Studies in the stable isotope ecology of coral-reef fish food-webs

Abstract

Coral-reef fish food-webs are complex and few studies have explored the range of production sources or how these support the higher trophic levels of coral-reef foodwebs. I collected muscle tissue samples of abundant coral-reef fish at Bahamian and Maldivian sites and used bulk stable isotope (δ13C and δ15N) data in bi-plot to analyse their isotopic niches in relation to their putative trophic guilds; some species (e.g. some corallivorous Chaetodontidae) are evidently confined to their guild, but others seem to be utilizing multiple food sources. I also analysed δ15N (as a proxy for trophic position) to body size relationships among all individuals, individuals of the same species, individuals sharing the same trophic pathway and of the community as a whole. At species, some fishes had relatively flat relationships whereas others varied greatly with size. Individuals relying on the same production source type (e.g. planktivorous Chaetodontidae and Acanthuridae, Maldivian data only) had similar body size-δ15N trends with variation potentially due to at least family-related traits. At community level, there was a positive linear relationship in the Bahamas sites but a parabolic relationship in the Maldives, suggesting trophic function changes at different size classes in different ways in the two locations. I further tested whether the putative trophic guilds precisely portrayed the strict feeding patterns among fishes and whether location affected source partitioning using both bulk (SIA) and compound-specific stable isotope data (CSIA). The results suggested that certain consumers (e.g. corallivores [SIA] and detritivores [SIA and CSIA]) were confined to the trophic guild to which they have typically been assigned, while others show some evidence of relying on multiple sources (e.g. diurnal planktivores [SIA and CSIA]) indicating that it is imprecise to assign them to the single guilds to which they have commonly been assigned. At outer-atoll sites, some ii fish fed more on diurnal plankton than those in the inner-atoll suggesting some geographic characteristics could affect the feeding preferences. I have applied stable isotope data to elucidate the importance of body size and geographic location in determining the feeding strategies of fish to better understand how these diverse food-webs work. The study provides pointers to future work such as on benthic-pelagic coupling, roles of sponges, contribution of SIA vs CSIA data and finer-detailed diet composition.