Low connectivity between shallow, mesophotic and rariphotic zone benthos

Abstract

Worldwide coral reefs face catastrophic damage due to a series of anthropogenic stressors. Investigating how coral reefs ecosystems are connected, in particular across depth, will help us understand if deeper reefs harbour distinct communities. Here, we explore changes in benthic community structure across 15–300 m depths using technical divers and submersibles around Bermuda. We report high levels of floral and faunal differentiation across depth, with distinct assemblages occupying each depth surveyed, except 200–300 m, corresponding to the lower rariphotic zone. Community turnover was highest at the boundary depths of mesophotic coral ecosystems (30–150 m) driven largely by taxonomic turnover and to a lesser degree by ordered species loss (nestedness). Our work highlights the biologically unique nature of benthic communities in the mesophotic and rariphotic zones, and their limited connectivity to shallow reefs, thus emphasizing the need to manage and protect deeper reefs as distinct entities.

Distribution of deep-water scleractinian and stylasterid corals across abiotic environmental gradients on three seamounts in the Anegada Passage

“…Several coral taxa encountered in the Anegada Passage are important species for understanding biogeographic patterns of the Caribbean bathyal zone. Madracis myriaster was observed both on Dog and Conrad Seamounts, but only between 253 and 311 m. This depth range is also similar to that observed in the western North Atlantic off Bermuda (90–300 m) ( Stefanoudis et al, 2019 ). In the Caribbean, M .…”

“…myriaster ( Fig. 8C ), which is more common at mesophotic and upper bathyal depths ( Reyes et al, 2005 ; Santodomingo et al, 2007 ; Hernández-Ávila, 2014 ; Stefanoudis et al, 2019 ). At lower bathyal depths, regional biogeographic differences have also been attributed to variation in the frequency of solitary scleractinian species (e.g., Stephanocyathus spp., Fungiacyathus sp.)…”

Isolated reefs support stable fish communities with high abundances of regionally fished species

“…For example, reef predators often show a preference for outer reef slopes (Dale et al, 2011;Friedlander et al, 2010), where planktivorous fish prey are abundant due to enhanced primary productivity driven by oceanic currents (Skinner et al, 2019). On deeper reefs (>30 m), fish assemblages are also often ecologically distinct, with low abundance of herbivores, a concentration of predatory fish biomass, and high abundance of planktivorous fishes (Rocha et al, 2018;Stefanoudis et al, 2019). Furthermore, although pressures such as pollution may extend throughout a coral reef (Rocha et al, 2018), the impact may vary among habitats.…”

Reef benthos of Seychelles - A field guide

“…Both shallow and deep water coral reefs are valued for their incredible diversity and species richness, yet little is known about the processes and functions of mesophotic coral ecosystems (MCEs; reefs ~ 30–150 m depth, as proposed by Rocha et al 2018 ) and rariphotic reefscapes (~ 150–300 m depth, as proposed by Baldwin et al 2018 ). While shallow and deeper coral reef ecosystems exist in close proximity to one another, the different conditions to which they are exposed have led to the formation of distinct forms of life and ecosystem functions within them ( Holstein et al 2019 , Stefanoudis et al 2019 ).…”