Trait-based approach reveals how marginal reefs respond to acute and chronic disturbance

Abstract

Ecosystems worldwide are becoming increasingly altered by environmental stress, yet little is known about how acute disturbances affect ecological communities that already persist under chronically stressful environmental conditions. Here, we use a trait-based approach to understand the effects of repeated flooding on the subtropical reefs of Hervey Bay, Australia (25˚ South) that are considered marginal both in terms of their chronically turbid and high-latitude settings. We quantify variation in taxonomic and functional composition of coral assemblages along spatial gradients of water quality, before and after three flooding events of the Mary River between 2010 and 2013. We demonstrate systematic variation in taxonomic and functional composition along distance from shore and rivers and show remarkable stability of patterns over the course of repeated flooding. This is likely because the reefs of Hervey Bay are regularly exposed to resuspension of sediments, limiting the types of species that are able to persist in these chronically turbid settings, with flood events reinforcing the longer-term stress regime. Greater similarity of co-occurring species on nearshore reefs than expected by chance also supports this hypothesis. Corallite size, coral morphology, the ability to remove sediment, and propagule development rate best explained variation in coral assemblage structure. Specifically, reefs closer to shore and to rivers were characterised by species with large corallites, high sediment removal ability and foliose morphology, strategies considered beneficial in turbid conditions. Among individual traits, symbiont transmission best explained variation in assemblage structure, and it is plausible that the acquisition of locally adapted symbionts from the environment is advantageous in the turbid and light-limited conditions of nearshore reefs. These findings highlight the importance of tolerance to turbidity and sedimentation in driving coral biodiversity patterns in Hervey Bay, with acute flooding reinforcing the long-term stress regime that controls the distribution of species on these marginal reefs.