Offshore pelagic subsidies dominate carbon inputs to coral reef predators.

Coral reefs were traditionally perceived as productive hot spots in oligotrophic waters. While modern evidence indicates that many coral reef food webs are heavily subsidized by planktonic production, the pathways through which this occurs remain unresolved. We used the analytical power of carbon isotope analysis of essential amino acids to distinguish between alternative carbon pathways supporting four key reef predators across an oceanic atoll. This technique separates benthic versus planktonic inputs, further identifying two distinct planktonic pathways (nearshore reef-associated plankton and offshore pelagic plankton), and revealing that these reef predators are overwhelmingly sustained by offshore pelagic sources rather than by reef sources (including reef-associated plankton). Notably, pelagic reliance did not vary between species or reef habitats, emphasizing that allochthonous energetic subsidies may have system-wide importance. These results help explain how coral reefs maintain exceptional productivity in apparently nutrient-poor tropical settings, but also emphasize their susceptibility to future ocean productivity fluctuations.

Methodological uncertainty in resource mixing models for generalist fishes.

Carbon and nitrogen stable isotope ratios are used to assess diet composition by determining bounds for the relative contributions of different prey to a predator's diet. This approach is predicated on the assumption that the isotope ratios of predator tissues are similar to those of dominant food sources after accounting for trophic discrimination (Δ(x)X), and is formulated as linear mixing models based on mass balance equations. However, Δ(x)X is species- and tissue-specific and may be affected by factors such as diet quality and quantity. From the different methods proposed to solve mass balance equations, some assume Δ(x)X to be exact values whilst others (based on Bayesian statistics) incorporate variability and inherent uncertainty. Using field data from omnivorous reef fishes, our study illustrates how uncertainty may be taken into account in non-Bayesian models. We also illustrate how dietary interpretation is a function of both absolute Δ(x)X and its associated uncertainty in both Bayesian and non-Bayesian isotope mixing models. Finally, collated literature illustrate that uncertainty surrounding Δ(x)X is often too restricted. Together, these data suggest the high sensitivity of mixing models to variation in trophic discrimination is a consequence of inappropriately constrained uncertainty against highly variable Δ(x)X. This study thus provides guidance on the interpretation of existing published mixing model results and in robust analysis of new resource mixing scenarios.

Habitat degradation and fishing effects on the size structure of coral reef fish communities.

Overfishing and habitat degradation through climate change pose the greatest threats to sustainability of marine resources on coral reefs. We examined how changes in fishing pressure and benthic habitat composition influenced the size spectra of island-scale reef fish communities in Lau, Fiji. Between 2000 and 2006 fishing pressure declined in the Lau Islands due to declining human populations and reduced demand for fresh fish. At the same time, coral cover declined and fine-scale architectural complexity eroded due to coral bleaching and outbreaks of crown-of-thorns starfish, Acanthaster planci. We examined the size distribution of reef fish communities using size spectra analysis, the linearized relationship between abundance and body size class. Spatial variation in fishing pressure accounted for 31% of the variation in the slope of the size spectra in 2000, higher fishing pressure being associated with a steeper slope, which is indicative of fewer large-bodied fish and/or more small-bodied fish. Conversely, in 2006 spatial variation in habitat explained 53% of the variation in the size spectra slopes, and the relationship with fishing pressure was much weaker (approximately 12% of variation) than in 2000. Reduced cover of corals and lower structural complexity was associated with less steep size spectra slopes, primarily due to reduced abundance of fish < 20 cm. Habitat degradation will compound effects of fishing on coral reefs as increased fishing reduces large-bodied target species, while habitat loss results in fewer small-bodied juveniles and prey that replenish stocks and provide dietary resources for predatory target species. Effective management of reef resources therefore depends on both reducing fishing pressure and maintaining processes that encourage rapid recovery of coral habitat.

Identifying reefs of hope and hopeful actions: contextualizing environmental, ecological, and social parameters to respond effectively to climate change.

Priorities for conservation, management, and associated activities will differ based on the interplay between nearness of ecosystems to full recovery from a disturbance (pristineness), susceptibility to climate change (environmental susceptibility [ES]), and capacity of human communities to cope with and adapt to change (social adaptive capacity [AC]). We studied 24 human communities and adjacent coral reef ecosystems in 5 countries of the southwestern Indian Ocean. We used ecological measures of abundance and diversity of fishes and corals, estimated reef pristineness, and conducted socioeconomic household surveys to determine the AC of communities adjacent to selected coral reefs. We also used Web-based oceanographic and coral mortality data to predict each site's ES to climate warming. Coral reefs of Mauritius and eastern Madagascar had low ES and consequently were not predicted to be affected strongly by warm water, although these sites were differentiated by the AC of the human community. The higher AC in Mauritius may increase the chances for successful self-initiated recovery and protective management of reefs of this island. In contrast, Madagascar may require donor support to build AC as a prerequisite to preservation efforts. The Seychelles and Kenya had high ES, but their levels of AC and disturbance differed. The high AC in the Seychelles could be used to develop alternatives to dependence on coral reef resources and reduce the effects of climate change. Pristineness weighted toward measures of fish recovery was greatest for Kenya's marine protected areas; however, most protected areas in the region were far from pristine. Conservation priorities and actions with realistic chances for success require knowledge of where socioecological systems lie among the 3 axes of environment, ecology, and society.

Effects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of fish tissues.

For accurate interpretation of fish trophodynamics from carbon stable isotope data it is necessary to extract tissue lipids. This is because lipid content varies within and among tissues in both space and time, and because lipids are 13C-depleted relative to proteins. However, lipid extraction may affect delta15N, thus requiring costly and time-consuming separation of delta13C and delta15N analyses. These problems have prompted the development of arithmetic correction techniques for delta13C, but the techniques and their underlying assumptions have not been systematically tested. This study compared the effects of lipid extraction and arithmetic correction techniques on delta13C and delta15N of European sea bass (Dicentrarchus labrax) tissues. Following Folch lipid extraction from muscle and liver, there was a mean increase in delta15N of 0.77 per thousand, but enrichment varied with lipid content such that effects on delta15N were hard to predict. Changes in delta13C and C:N between untreated and lipid-extracted samples reflected the quantity of lipid removed. The arithmetic correction techniques of mass balance and lipid correction were sensitive to the C:N of the lipid-extracted tissue and to the assumed depletion of lipid delta13C relative to protein delta13C. However, the mass balance approach was appropriate for the mathematical correction of bulk tissue data in most circumstances, provided that the C:N of lipid-extracted tissue could be determined for a small proportion of samples. Application of mass balance arithmetic correction can lead to significant time and cost savings in trophodynamic studies, because the majority of delta13C and delta15N analyses would not need to be run separately.

Tissue and fixative dependent shifts of delta13C and delta15N in preserved ecological material.

Carbon and nitrogen stable isotope analyses are routinely used to investigate aquatic food webs, and have potential application in retrospective investigations using archived materials. However, such analyses assume that storage does not alter isotopic signatures of materials preserved, or that changes in isotopic composition during storage are predictable. Here we examine preservation shifts on cod (Gadus morhua) muscle, roe and liver tissue over 21 months following preservation in 80% ethanol, in 4% formaldehyde, and by freezing. Preservation shifts were not consistent among tissues. High protein tissues exhibited greater delta(15)N shifts than low protein tissues in 4% formaldehyde, while greater delta(13)C shifts occurred in relatively higher fat tissues when preserved in alcohol. Freezing did not change isotopic signatures. Responses of delta(15)N and delta(13)C are explained by differences in the preservative's isotopic signature and the reaction properties and biochemical composition of the tissues preserved. The results clarify some of the processes that lead to isotopic change during preservation.

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes.

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ13C and δ15N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ13C of plants ranged from -8.59‰ to -33.74‰, of benthic invertebrates from -17.0‰ to -20.52‰, of planktonic invertebrates from -20.08‰ to -22.34‰ and of fishes from -16.27‰ to -19.59‰. δ15N was generally greater at higher trophic levels. δ15N of plants was 0.95-2.92‰, of benthic invertebrates 1.69-6.54‰, of planktonic invertebrates 3.51-6.82‰ and of fishes 4.63-9.77‰. 13C enrichment tended to be associated with benthic food chains and 13C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ15N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected.