Most people envision coral reefs as colorful underwater ecosystems teeming with fish in shallow crystal blue water. There is another, lesser know type of coral reef – in deeper and darker waters – that is of great interest to scientists called mesophotic reefs.
Continual global degradation of shallow water reefs and improvements in underwater exploration have renewed a scientific interest in deep reef communities (30-150 m) called mesophotic coral ecosystems (MCEs). These reefs are sometimes said to exist in “the Twilight zone” due the limited amount of light. Recent technological advances in SCUBA have facilitated studies of mesophotic biology and ecology. However, there is little known about basic MCE sedimentology and how the variability of sedimentary processes affects MCE architectural integrity and structural sustainability. A better scientific understanding of the basic physical structure of these deep reefs is necessary for governments to develop environmental management practices, especially now as shallow water reefs continue to degrade worldwide.
Mesophotic reef in the U.S. Virgin Islands 130 ft, with bioerosion experiment blending into the reef in the lower right part of the picture.
The deeper depths in which mesophotic reefs are located require scientists either use technical diving or submersibles to directly sample the system. My particular research requires diving beyond the depth and bottom time limits set for the recreational community. To prepare for the research, Rick Gomez, the UM Diving Safety officer, spent over a year training the team technical decompression and tri-mix diving techniques.
Bioerosion, the removal or coral skeletal material by the actions of other organisms, is a key sedimentalogical process impacting reef development, accretion, destruction, and preservation. To study how bioerosion rates change over time, one major component of my research involves analyzing 216 pre-weighed and imaged pristine coral substrate disks (made from the coring of recently dead coral). These coral disks were attached to the bottom of the sea at six reef locations in waters just south of St. Thomas in the U.S. Virgin Islands in Aug. 2010. The research plan entails collecting 54 coral disks a year to quantify how much material has been removed from these disks through the bioerosion of various reef organisms such as grazing fish and boring sponges.
The first set of substrate disks were collected in Aug. 2011. They are currently being analyzed in our lab on main campus. By weighing, photographing, cut each disk into slices like a pizza for image analysis, we can figure out how much material has been removed from these disks and what types of organisms are removing the reef material. The analysis offers a quantification of the overall bioerosion process in these different reef habitats, and may offer new ideas on the heterogeneous construction of these reefs. Our next collection trip to the USVI is schedule for the summer of 2012.
In addition to this ongoing research, my team is preparing a manuscript to publish results of a taphonomic analysis of coral rubble from the USVI mesophotic reefs. The paper will identify distinct preservation zones (taphofacies) in different deep reef habitats. These taphofacies may eventually be utilized to better understand the degree of structural and habitat complexity and diversity of ancient reefs preserved as geological limestone deposits. Hopefully the work will allow us to start understanding what mesophotic reefs were like in the past, how diverse they were, and how resilient they might have been.
More information on my research can be found at my website. On March 1, 2012, I will be giving a presentation in the Ungar building, suite 230D, beginning at 5pm. Everyone is welcome to attend. For more information on the event click here.
-David Weinstein
David is a 3rd year Ph.D. student in the Marine Geology and Geophysics division, and one of the first geologists to study the sedimentology of near-horizontal mesophotic reefs.