Re-visiting the paradigm of coral reef depth zonation on contemporary reefs

Abstract

The influence of depth and associated gradients in light, nutrients, and plankton on the ecological organisation of tropical reef communities was first described over six decades ago but remains untested across broad geographies. Humans have also become the dominant driver of planetary change, requiring we re-visit historic ecological paradigms to ensure they capture the dynamics of contemporary ecological systems. Analysing >5,500 in-water reef fish surveys between 0–30 m depth across reef slopes at 35 islands in the Pacific, we assess whether a depth gradient consistently predicts variation in reef fish biomass. We reveal predictable ecological organisation in the absence of local human populations, with increased biomass of planktivores and piscivores, and decreased biomass of primary consumers with increasing depth. Bathymetric steepness also had a striking influence on biomass patterns, primarily for planktivores, emphasising potential links between local hydrodynamics and the upslope propagation of pelagic subsidies to the shallows. However, signals of resource-driven depth zonation were altered or lost at populated islands, likely due to depleted fish biomass baselines. While principles of depth zonation broadly held, our findings expose limitations of the paradigm for predicting ecological dynamics where human-mediated impacts confound the connections between ecological communities and their surrounding environment.