Wave exposure shapes reef community composition and recovery trajectories at a remote coral atoll

Abstract

In a time of unprecedented ecological change, understanding natural biophysical relationships between reef resilience and physical drivers is of increasing importance. This study evaluates how wave forcing structures coral reef benthic community composition and recovery trajectories after the major 2015/2016 bleaching event in the remote Chagos Archipelago, Indian Ocean. Benthic cover and substrate rugosity were quantified from digital imagery at 23 fore reef sites around a small coral atoll (Salomon) in 2020 and compared to data from a similar survey in 2006 and opportunistic surveys in intermediate years. Cluster analysis and principal component analysis show strong separation of community composition between exposed (modelled wave exposure > 1000 J m−3) and sheltered sites (< 1000 J m−3) in 2020. This difference is driven by relatively high cover of Porites sp., other massive corals, encrusting corals, soft corals, rubble and dead table corals at sheltered sites versus high cover of pavement and sponges at exposed sites. Total coral cover and rugosity were also higher at sheltered sites. Adding data from previous years shows benthic community shifts from distinct exposure-driven assemblages and high live coral cover in 2006 towards bare pavement, dead Acropora tables and rubble after the 2015/2016 bleaching event. The subsequent recovery trajectories at sheltered and exposed sites are surprisingly parallel and lead communities towards their respective pre-bleaching communities. These results demonstrate that in the absence of human stressors, community patterns on fore reefs are strongly controlled by wave exposure, even during and after widespread coral loss from bleaching events.

Remote reef cryptobenthic diversity: Integrating autonomous reef monitoring structures and in situ environmental parameters

“…We find that together, plate face and site choice significantly contribute to differences observed in the abundance of CCA, fleshy red encrusting algae and green encrusting algae across sampled reefs. Whilst the percentage cover of fleshy macroalgae has been estimated to be between 1% and 4% across shallow reefs of the Chagos Archipelago (Head et al, 2019;Lange et al, 2021), they represent around a third of benthic communities across ARMS devices in this study. Unlike some cryptobenthic invertebrates (e.g., bryozoan, ascidian, and encrusting sponges), macroalgae are often included in reef biomonitoring protocols and surveys.…”

“…Our image analysis recovers a diverse community of benthic invertebrate and algal groups after 1 year. Other studies have investigated the sessile benthic community composition and condition of exposed reef surfaces across the Chagos Archipelago MPA (Head et al, 2019;Lange et al, 2021;Pilly et al, 2022), but none have previously investigated sessile cryptobenthic communities in this region. We find that sponges, fleshy and calcareous macroalgae, bryozoans, and annelid worms dominated recruitment surfaces, whilst colonial ascidians, molluscs, colonial ascidians, tunicates, and large foraminifera were less frequently observed.…”

“…Physical factors such as current velocity and wave forcing are also important for shaping the structure of coral reefs (Lange et al, 2021), but benthic reef communities have been shown to have non-linear threshold responses to changes in these parameters (Gove et al, 2015). Our understanding of the combined effect of physical and environmental factors on whole sessile communities within cryptic reef cavities is limited (but see Scheffers et al, 2010 and.…”

Decadal changes in parrotfish assemblages around reefs of Guam, Micronesia

“…While depth, fishing pressure, and adjacent habitat type were all important, wave exposure was the most important factor delineating populations at the within-island site scale. Wave exposure is an important environmental factor that not only physically structures coral reef environments, but also influences the behavioral aspects of herbivore communities (Bejarano et al 2017), the detrital and microbial composition of benthic substrates (Crossman et al 2001, Tribollet et al 2008, and their trajectories of recovery after disturbance (Lange et al 2021). High-wave exposure limits the diversity of corals and the structural complexity of shallow water environments (Dollar 1982), and this influences fish populations at the island scale.…”

Size and wave exposure shape parrotfish distributions and function in shallow coral reefs

“…For instance, turbulent conditions have been shown to in uence the growth and mortality rates of sessile intertidal organisms 20 . Many reefs show very different benthic compositions on windward and leeward aspects, and can have divergent post-disturbance recovery trajectories 21,22 . For mobile organisms, wave exposure presents a particular challenge; navigating through the turbulent waters of highly exposed environments can represent signi cant energetic costs 23,24 .…”