Ph.D. Hinojosa-Toledo, Ivan

1. The structure of food webs provides important insight into biodiversity, organic matter (OM) pathways, and ecosystem functioning. 2. Stable isotope analysis (δ13C and δ15N) was used to characterize the trophic structure and the main OM pathways supporting food webs in the Rapa Nui coastal marine ecosystem. 3. The trophic position of consumers and isotopic niche metrics were estimated for different assemblages (i.e. mesozooplankton, emergent zooplankton, reef invertebrates, reef fishes, pelagic fishes, and seabirds). Furthermore, the relative importance of different OM sources (i.e. macroalgae, zooxanthellate corals, and particulate OM [POM]) was assessed for heterotrophic consumers using Bayesian mixing model (MixSIAR). 4. Results show a clear pattern of 13C and 15N enrichment from small‐sized pelagic and benthic invertebrates, to reef and pelagic fishes, and seabirds. Most invertebrates were classified as primary consumers, reef fishes as secondary consumers and pelagic predators and seabirds as tertiary and quaternary consumers. 5. Isotopic niche metrics indicate a low trophic diversity for pelagic assemblages (mesozooplankton and pelagic fishes), in contrast to reef fauna (invertebrates and fishes), whose higher trophic diversity suggest the exploitation of a wider range of trophic resources. Overlapping of standard ellipses areas between reef invertebrates and reef fishes indicates that both assemblages could be sharing trophic resources. 6. Mixing models results indicate that POM is the main trophic pathway for mesozooplankton, macroalgae (Rhodophyta) for emergent zooplankton, and a mix of coral‐derived OM and Rhodophyta for coral reef assemblages such as macrobenthos and reef invertebrates. In contrast, POM contribution was notably more important for some pelagic fishes and seabirds from upper trophic levels. 7. This study provides key elements for conservation efforts on coral reefs, management planning and full‐implementation of the recently created Rapa Nui Multiple Use Marine Protected Area.

Coastal marine upwelling famously supports elevated levels of pelagic biological production, but can also subsidize production in inshore habitats via pelagic‐benthic coupling. Consumers inhabiting macroalgae‐dominated rocky reef habitats are often considered to be members of a food web fuelled by energy derived from benthic primary production; conversely, they may also be subsidized by materials transported from pelagic habitats. Here, we used stable isotopes (δ13C, δ15N) to examine the relative contribution of pelagic and benthic materials to an ecologically and economically important benthivorous fish assemblage inhabiting subtidal macroalgae‐dominated reefs along ~1,000 km of the northern Chilean coast where coastal upwelling is active. Fish were isotopically most similar to the pelagic pathway and Bayesian mixing models indicated that production of benthivorous fish was dominated (median 98%, range 69–99%) by pelagic‐derived C and N. Although the mechanism by which these materials enter the benthic food web remains unknown, our results clearly highlight the importance of pelagic‐benthic coupling in the region. The scale of this subsidy has substantial implications for our basic understanding of ecosystem functioning and the management of nearshore habitats in northern Chile and other upwelling zones worldwide.