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Submarine Caves and Cave Biology of Bermuda text and ph(Jt(J9raphs blJ Th(Jmas M. Ifi((e Department of Marine Biology, Texas A & M University at Galveston, Galveston, TX 77553-1675
My scientific interest in caves began quite by accident. After finishing my PhD in June 1977, my first job was as a research scientist at the Bermuda Biological Station, where I conducted studies of tar being washed up on the island's beaches. Up to that time, my interest in caves was from a purely recreational viewpoint. I had done some cave diving while obtaining my Masters degree in Oceanography at Florida State and later when teaching at the Florida Institute of Technology, When I moved to Texas to work on my PhD, I found diving opportunities to be quite limited and so I contacted Houston cavers who invited me along on their trips to caves in central Texas and northern Mexico. Upon arriving in Bermuda to start my new job, I was excited to learn about the numerous and well-decorated limestone caves the island had to offer. Since Bermuda is a rather small, narrow island, most of its approximately 150 known caves are within a few hundred meters of the coast (Figure 1) and many contain sea level pools in their interior. Such pools. both at entrances and in the interior of otherwise dry caves, lack above ground connection with the sea and contain tidally fluctuating brackish or salt water. They are properly referred to as "anchialine." Bermuda's anchialine cave pools contain exceptionally clear, dark blue, and often quite deep waters. Although large submerged stalactites and stalagmites can be seen from the surface, there are typically no obvious signs of cave-adapted aquatic animals from above. Curious about the apparent lack of life in the cave pools, I asked Dr. Wolfgang Sterrer, the director of the Biological Station, if anyone had ever looked for animals in the saltwater caves. He told me that several years
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Fig. 1: Map of Bermuda showing the location ofprinciple caves: Green Bay, Admiral's, Walsingham, Palm, Government Quarry, and Tucker's Town Caves.
prior, a biologist had attempted to collect from several cave pools but found nothing of interest and concluded that the caves were lifeless habitats. Indeed at that time, Virtually all investigations of aquatic cave life were conducted on freshwater habitats and some of the world's most prominent cave biologists had stated that, with few exceptions, marine caves were not important to the field of biospeleology. Not to be deterred, I contacted Dr. Boris Sket of the University of Ljubljana in Slovenia, who was studying caves along the Adriatic coast, and invited him to come to Bermuda. Dr. Sket accepted and spent two weeks working with me in September 1978. We made dives in several of the more open cave pools and found that salinity increased with depth so that at 10 to 20 foot depths, the salinity approached that of the open sea. It was in these deeper, fully marine waters that we discovered a wealth of marine life including sponges, gastropods, various worms, and crustaceans, including
copepods, ostracods, isopods, amphipods, mysids, and cumaceans, as well as tanaidaceans, echinoderms, tunicates and fish. While many of these animals, such as sponges, echinoderms, tunicates and fish, are normal inhabitants of the near-shore zane, others are known only from the caves and are restricted to this habitat. It was soon apparent that most cave-adapted (i.e., eyeand pigment-reduced) animals inhabited the deep, marine waters in the interior of caves and were thus only accessible by cave divers. Therefore, specialized training in cave diving would be required to investigate and catalog the fauna of Bermuda's caves. In September 1979, I invited Paul Meng, a Florida-based cave diving instructor with the NSS Cave Diving Section, and his friend Barry Warner to come to Bermuda and teach a course for several friends, Paul Hobbs, Rob Power, and me. Once properly trained and equipped, we organized ourselves as the Bermuda Cave Diving Association and began to systematically explore, map, and scientifically document the island's anchialine caves. Bermuda is a volcanic seamount located 1000 km off the east coast of the United States in that part of the Western Atlantic known as the Sargasso Sea. The island was created by a series of mid-ocean volcanic eruptions that began about 60 million years ago, at a time when the Atlantic Ocean was much narrower. Subsequent plate tectonics NSS NEWS, August 2003
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Marine biologist Tom IIiffe with plankton net bY'S!alagmltl! Blue Cave. Submerged speleotlilflns originally 7/ftmed ng tbe Ice Ages when sea level wastpwer and the c,ves e dry. . I J!
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and sea floor spreading have maintained Bermuda's location relative to North America, while ever increasing its distance from Europe and Africa as the Atlantic Ocean
enlarged. Thus, Bermuda has never been part of, or closer to, a continental landmass. As the top of the volcanic seamount was eroded down below sea level, corals began to grow around the margins, thus producing the only atoll in the North Atlantic. Coral reefderived limestone, first deposited as coastal sand dunes, caps most of present-day Bermuda. Approximately one million years ago, limestone caves began forming during glacial periods, when sea level was 100 rn or more tower. Later, as glaciers on the continents melted and post-glacial sea levels rose, encroaching seawater drowned large portions of the caves. Continuing collapse of overlying rock into the large, solutionallyformed voids created the irregular chambers and fissure entrances that are commonly seen in Bermuda's caves. Extensive networks of submerged passageways, developed primarily at depths between 17 and 20 m below present sea level, interconnect otherwise isolated cave pools. These passages, only accessible to divers, are well decorated at all depths with impressive 218
NSS NEWS, August 2003
stalactites and stalagmites, confirming that the caves must have been dry and air-filled for much of their history. Green Bay Cave is presently the longest cave in Bermuda, with more than 2 km of surveyed passage. The main entrance is a wide, submerged passageway extending inland from the end of Green Bay on Harrington Sound. From shallow depths at the Green Bay entrance, the cave slopes progressively deeper to the Rat Trap, a low but wide section at 17 m depth. At this point, a low side passage turns south to the Connection Passage and the major part of the cave, while the Rat Trap continues and opens out into the Green Bay Passage. This passage climbs over breakdown at the Letterbox and passes through a tight restriction between collapse blocks only to enlarge again. This further extension of the Green Bay Passage bends back toward Harrington Sound and extends to a small air bell in ceiling breakdown. An underwater breakdown slope on the left side of the air bell descends to a spacious, deeper chamber with a massive boulder choke at one end and a room with distinctly lower visibility-the Fog Room at the other. From the Rat Trap, the Connection
Passage opens into a large silt floored chamber-the Desert-before passing under a low arch into the Trunk Passage. This is the largest passage in the cave, averaging 15 m wide and 10 m high. The far end of the Trunk passage terminates in a breakdown stope to the surface and a murky inland sinkhole-Cliff Pool -the second and only other entrance to the Green Bay System. On the east side of the Trunk Passage, the Harrington Sound Passages comprise two interconnecting passageways with much clearer water than adjacent sections of the cave. The Bath Tub Ring Room at 15 m depth near the end of the Harrington Sound Passage contains a horizontal bleached band of bedrock about 50 em in width cutting across the rock strata (Figure 2). In the same area are bones of a sea turtle that apparently became lost and died in the cave at some time in the distant past. The North Shore Passage is the longest single passage in the cave. It begins on the west side of the Trunk Passage and extends for nearly 500 m to a point past the northern shoreline of the island, where the passage becomes too low for divers to follow. Several extended interconnecting loops characterize this part of the cave. Undercut walls and the level nature of this part of the cave at 18 m depth indicate an underground stream must have flowed through these tunnels during glacial low stands of sea level. Massive stalactites and stalagmites, present in virtually all parts of the underwater cave, are another indication of the cave's long history as a dry cave. Biological zonation is evident as the diver progresses farther into the cave from the Green Bay entrance. Brightly colored sponges, hydroids, tunicates, and other encrusting organisms literally cover the walls and ceiling in areas close to the entrance. As a consequence of decreasing tidal currents and particulate matter suspended in the water, the density of these organisms declines with distance into the cave. In the much clearer waters of the deep cave interior, troglobitic species predominate. Other larg~ anchialine caves are located on the opposite side of Harrington Sound from Green Bay Cave. Two major cave systems in this area, called the Walsingham Tract, are the Walsingham and Palm Cave Systems. The Walsingham System is about 1.3 km long and comprises seven separate entrances: Deep Blue, Vine, Old Horse, Walsingham, Fern Sink, Crystal and Wonderland Caves. Of these, Walsingham was a former commercial cave, while adjacent Crystal and Wonderland (now renamed Fantasy) are major tourist attractions in Bermuda. Deep Blue Cave consists of an isolated pool in the Bermuda jungle at the base of a 10 m high limestone cliff face. The pool is particularly impressive due to its crystal clear
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