Deep Reefs

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Mesophotic Reefs

The term "mesophotic" has been used to describe a pelagic life zone and has been applied benthically to British subtidal seaweed assemblages. Recently, "mesophotic" has been used to describe a class of coral reefs living at the deeper extents of the photic zone.

Mesophotic reefs are relatively unexplored due to the time limitations inherent to SCUBA surveys. They are, however, of increasing scientific interest because they have the potential to cover large areas, harbor ecologically important species, and provide a potential refuge from warm surface waters.

Here we provide two examples of mesophotic reefs surveyed with landscape mosaics. Because mosaic surveys of up to 400 square meters can be accomplished in under an hour, mosaics are one of the easiest ways to survey hard to reach areas where survey time is limited. The images here were acquired by divers, but we have also made mosaics using video acquired by a remotely operated vehicle (ROV). The abitility to explore and monitor mesophotic reefs either remotely, or with minimum dive time, greatly increases the ability to study and detect future changes in this relatively unexplored reef habitat.

The first area where mosaics were used to map a portion of a mesophotic reef was in the "Sherwood Forest" area of Dry Tortugas National Park, Florida (Fig. 1). These data were acquired to assess the practicality of assembling mosaics from naturally illuminated imagery at the deeper limits of most scientific diving expeditions.

Figure 1: Mosaic of a deep reef community (depth = 30 m) dominated by stony corals (primarily Montastrea spp.) in the Dry Tortugas, Florida. White squares are PVC quadrats 0.5 m on a side.

Based on the success of the Sherwood Forest mosaic, we began to support Tyler Smith and colleagues at the University of the Virgin Islands who are monitoring an extensive area of mesophotic reefs along the southern Puerto Rican shelf. Four permanent monitoring stations have been established between 25 to 50 m depth within the Marine Conservation District, St. Thomas, US Virgin Islands (Fig. 2). A mosaic from one of these sites (at reduced resolution to accommodate downloading) is shown below (Fig. 3).


Figure 2: Locations of monitoring sites. Top: Map of the Caribbean. Center: Map of the Puero Rican shelf and surrounding bathymetry. Yellow box indicates area cut-out for the bottom panel. Bottom: Permanent mosaic image sites were established at MCD S166 (2 sites), College Shoal E, and MCD Mosaic 3. North is up

Figure 3: A deep coral hillock (42 m) in the Marine Conservation District, St. Thomas, US Virgin Islands. This image is from the site labeled "Mcd Mosaic" in Fig. 2. The red-form Montastraea cavernosa in the upper center is at the top of the hillock, about 2 m above the corals on the side of the hillock, at the bottom of the picture. Mosaic images were used in this no-take marine protected area to establish long-term monitoring plots in an effort to understand a wide-spread coral mortality event and other recurring processes that may affect reef development in these poorly studied mesophotic reefs. PVC quadrat in the center of the mosaic is 0.5 x 0.5 m.

References:

Nemeth RS, Smith TB, Blondeau J, Kadison E, Calnan JM, Gass J (2008) Characterization of deep water reef communities within the marine conservation district, St. Thomas, U.S. Virgin Islands. Submitted to the Caribbean Fisheries Management Council. University of the Virgin Islands, St. Thomas 87 + appendices

Smith TB, Blondeau J, Nemeth RS, Calnan J, Kadison E (in prep.) Benthic structure and cryptic mortality in a Caribbean mesophotic coral reef system, the Marine Conservation District, U.S. Virgin Islands