Diving deep: benefits of deep sea coral refuges in the Atlantic

Reference: Pereira PHC, Macedo CH, Nunes JdACC, Marangoni LFdB, Bianchini A (2018) Effects of depth on reef fish communities: Insights of a deep refuge hypothesis from Southwestern Atlantic reefs. PLoS ONE 13(9): e0203072. https://doi.org/10.1371/journal.pone.0203072


There are coral reefs in the Atlantic?

Yes there are!

Coral reefs are recognized as some of the most diverse ecosystems on the planet. When you think coral reefs, what pops into mind is most likely reefs in the Pacific Ocean (i.e. Hawaii!) or the Caribbean Sea. However, coral reefs are found in the South Atlantic Ocean as well (Fig 1). Somewhat understudied compared to their well-known counterparts, coral reefs off the Brazilian coast are ideal systems for understanding the differences between shallow and deep-water reef types and fish populations in the South Atlantic.

Figure 1. Corals in the Atlantic Ocean. Photo credit: South Atlantic Fishery Management Council 

Pereira and colleagues examined the importance of deep water coral reefs by comparing shallow reefs (average depth 4m) and deep reefs (depth ranging from 25-30m) along the northeastern Brazil (Fig 2). They studied differences in the coral reef community, bottom-dwellers (benthic community), and reef fishes. For the reef fishes they identified the various species, recorded their sizes, and classified them according to their feeding habits (e.g., omnivore).

Figure 2. Shallow (upper left) and deep reefs (upper right) included in this study, and the coral species studied (bottom panels) by Pereira et al. (2018). Photo credits: PlosOne, 2018 Pereira et al
Are deep refuges important?

Pereira and colleagues found no significant differences in the coral and attached-algae communities among the different reef-types. But the reef fish communities were a different story.

The scientists found a higher abundance of fish in the deeper sites, and also noted higher species richness (the number of species found in the community). Additionally, they found significant differences when comparing the various groups of interest. Abundance of ornamental, great herbivores, and groupers were all significantly higher in the deep reefs. Larger fishes (>15-30 cm) of great herbivores, groupers, and snappers were only observed in the deep sites. These results are especially interesting given that there were no significant differences habitat quality or features.

So what? Should we care about deep reefs?

Overall, this study supports the “deep refuge hypothesis” for South Atlantic coral reef communities. This hypothesis suggests that deeper reefs are generally less impacted by human-induced stressors like overfishing, pollution, and climate change due to their location. The differences in abundance, species richness, and size classes for the groups of interest are most likely strongly tied to impacts of the above stressors. For example, easier access to shallow reefs compared to deep reefs means large individuals are disproportionately removed from the shallow sites, while deep reefs can act as refuges that are safe from fishing pressure.

In addition to the ecosystem data collection carried out by the authors, they complimented their results with local knowledge. Interviews with local fisherfolk confirmed the observations that certain species of reef fishes not found in shallow reefs are abundant in deep waters. Given that deep reefs appear to be acting as refuges, it is important to consider this specific role when designing Marine Protected Areas (MPAs). Generally, protections are predominantly extended to shallow reefs. Strict protections and ‘no take zones’ explicitly designated for deep reefs may need to be implemented, especially since their importance will undoubtedly increase as climate change progresses and oceans continue to warm.

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Lushani Nanayakkara

Lushani Nanayakkara

I completed my PhD at the University of Regina in Saskatchewan, Canada. I study both the human dimensions (via stakeholder surveys) and ecological dynamics (via ecosystem surveys and stable isotopes) of aquatic ecosystems. Prior to this I completed my MSc in Environmental Sciences and Policy at Johns Hopkins University. I currently live in Ottawa, and in my spare time I love hanging out with my dog Piper, travelling, cooking and listening to podcasts. Find me on Twitter @SciPoliBoundary

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