Tropical scleractinian corals are considered autotrophic as they rely mainly on photosynthesis-derived nutrients transferred from their photosymbionts. Corals are also able to capture and ingest suspended particulate organic matter, so heterotrophy can be an important supplementary trophic pathway to optimize coral fitness. The aim of this in situ study was to elucidate the trophic status of 10 coral species under contrasted environmental conditions in a French Polynesian lagoon. Carbon (δ13C) and nitrogen (δ15N) isotopic compositions of coral host tissues and photosymbionts were determined at 3 different fringing reefs during wet and dry seasons. Our results highlighted spatial variability in stable isotopic compositions of both coral host tissues and photosymbionts. Samples from the site with higher level of suspended particulate matter were 13C-depleted and 15N-enriched relative to corals and photosymbionts from less turbid sites. However, differences in both δ13C and δ15N between coral host tissues and their photosymbionts (Δhost-photosymbionts 13C and Δhost-photosymbionts 15N) were small (0.27 ± 0.76‰ and 1.40 ± 0.90‰, respectively) and similar at all sites, thus indicating no general increases in the heterotrophic pathway. Depleted δ13C and enriched δ15N values of coral host tissues measured at the most turbid site were explained by changes in isotopic composition of the inorganic nutrients taken up by photosymbionts and also by changes in rate of isotopic fractionation with environmental conditions. Our results also highlighted a lack of significant temporal variations in δ13C and δ15N values of coral host and photosymbiont tissues and in Δhost-photosymbionts 13C and Δhost-photosymbionts 15N values. This temporal stability indicated that corals remained principally autotrophic even during the wet season when photosymbiont densities were lower and the concentrations of phytoplankton were higher. Increased coral heterotrophy with higher food availability thus appears to be species-specific.

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Fatty acid profiles of food sources, dominant zooplankton, macrofauna and a few small ichthyofauna in a temperate estuary on the southeastern coast of South Africa were used to trace the original carbon sources and the degree of feeding plasticity of consumers within an oligotrophic environment. Fatty acid analysis is seldom used to examine food web ecology in estuaries, as these systems are typically highly complex owing to the variety of potential food sources and the variable nature of detritus. We hypothesized that generalist consumers would exhibit similar and overlapping fatty acid compositions, whereas any specialist consumers would 'stand out' as they may contain high proportions of fatty acids specific to their preferred food sources. Within-species variation was very small in the food sources, and Principal Component Analysis (PCA) allowed resolution of most species and sample types into distinct clusters. Variation of fatty acid composition was greater within the consumers, with PCA resolving several clusters by collection location within the site and feeding mode of the animals (suspension feeders, grazers and omnivores). We considered the fatty acid profiles (with particular reference to polyunsaturated fatty acids and essential fatty acids) of the consumers in relation to the food sources to determine which organisms played significant roles in the sequestration and cycling of high quality organic material within the estuary. KEY WORDS: Fatty acids · Food web · Biomarker · Salt marsh · Estuary · Trophic ecology · South AfricaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 357: 121-137, 2008 transferred without modification into higher trophic levels (Dalsgaard et al. 2003). When used together, fatty acid and stable isotope techniques show promise in resolving the trophic ecology of complex aquatic ecosystems (Kharlamenko et al. 2001, Alfaro et al. 2006, Perga et al. 2006.Consumers derive all their lipid requirements either directly from the diet or indirectly by the transformation of protein and carbohydrate precursors. Lipids, particularly the polyunsaturated fatty acid (PUFA) components, have vital structural and functional roles in membranes and are therefore required for growth and reproduction (Arts 1999). Fatty acids that are required for the proper biological functioning of organs, but cannot be effectively synthesized de novo by animals (the essential fatty acids, EFAs), include the PUFAs 20:5ω3, 22:6ω3 and occasionally 20:4ω6 (Olsen 1999). These fatty acids must therefore be obtained in the diet, and, as such, EFAs are highly conserved in aquatic systems and are fundamental drivers of ecosystem fitness and stability (Arts et al. 2001). Trophic ecologists are interested in the sources of PUFAs, and EFAs in particular, and in the consumers that most effectively sequester these molecules within an aquatic ecosystem (Parrish et al. 2000). Most research into fatty acid composition has been focussed on marine fish and shellfish owing ...

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Sub-Antarctic islands represent critical breeding habitats for land-based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub-Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long-term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long-term decline in enhanced phytoplankton productivity at the islands in response to a climate-driven shift in the position of the sub-Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore-feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore-feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.