Depth-Dependent Structuring of Reef Fish Assemblages From the Shallows to the Rariphotic Zone

Abstract

Shallow coral reef ecosystems worldwide are affected by local and global anthropogenic stressors. Exploring fish assemblages on deeper reefs is therefore important to examine their connectivity, and to help understand the biodiversity, ecology, distinctiveness, evolutionary history and threats in this sparsely studied environment. Conducting visual surveys on the Bermuda slope and a nearby seamount at depths from 15 to 300 m, we document decreasing fish biomass and diversity with increasing depth. Fish assemblages were primarily depth-stratified, with distinct suites of species inhabiting shallow (<30 m depth) and upper (60 m) and lower (90 m) mesophotic coral ecosystems, and confirming the presence of a distinct rariphotic (∼150-300 m) assemblage. We also report evidence of anthropogenic pressures throughout our surveyed depths. Our results highlight the novelty of deeper reef fish faunas, therefore suggesting limited applicability of the deep reef refuge hypothesis, and showcase the vulnerability of deep reefs to targeted fishing pressure and invasive species.

Low connectivity between shallow, mesophotic and rariphotic zone benthos

“…A dramatic decline of herbivorous fish abundance and biomass below shallow depths [33] is probably associated with the reduced hard coral cover mentioned earlier. The widespread occurrence of invasive lionfish in Bermuda's MCEs [33,36] may affect juvenile herbivorous fishes at depth [37] and probably reduces grazing pressure on macroalgae allowing them to find refuge in MCEs.…”

“…It is also worth noting the steep decline in benthic organism abundance and morphotype richness with depth (figure 3). The drop in abundance is most likely explained by the concordant decrease in primary production that is reflected in the disappearance of zooxanthellate scleractinian corals and decline of macroalgae cover (figures 2 and 3 a ; also figure 6 in [33]). The decline in morphotype richness with depth is probably linked to temperature, which was also negatively correlated with depth, as has been previously suggested for tropical ophiuroid diversity in continental shelf (20–200 m) and upper-slope waters (200–2000 m) [39].…”

Ngaraard Pinnacle, Palau: An Undersea “Island” in the Flow

“…Investigations of lower mesophotic and rariphotic zones in the tropics have been limited due to their steep profiles and difficult working environments. Submersibles (Baldwin et al, 2018;Stefanoudis et al, 2019), remotely operated vehicles, multibeam sonar, and autonomous underwater vehicles (Armstrong et al, 2019) have substantially increased knowledge of the structure and communities of many deep habitats. However, scant attention has been directed at deep photic environments, such as our study site at NP, where connected shallow environments (altiphotic and upper mesophotic reefs) do not also occur.…”

“…Baldwin et al (2018) describe three reef faunal zones based loosely on depth and light penetration: altiphotic (high light, surface to 30-40 m depth), mesophotic (intermediate light, 30-150 m depth), and, coining a new term, "rariphotic" (low light, 150-300 m depth in clear tropical waters), and the aphotic zone, with no surface light, lies beneath 300 m. The mesophotic has also been divided into poorly defined upper (30-40 to 70 m depth) and lower (70-150 m) zones whose depth limits differ by location (Baker et al, 2016). In addition to light, the zones may be distinguished based on temperature (Colin, 2018) and faunal composition (Baldwin et al, 2018;Stefanoudis et al, 2019).…”

Faculty Opinions recommendation of Low connectivity between shallow, mesophotic and rariphotic zone benthos.

“…It is also worth noting the steep decline in benthic organism abundance and morphotype richness with depth (figure 3). The drop in abundance is most likely explained by the concordant decrease in primary production that is reflected in the disappearance of zooxanthellate scleractinian corals and decline of macroalgae cover (figures 2 and 3a; also figure 6 in [33]). The decline in morphotype richness with depth is probably linked to temperature, which was also negatively correlated with depth, as has been previously suggested for tropical ophiuroid diversity in continental shelf (20-200 m) and upper-slope waters (200-2000 m) [39].…”