Feature Paper: Locker SD, Armstrong RA, Battista TA, Rooney JJ, Sherman C, Zawada DG (2010). Geomorphology of mesophotic coral ecosystems: current perspectives on morphology, distribution, and mapping strategies. Coral Reefs, 29:329-345.
Author Abstract: This paper presents a general review of the distribution of mesophotic coral ecosystems (MCEs) in relationship to geomorphology in US waters. It was specifically concerned with the depth range of 30–100 m, where more than 186,000 km2 of potential seafloor area was identified within the US Gulf of Mexico/Florida, Caribbean, and main Hawaiian Islands. The geomorphology of MCEs was largely inherited from a variety of pre-existing structures of highly diverse origins, which, in combination with environmental stress and physical controls, restrict the distribution of MCEs. Sea-level history, along with depositional and erosional processes, played an integral role in formation of MCE settings. However, mapping the distribution of both potential MCE topography/substrate and existing MCE habitat is only beginning. Mapping techniques pertinent to understanding morphology and MCE distributions are discussed throughout this paper. Future investigations need to consider more cost-effective and remote methods (such as autonomous underwater vehicles (AUVs) and acoustics) in order to assess the distribution and extent of MCE habitat. Some understanding of the history of known MCEs through coring studies would help understand their initiation and response to environmental change over time, essential for assessing how they may be impacted by future environmental change.
Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.
Review: This is a review paper, meaning that it collates current knowledge about a certain topic. These kinds of papers are very useful to scientists and students wishing to know a base of information on a given topic that they are unfamiliar with, as well as providing a good article on a discipline of interest that can be "bookmarked" for following up on specific theories posed.
This paper is important because it deals with a kind of coral reef environment that has generally gone unexplored: the mesophotic coral reef. Colloquially this region is known as the "twilight zone" (a term coined by Dr. Richard Pyle, one of a small group of regular explorers of this zone) because it lies below the normal photic zone (region of highest photosynthetic activity, generally between 0 - 30 meters depth) of reefs, but still has enough light (generally defined as at least 1% of equivalent solar radiation at the surface) to allow some photosynthesis. But because the amount of light is so small, there are only a few zooxanthellate corals (those containing symbiotic photosynthetic algae within their tissues) and algae that can survive. Most of the reef looks quite different, dominated by whip corals and sponges. Fish life, however, is abundant.
As a side note, there are other deep reef communities that survive in cold waters absent from light (generally below 300 meters depth) but we won't be reviewing those communities this week. However, it should be noted that deep water coral communities likely have as much diversity as shallow water coral reefs (with over 2000 species of "hard" corals, only about 850 species are known from tropical coral reefs) and are extremely important for fish communities (and subsequently suffer damage from trawling by deepwater fishermen). For scientists working in the mesophotic deep reef realm, there is now a Mesophotic Research Organization that collects and posts information about that research community.
Okay, so now that the background is out of the way, on to the key points of the paper:
1. Mesophotic reefs are poorly studied. This is because they are too deep for conventional scuba diving (the main research tool of coral reef scientists) but too shallow to be cost-effective for research submersibles. A good overview of this point was made by Dr. Richard Pyle at the TED 2004 conference:
2. The paper gives an overview of the current geographical distribution of known mesophotic reefs, concentrating heavily on the United States (northern Gulf of Mexico, Florida, US Caribbean, the main Hawaiian Islands, American Samoa, and the Mariana Islands). It is clear from the paper that the potential habitat for mesophotic reefs is large, but known mesophotic reefs are few and far between (because of point #1 above).
3. The paper discusses the various habitats where mesophotic reefs can be found, including: continental shelves, isolated banks, terraces along sloping drop offs, deep buttresses and continental shelf-edge spurs and grooves, and relic reefs (often from old sea stands during glaciation periods).
4. "Until recently, little effort has been made to systematically identify, map, and study scleractinian [reef-building] coral reefs found at depths below ~ 30 m."
5. The paper overviews various methodologies used for sampling and mapping mesophotic reefs, noting that they are below the limit of conventional scuba diving and "beyond the scope of airborne and satellite remote-sensing instrumentation." Successful research methods used are acoustic technologies (to map reefs based on distance traveled from surface ship based sounds, such as with swath-mapping systems, multibeam and side-scan sonar, and electromechanical "boomers" such as seismic-reflection profiles), underwater still and video cameras (either with technical scuba divers or remotely operated vehicles, or ROVs), pulsed-laser line imaging to construct a "3D digital representation of the seafloor," other submerged vehicles (ROVs as noted as well as autonomous underwater vehicles, or AUVs, and towed vehicles or sleds with attached instrumentation), and technical scuba diving techniques (discussed by Dr. Richard Pyle in the video above).
6. Geomorphology can limit the "occurrence and distribution of [mesophotic reefs] by providing favorable hard substrates for colonization and by directing the down-slope transport of sediment" that would smother reef corals. Corals and many benthic organisms need hard substrates to survive and grow.
7. And in a sobering note, the authors state that "the single most important issue from a mapping perspective is the lack of knowledge on the potential extent" of mesophotic reefs. Dr. Pyle has also noted that more is known about the surface of the moon than mesophotic reefs and more scientists have surveyed the abyssal deep ocean bottom than "deep reefs."
8. Continuing with mapping and surveying mesophotic reefs will require the use of emerging technologies. Mesophotic reef scientists who use scuba-based technologies either require multiple staged bottles with different gas mixtures (to lessen the effects of nitrogen narcosis or "getting the bends") and technical diving skills or they have switched over to closed-circuit rebreather technology adapted for deep diving.
9. The thickness of mesophotic reef benthic communities is unknown, meaning that the rate of calcification and accretion of corals and other calcifying organisms is also unknown at depths below 30m. This is important because as ocean acidification due to increasing carbon dioxide concentration continues, the effect on mesophotic reefs may be more acute than on shallow-water reefs since calcification is often tied (in corals) to photosynthesis and metabolic rates. In layman terms, too much carbon dioxide will dissolve coral skeletons faster than they can be laid down, destroying reefs even without conventional pollutants like increased nitrogen or phosphorous.
And on that note, next week we'll look at labratory experiments meant to determine the harm that decalcification of living coral skeletons has on coral organisms.
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