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Seafloor Characterization of the U.S. Caribbean |
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Day 7: 2.7.2005
Nancy Foster Mission Cruise Update #8
The Nancy Foster and her crew continued multibeam acquisition of the area labeled NPS Monument inshore (see cruise plan). This work is being conducted in conjunction with NOAA's Office Coastal Survey (OCS). Although the multibeam data is primarily being collected for seafloor characterization efforts, NOS is collecting the NPS Monument Inshore data at a higher level of confidence to meet acquisition requirements for OCS. This area was identified by OCS Charting Priority Level 2. This exercise will attempt to demonstrate the utility of NOAA scientific oceanographic vessels to conduct Integrated Ocean Mapping that is collecting oceanographic data to simultaneously satisfy multiple purposes (i.e. charting and seafloor characterization).
Although collection has not been 100% completed due to avoidance of moorings, Figure 1 depicts an area of interesting seafloor composition within a shoal just northwest of Ram's Head, St. John . The features captured appear to be Spur and Groove, hard bottom. Spur and group is defined as habitat having alternating sand and coral formations that are oriented perpendicular to the shore or bank/shelf escarpment. The coral formations (spurs) of this feature typically have a high vertical relief compared to pavement with sand channels and are separated from each other by 1-5 meters of sand or bare hardbottom (grooves), although the height and width of these elements may vary considerably. This habitat type typically occurs in the forereef or bank/shelf escarpment zone.
The second day of ROV work south of St. John produced exceptional video and photographs for two of the longest transects on our 12 day expedition. One transect crossed the mid shelf ridge inside the US Virgin Island National Coral Reef Monument, the other spanned two areas of high relief outside the Monument between the mid shelf reef and the shelf edge.
Diverse assemblages of fish species abounded. In one snapshot encompassing a square meter of benthic habitat six fish species were counted. This is extraordinary considering that there are likely more species hiding among the complex assortment of coral colonies observed in the shot. We have seen that many distant fish swim away or hide as the ROV approaches.
The first impression of the ROV team was that the reefs outside the Monument had much more coral than reefs inside. This perception was short lived. As the ROV progressed and the series of still photos flashed across the monitors in the lab aboard the Foster, the science crew noticed a disturbing trend. What was once a reef composed of 80% live coral had changed to a reef with less than 20% live coral. More alarming was the apparently recent and simultaneous loss of so much coral. Careful observation of all the photos along this transect revealed that the massive decline in live coral had occurred only within the last couple months or so. A sickly green hue covered many large colonies of agaricia, montastrea, and other plate forming species. The dead coral all had a consistently sparse growth of a thin turf algae rather than the more fleshy algal growth indicative of long dead coral. It was puzzling to see the decline of such a large amount of live coral on a reef in such deep water and so far offshore. The depth and distance from shore should have insulated this reef from most sources of stress that have resulted in the demise of so many nearshore/shallow reefs. What disease, pollution, or natural disturbance could have resulted in the loss of so much live coral at this once magnificent reef? By the end of the dive, the science crew anxiously eyed a transect scheduled for tomorrow which passes through the same area, hopeful that damage is not widespread.
Figure 2 demonstrates numerous colonies of coral competing for space. A small gap, a few centimeters wide, separates each colony and delimits the battle zone.
What was once a flourishing coral reef has now succumbed to some unknown ailment, as shown in Figure 3. Algal turf has grown over the dead coral colonies. The extent and type of algae indicate a sudden massive loss of coral in the last few months.