Investigating the global importance of benthic-pelagic coupling for the ecosystem function of coastal reefs

Coastal reefs are some of the most diverse ecosystems on the planet, hosting over 3000 fish species that fulfil important ecological roles (Bellwood et al. 2013). This diversity sustains important artisanal fisheries as well as economically valuable ecotourism industries, ensures ecosystem stability and can also accelerate coral reef recovery from disturbances such as cyclones and bleaching (Bellwood et al. 2013). 

However, the productivity, species composition and growth rates of fish in coastal reefs is changing in response to habitat degradation and climate change (Munday et al. 2008), yet we do not understand the role of pelagic and benthic productivity in sustaining these ecosystems.

Recent studies in coastal upwelling systems in South America have demonstrated that coastal reefs almost completely (up to 99%) depend on pelagic productivity (Docmac et al. 2017). However, it is entirely unclear to what extent this applies to other coastal ecosystems, including those in Australia (Truong et al. 2017).

Are coral and rocky reefs or temperate kelp forest ecosystems similarly dependent on pelagic production or do they act as largely self-sustained systems? And how will the climate driven shifts in oceanic circulation, changes in species distribution, rates of pelagic production and coastal habitat degradation affect the relative importance of benthic (mostly local) versus pelagic (mostly imported through water exchange) food sources and ecosystem dynamics?

The broad goals of this project are therefore to quantify the role of pelagic and benthic carbon sources in sustaining coastal reef productivity, and how/if the ratio of benthic:pelagic carbon sources changes with latitude for overall reef biomass, and/or for particular species where applicable. Further, in order to support conclusions drawn from isotopic signatures, this project will examine the gut contents of all fish collected. In addition to identifying gut contents to the finest taxonomic resolutions feasible, gut contents will also be measured providing valuable data for size-based models.

The specific aims of this study are therefore as follows: Investigate the isotopic signatures of fish and basal food sources, to determine the ratio of pelagic/benthic carbon supporting the biomass of key fish functional groups on shallow reefs. Examine the gut contents of fish to provide size spectrum diet data for size-based ecosystem models; and, to validate conclusions drawn from isotopic analyses. Determine the congruence of methods for deriving fish trophic level, by comparing the overlap in statistical clustering of fish species via gut content and stable isotope methods, as per Cresson et al. (2014).

This is a study led by Amy Coghlan from the Institute for Marine and Antarctic Studies, University of Tasmania and supported by researchers from UNSW who have collaborated in fieldwork.