Home >Documents >Submarine Caves and Cave Biology of Bermuda

Submarine Caves and Cave Biology of Bermuda

Date post:23-Jan-2017
Category:
View:215 times
Download:0 times
Share this document with a friend
Transcript:
  • (

    'rI U"_I

    ~_..-rl

    CastleHarbour

    copepods, ostracods, isopods, amphipods,mysids, and cumaceans, as well astanaidaceans, echinoderms, tunicates andfish. While many of these animals, such assponges, echinoderms, tunicates and fish, arenormal inhabitants of the near-shore zane,others are known only from the caves andare restricted to this habitat. It was soonapparent that most cave-adapted (i.e., eye-and pigment-reduced) animals inhabited thedeep, marine waters in the interior of cavesand were thus only accessible by cave divers.

    Therefore, specialized training in cavediving would be required to investigate andcatalog the fauna of Bermuda's caves. InSeptember 1979, I invited Paul Meng, aFlorida-based cave diving instructor with theNSS Cave Diving Section, and his friendBarry Warner to come to Bermuda and teacha course for several friends, Paul Hobbs, RobPower, and me. Once properly trained andequipped, we organized ourselves as theBermuda Cave Diving Association andbegan to systematically explore, map, andscientifically document the island's anchialinecaves.

    Bermuda is a volcanic seamount located1000 km off the east coast of the UnitedStates in that part of the Western Atlanticknown as the Sargasso Sea. The island wascreated by a series of mid-ocean volcaniceruptions that began about 60 million yearsago, at a time when the Atlantic Ocean wasmuch narrower. Subsequent plate tectonics

    HarringtonSound

    N

    A

    Green BayCave

    prior, a biologist had attempted to collectfrom several cave pools but found nothingof interest and concluded that the caves werelifeless habitats. Indeed at that time, Virtuallyall investigations of aquatic cave life wereconducted on freshwater habitats and someof the world's most prominent cave biologistshad stated that, with few exceptions, marinecaves were not important to the field ofbiospeleology.

    Not to be deterred, I contacted Dr. BorisSket of the University of Ljubljana inSlovenia, who was studying caves along the

    Adriatic coast, andinvited him to come toBermuda. Dr. Sketaccepted and spent twoweeks working with mein September 1978. Wemade dives in several ofthe more open cavepools and found thatsalinity increased withdepth so that at 10 to 20foot depths, the salinityapproached that of theopen sea. It was in thesedeeper, fully marinewaters that wediscovered a wealth ofmarine life includingsponges, gastropods,various worms, andcrustaceans, including

    Fig. 1: Map ofBermuda showing the location ofprinciple caves: Green Bay, Admiral's,Walsingham, Palm, Government Quarry, and Tucker's Town Caves.

    Submarine Caves and Cave Biology of Bermudatext and ph(Jt(J9raphs blJ Th(Jmas M. Ifi((e

    Department of Marine Biology, Texas A & M University at Galveston, Galveston, TX 77553-1675My scientific interest in caves began quite

    by accident. After finishing my PhD in June1977, my first job was as a research scientistat the Bermuda Biological Station, where Iconducted studies of tar being washed upon the island's beaches. Up to that time, myinterest in caves was from a purelyrecreational viewpoint. I had done some cavediving while obtaining my Masters degree inOceanography at Florida State and laterwhen teaching at the Florida Institute ofTechnology, When I moved to Texas to workon my PhD, I found diving opportunities tobe quite limited and so I contacted Houstoncavers who invited me along on their trips tocaves in central Texas and northern Mexico.

    Upon arriving in Bermuda to start my newjob, I was excited to learn about thenumerous and well-decorated limestonecaves the island had to offer. Since Bermudais a rather small, narrow island, most of itsapproximately 150 known caves are withina few hundred meters of the coast (Figure 1)and many contain sea level pools in theirinterior. Such pools. both at entrances andin the interior of otherwise dry caves, lackabove ground connection with the sea andcontain tidally fluctuating brackish or saltwater. They are properly referred to as"anchialine." Bermuda's anchialine cavepools contain exceptionally clear, dark blue,and often quite deep waters. Although largesubmerged stalactites and stalagmites can beseen from the surface, there are typically noobvious signs of cave-adapted aquaticanimals from above.

    Curious about the apparent lack of life inthe cave pools, I asked Dr. Wolfgang Sterrer,the director of the Biological Station, ifanyone had ever looked for animals in thesaltwater caves. He told me that several years

    NSS NEWS, August 2003 217

  • -

    Passage opens into a large silt flooredchamber-the Desert-before passing undera low arch into the Trunk Passage. This isthe largest passage in the cave, averaging 15m wide and 10 m high. The far end of theTrunk passage terminates in a breakdownstope to the surface and a murky inlandsinkhole-Cliff Pool -the second and onlyother entrance to the Green Bay System. Onthe east side of the Trunk Passage, theHarrington Sound Passages comprise twointerconnecting passageways with muchclearer water than adjacent sections of thecave. The Bath Tub Ring Room at 15 mdepth near the end of the Harrington SoundPassage contains a horizontal bleached bandof bedrock about 50 em in width cuttingacross the rock strata (Figure 2). In the samearea are bones of a sea turtle that apparentlybecame lost and died in the cave at sometime in the distant past.

    The North Shore Passage is the longestsingle passage in the cave. It begins on thewest side of the Trunk Passage and extendsfor nearly 500 m to a point past the northernshoreline of the island, where the passagebecomes too low for divers to follow. Severalextended interconnecting loops characterizethis part of the cave. Undercut walls and thelevel nature of this part of the cave at 18 mdepth indicate an underground stream musthave flowed through these tunnels duringglacial low stands of sea level. Massivestalactites and stalagmites, present in virtuallyall parts of the underwater cave, are anotherindication of the cave's long history as a drycave.

    Biological zonation is evident as the diverprogresses farther into the cave from theGreen Bay entrance. Brightly coloredsponges, hydroids, tunicates, and otherencrusting organisms literally cover the wallsand ceiling in areas close to the entrance. Asa consequence of decreasing tidal currentsand particulate matter suspended in thewater, the density of these organisms declineswith distance into the cave. In the muchclearer waters of the deep cave interior,troglobitic species predominate.

    Other larg~ anchialine caves are locatedon the opposite side of Harrington Soundfrom Green Bay Cave. Two major cavesystems in this area, called the WalsinghamTract, are the Walsingham and Palm CaveSystems. The Walsingham System is about1.3 km long and comprises seven separateentrances: Deep Blue, Vine, Old Horse,Walsingham, Fern Sink, Crystal andWonderland Caves. Of these, Walsinghamwas a former commercial cave, whileadjacent Crystal and Wonderland (nowrenamed Fantasy) are major touristattractions in Bermuda.

    Deep Blue Cave consists of an isolatedpool in the Bermuda jungle at the base of a10 m high limestone cliff face. The pool isparticularly impressive due to its crystal clear

    stalactites and stalagmites, confirming thatthe caves must have been dry and air-filledfor much of their history.

    Green Bay Cave is presently the longestcave in Bermuda, with more than 2 km ofsurveyed passage. The main entrance is awide, submerged passageway extendinginland from the end of Green Bay onHarrington Sound. From shallow depths atthe Green Bay entrance, the cave slopesprogressively deeper to the Rat Trap, a lowbut wide section at 17 m depth. At this point,a low side passage turns south to theConnection Passage and the major part ofthe cave, while the Rat Trap continues andopens out into the Green Bay Passage. Thispassage climbs over breakdown at theLetterbox and passes through a tightrestriction between collapse blocks only toenlarge again. This further extension of theGreen Bay Passage bends back towardHarrington Sound and extends to a small airbell in ceiling breakdown. An underwaterbreakdown slope on the left side of the airbell descends to a spacious, deeper chamberwith a massive boulder choke at one end anda room with distinctly lower visibility-theFog Room at the other.

    From the Rat Trap, the Connection

    'J,!-., "

    ~ -L~,~' ','1\ ~ ,Marine biologist Tom IIiffe with plankton net bY'S!alagmltl!

    Blue Cave. Submerged speleotlilflns originally 7/ftmedng tbe Ice Ages when sea level wastpwer and the c,vese dry. . I J!

    -----~and sea floor spreading have maintainedBermuda's location relative to NorthAmerica, while ever increasing its distancefrom Europe and Africa as the Atlantic Oceanenlarged. Thus, Bermuda has never beenpart of, or closer to, a continental landmass.

    As the top of the volcanic seamount waseroded down below sea level, corals beganto grow around the margins, thus producingthe only atoll in the North Atlantic. Coral reef-derived limestone, first deposited as coastalsand dunes, caps most of present-dayBermuda. Approximately one million yearsago, limestone caves began forming duringglacial periods, when sea level was 100 rn ormore tower. Later, as glaciers on thecontinents melted and post-glacial sea levelsrose, encroaching seawater drowned largeportions of the caves. Continuing collapse ofoverlying rock into the large, solutionally-formed voids created the irregular chambersand fissure entrances that are commonly seenin Bermuda's caves. Extensive networks ofsubmerged passageways, developedprimarily at depths between 17 and 20 mbelow present sea level, interconnectotherwise isolated cave pools. Thesepassages, only accessible to divers, are welldecorated at all depths with impressive

    218 NSS NEWS, August 2003

  • Hamilton Parish, Bermuda~RMUOA

    *.

Click here to load reader

Reader Image
Embed Size (px)
Recommended