Benthic processes and overlying fish assemblages drive the composition of benthic detritus on a central Pacific coral reef

Abstract

While detrital material is recognized as an important food source on coral reefs, its role in reef food webs remains unclear. We quantified standing stock and input rates to the detrital resource pool in exposed forereef and protected backreef habitats of Palmyra Atoll National Wildlife Refuge and measured the trophic structure of the overlying fish assemblage. While detrital standing stock was 1.6 times higher on the backreef than on the forereef, detrital input rates were 1.7 to 2.9 times higher on the forereef. Planktivores were the most abundant guild in the fore reef habitat, and stable isotope signatures of detritus reflected a greater input from pelagic sources (i.e. depleted in 13C). In contrast, herbivores and detritivores numerically dominated the backreef habitat and detrital stable isotope signatures appeared to be predominately of benthic origin (i.e. enriched in 13C). Through total organic carbon (TOC) and nitrogen analyses we found that benthic detritus may represent a significant nutritional source. Converting total nitrogen into maximum protein estimates, we found high benthic deposition of protein (104 to 124 mg m-2 d-1) and organic carbon (184 to 190 mg m -2 d-1), but very low standing stocks of these materials (protein: 5 to 6 mg m-2, organic carbon: 46 to 63 mg m-2). While high water flow rates may explain low standing stocks of detritus in forereef habitats, the lower flow rates in backreef habitats suggest that removal of this material is via consumption by abundant roving detritivorous fishes. Our results provide support for the hypothesis that reef fish detritivory represents a significant consumer-mediated energy pathway, promoting nutrient recycling by linking many elements of a complex food web. © Inter-Research 2013.