. van Hees, K. van Hees, I. Moffit, K. Kopecky, S. Sampson, A. Macleod, and A. Olson for field help. I also thank M. Graham, S. Hamilton, and D. Steller for advice and comments during the development and completion of this work. I would also like to thank T. Oudin, L. Oudin, and K. Spafford at the USC Wrigley Institute for Environmental Studies. I thank Dr. Patrick Gagnon and four anonymous reviewers for their time and constructive feedback that improved this manuscript.


This work was funded by The American Academy of Underwater Sciences (AAUS) Kevin Gurr Scholarship Award, Moss Landing Marine Laboratories (MLML) Signe Lundstrom Memorial Scholarship, MLML Wave Award, Council on Ocean Affairs, Science & Technology (COAST) Student Award for Marine Science Research, David and Lucile Packard Foundation Award, and the Dr. Earl H. Myers and Ethel M. Myers Oceanographic and Marine Biology Trust.

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Correspondence to Scott S. Gabara.

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All applicable national and/or institutional guidelines for the care and use of animals were followed. I thank the California Department of Fish and Wildlife for permit SCP-10399 to conduct this work.

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Gabara, S.S. Trophic structure and potential carbon and nitrogen flow of a rhodolith bed at Santa Catalina Island inferred from stable isotopes. Mar Biol 167, 30 (2020).

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