Pelagic Subsidies Underpin Fish Productivity on a Degraded Coral Reef

Abstract

Coral reefs harbor high productivity in nutrient-poor tropical oceans. This exceptional productivity can be explained by high recycling rates [1, 2], deep-water nutrient enrichment [3], and assimilation of external production [4]. Fishes consume this productivity through multiple trophic pathways and, as a result, dominate consumer biomass. Their reliance on pelagic versus benthic productivity pathways has been quantified from the tissues of individual fish [5, 6], but the contribution of different energetic pathways to the total productivity of coral reef fishassemblages remains unquantified. Here, we combined high-resolution surveys and individual biomass production estimates to generate the first energetic map of a full coral reef fish assemblage, from the smallest to the largest fishes [7, 8]. We found that the windward section of a coral reef on the Great Barrier Reef delivered an average fish productivity of 4.7kg ha-1 day-1, of which 41% was derived from water column photosynthesis, 29% by the epibenthic reef surface, 14% from cryptobenthic microhabitats, and 11% from adjacent sandy areas. The critical energetic contribution of pelagic subsidies would remain undetected if considering fish standing biomass alone, because the high productivity of reef planktivores originated from a relatively small biomass. Importantly, this study took place on a reef with only ∼6% of coral cover following multiple coral mortality events. Thus, our study offers hope that reefs subject to coral loss can still maintain considerable fish productivity, with planktivorous fishes providing major pelagic subsidies.