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Last Updated: November 19, 2003
DRY TORTUGAS
BENTHIC COMMUNITY CHARACTERIZATION
December 1998
Walter C. Jaap1, Jennifer L. Wheaton1, James Forqurean2
Pulaski Shoal, Dry Tortugas National Park: Turtle and benthic resources
1: Florida Marine Research Institute,100 8th Ave SE, St. Petersburg, FL 33701-5095
2: Florida International University, Dept. of Biology, Miami, FL 33199
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INTRODUCTION The Dry Tortugas is the western terminus of the Florida Reef Tract and lies 100 to 112 km west of Key West, Florida; the geographic coordinates are 24° 33' to 24° 44' N latitude, 82° 46' to 83° 15’ W longitude (Figure 1). At the convergence of the Gulf of Mexico, Caribbean Sea, and the Atlantic Ocean, the area is characterized as a mosaic of coral reefs, sedimentary shoals, seagrass meadows, and small islands. Within the Proposed Tortugas 2000 Ecological Preserve (T2EP) there are several prominent features. These are Dry Tortugas National Park, and deep-reef structures identified in the (T2EP) literature as: Tortugas Bank, Eight Fathom Bank, and Little Bank (Figure 1). Dry Tortugas was discovered by Ponce de Leon in 1513. The area was very much a graveyard of ships (Murphy 1993). The sailing instructions in the eighteenth century warned mariners to be cautious in traversing the area (Gauld 1796). Natural history expeditions to the area in the nineteenth century include Louis and Alexander Agassiz, and Louis Pourtales. The greatest contribution in documenting marine benthic resources during this era is a map of submerged habitats published by Alexander Agassiz (1882). In 1904, the Carnegie Institution established a marine laboratory on Loggerhead Key, Dry Tortugas (Mayer 1902). Under Alfred G. Mayer's direction, the Tortugas laboratory (Figures 3, 4) was a leading research facility studying the biology, geology, and the environmental conditions of the Dry Tortugas and adjacent area (Davenport 1926, Colin 1980). Papers Tortugas Laboratory published by the Carnegie Institution, Washington, D.C., contains a complete set of the publications resulting from this research. Seminal coral reef work includes: Vaughan 1911, 1914, 1915, 1916; Mayer 1914,1918; and Wells 1932. Subsequent publications on Tortugas coral reefs include Shinn et al. 1977, Thompson and Schmidt 1977, Davis 1979, 1982, Halley 1979, Dustan 1985, Jaap et al. 1989, Jaap and Sargent 1993. See Schmidt and Pikula (1997) for annotated bibliography of scientific studies within Dry Tortugas National Park. An excellent history of the Dry Tortugas island dynamics and status is found in Robertson (1964). As an example, Robertson reported that Bird Key was a major island with a large rookery of terns (documented by Audubon in1832). Severe hurricanes in 1910 and 1919 destroyed the vegetation (eight ft high bay cedar). This was followed by chronic erosion of the island; by 1929, the Audubon warden abandoned his house on Bird Key and moved to Garden Key. In 1908, by executive order, President Theodore Roosevelt designated the Dry Tortugas as a wildlife refuge, principally to protect bird rookeries. In 1935, the Dry Tortugas National Monument was established. At the time, bird rookeries and cultural resources (Fort Jefferson) were still the principal concerns. In 1980, the enacting language was amended to include coral reefs and resident marine life. The area was designated as Dry Tortugas National Park in 1992; however, boundaries were not changed. The park is administered by the director of Everglades National Park and the site manager and a small staff live on Garden and Loggerhead Keys. The deed transferring lands from the State of Florida to the Department of the Interior, National Park Service, stipulated that the State of Florida retained the rights of the sea bed and associated resources. Recently, visitor utilization has increased dramatically at Dry Tortugas as a result of scheduled tour boats from Key West, Ft. Myers, and Marco Island. The number of live-aboard sailboats and yachts visiting Dry Tortugas has also increased in the last decade. Several dive operators are offering multi-day tours to Dry Tortugas from the Key West, Naples, and Ft. Myers. In 1998, an estimated 72,000 people will have visited the park. This number is a four-fold increase since 1984 (NPS 1998). The resources and infra structure at DTNP are not able to sustain a growth rate of that magnitude and be expected to provide visitors with a meaningful wilderness experience. A conservative approach and monitoring programs with rapid feed-back to the management system are important to ensure we provide sufficient protection for the resources. The monitoring component provides information on status and trends of resources and cause and effect relationships relative to visitor use and natural phenomenon (Likens 1988, Rogers 1997). Research at Dry Tortugas benefits from the historical data base, relative isolation, and being a National Park with a history of protecting natural resources. Within DTNP, commercial fishing is prohibited and recreational fishing is limited to hook and line fishing for fin-fish (Florida Fishing Regulations apply). Lobster, conch, and other benthic resources are totally protected within the park boundaries. The physiography-bathymetry of the Dry Tortugas is complex and dynamic. The DTNP is an elliptical area with a northeast to southwest axis; the approximate dimensions are 11 nmi NE to SW and 5.5 to 6 nmi SE to NW (Figure 1). Depth outside the ellipse is 18 m (60 ft) or greater. The park boundaries are designated by buoys (listed on the charts as: A, C, E, H, I, J, K, L, N, O). The park includes approximately 1002 miles (25,900 hectares), less than one percent is terrestrial (Davis, 1982). This ellipsoid area has three major components: a crescent-shaped shoal on the east that includes East and Middle keys; a shoal that extends from Iowa Rock in a southwestern trend for approximately 4 nmi and includes Bush, Garden, and Long Keys; and a western shoal, including Loggerhead Key, extending northeast to southwest approximately 5.4 nmi. A relatively deep basin (12 to 20 m (40 to 67 ft) occupies the central portion of the ellipse. Three channels to the outside-deeper waters (Southeast, Southwest, and Northwest) converge in the basin (Figure 1). Smaller shoal-water banks (emergent or semi-emergent at low tides) and reefs are found throughout the basin (including Hospital Key, Middle Ground, White Shoal, and Texas Rock). Three deep-water reef structures west of Loggerhead Key are included in the proposed T2ER. Tortugas Bank is approximately 7 nmi west of Loggerhead Key; 8 Fathom Rock is located north of Tortugas Bank and approximately 5.5 nmi WNW of Loggerhead Key; and Little Bank is north of 8 Fathom Rock and approximately 6.6 nmi NW of Loggerhead Key. In contrast to the DTNP, these deep reefs have not been well studied or mapped. Water depths surrounding the banks are 20 to 24 m (66 to 78 ft), the shallowest portions of these banks are 11 to 15 m (36 to 48 ft) deep. Diving observations reveal a complex karst-like limestone with abundant attached reef organisms (sponges, corals, octocorals). The Banks are within State of Florida jurisdiction and the FKNMS. BENTHIC COMMUNITIES DRY TORTUGAS NATIONAL PARK. The DTNP has the best documentation of marine habitats within the Florida Keys. The earliest estimate of area and classification of marine habitats was published by Alexander Agassiz (1883). This work reported the spatial coverage of habitats as follows: Terrestrial and marine habitats, Dry Tortugas, Agassiz, 1883.
Davis (1982) mapped the DTNP benthic communities and compared his findings with Agaissiz’s with modified mapping units. The following table presents Davis’ findings. Terrestrial and marine habitats, Dry Tortugas, Davis, 1983.
The principal differences between Agassiz’s and Davis’s evaluations are.
A seagrass and coral reef inventory of the Florida Keys was sponsored by U.S. U.S. Fish and Wildlife Service and Minerals Management Service and volume six (1983) of this inventory focuses on the Dry Tortugas. It includes the area within the park and slightly beyond the DTNP boundaries. The atlas of aerial photographs with acetate overlays indicates presence or absence of coral reefs and seagrass beds. The reported purpose for this inventory was to assist agencies in offshore petroleum development and for oil spill management. A recent collaborative effort by the Florida Marine Research Institute and NOAA (Benthic Habitats of the Florida Keys, FMRI Tech Report, TR-3, in press) provides a recent estimate of benthic habitats in the Dry Tortugas )DTNP, and adjacent areas outside the park boundaries). The following table includes the estimates. Marine habitats, Dry Tortugas, FMRI and NOAA, in press.
*The large unmapped area (approximately 60%) in this product makes comparisons with historical estimates very difficult. BENTHIC COMMUNITIES Algae Algal communities are the most ephemeral of the benthic communities. Davis (1983) reported that distribution of the brown algae was on rocks or rubble in areas of high wave energy, such as the reef flats. The conspicuous genera include: Laurencia, Dictyota, Sargassum, Cladophora, and Padina. In deeper areas, there are often abundant algae that are attached to the hard substrate or sedimentary deposits. Common genera include: Halimeda, Avrainvillea, Penicillus, Udotea. Crustose coralline algae (Rhodophyceae) form thin-branched or unbranched crusts typically attached to the limestone. These algae proliferate in shallow areas with high wave energy (Humm, 1984). The benthic algae and seagrasses function as primary producers contributing biomass and oxygen to the system. The algae are consumed by invertebrate and vertebrate herbivores ranging from microscopic crustaceans to large sea turtles. Some organisms such as the damselfish, lay their eggs in the algae. The life cycles of the algae are very rapid compared to sponges, corals and fish. The marine algae at Dry Tortugas include at least 377 species (Taylor 1928). Taylor found 50 species of algae within a few yards off the northwest beach of Loggerhead Key. Work to describe the marine algae at Dry Tortugas continues: Ballantine and Aponte (1995) and Ballantine (1996) described eight new species near Pulaski Shoal (northeastern DTNP). Algae such as Halimeda contribute significant amounts of carbonate sediments.
Seagrass meadows Seagrass beds are one of the most common benthic habitats in the Dry Tortugas; they are found in water as deep as 30 m (100 ft) whenever there is sufficient light and unconsolidated sediment to support their root systems. Five species of seagrass have been recorded from the Dry Tortugas: Turtle grass, Thalassia testudinum (Banks ex König) Manatee grass, Syringodium filiforme (Kützing) Shoal grass, Halodule wrightii (Ascherson) APaddle grass@, Halophila decipiens (Ostenfeld) AStar grass@, Halophila engelmannii (Ascherson) Two other species of seagrass occur in south Florida, but have not been reported from the Dry Tortugas (Halophila johnsonii (Eiseman) and Ruppia maritima (Linne.). Seagrasses are valued for their roles as nursery grounds, foraging habitat, shelter, sediment stabilization, energy attenuation, and primary production (Zieman 1982). Energy fixed by seagrasses as primary producers predominantly reaches higher trophic levels through the detritus pathway - seagrass blades generally die and are colonized by bacteria and fungi before being consumed by animals. Few organisms graze directly on living seagrass blades, but some of these are conspicuous. Green sea turtles (Chelonia mydas) feed almost exclusively on seagrass, and the Dry Tortugas is an important refuge for this endangered species. In 1998, 165 green turtle nesting attempts (and 78 actual nests) were recorded in DTNP (Reardon, 1998). Many other valued animals are dependent on seagrass beds during part of their life cycle, like the pink shrimp (Penaeus duorarum), the spiny lobster (Panulirus argus) and the queen conch (Strombus gigas). Many herbivorous fishes that find shelter on coral reefs during the day feed in seagrass beds at night. Predatory fishes of the reef also forage in seagrass beds. Vast schools of grunts and snappers migrate off of daytime resting areas around reefs to feed at night in seagrass beds (Robblee and Zieman 1984). The distribution of seagrass beds is a determined by exposure to air, penetration of light in the water column, availability of nutrients, suitable sandy or muddy sediments, and disturbance (Zieman 1982). The Dry Tortugas lay at the western end of a nearly continuous shallow-water seagrass bed that covers over 14,000 km2 (Fourqurean et al. in press). The primary factor limiting the distribution of seagrasses within DTNP is the presence of suitable unconsolidated substratum; otherwise water quality in the park is sufficient to support seagrass growth on the bottom. In fact, the depth record for the seagrass Thalassia testudinum is **, recorded by ** in ** from west of the old Carnegie Laboratory ruins on Loggerhead Key. This compares to a maximum depth of T. testudinum of 18 m (59 ft) and a mean depth of 3 m (10 ft) from 898 randomly-sampled sites south Florida (Fourqurean et al in press). These findings indicate that deeper waters in Dry Tortugas are generally clear enough to support growth of seagrass beds. In shallow water, Thalassia testudinum forms dense seagrass meadows. As depth increases, other species can coexist with T. testudinum. For example, as one swims down the slope of the bank north of Loggerhead Key, a dense Thalassia bed grades into a mixed Thalassia-Syringodium bed, then Thalassia drops out, and Halodule becomes common with the Syringodium. Deeper still, Syringodium drops out, and Halophila engelmannii and Halophila decipiens occur interspersed with Halodule. Once you reach 23 m (75 ft), the dominant seagrass is Halophila decipiens. The seagrass beds of DTNP are relatively diverse when compared to other beds in south Florida. It is not uncommon to find three or four seagrass species growing in close association; and 5 species have been found in the same 0.25 m2 area.
Porifera Summarized from Schmahl (1984). The sponge fauna at Dry Tortugas was studied by deLaubenfels during the Carnegie Laboratory period. He described 76 species including five dredged from 1,047 m. Schmahl (1984) reported 85 species with DTNP. Sponges create ecological space (niches) thus they are an important asset to the area. The numbers of species and the broad range of habitat that sponges occupy gives testament to their importance (Figure13). Functionally sponges provide space, filter water, are a food source for a wide variety of animals from invertebrates to turtles, and provide other ecological services. In the context of reefs and carbonate rock, sponges can be an important structural buttress in that they hold the reef together. Carbonate producing sponges provide structure and demosponges provide an interstitial fabric which holds the materials together. The boring sponges are destructive to the reef because they excavate coral limestone skeletons. Over time the weakened skeletons may break loose from the reef platform. Coral Reef Habitats The term coral reef is a broad category used to define habitats where massive corals are conspicuous. In other cases, the existing community is a mixture of smaller corals, octocorals, and sponges, but the underlying foundation was built in the recent past by massive corals. The major reef types at Dry Tortugas include bank reefs, patch reefs, and thickets of staghorn corals. The once abundant elkhorn coral (Acropora palmata) assemblages (44 hectares by Agassiz’s estimate in 1882) have virtually disappeared from the area (Davis 1982, Jaap and Sargent 1993). Since Davis published his map, some of the staghorn (Acropora cervicornis, A. prolifera) coral populations have declined due to hypothermal stress (Roberts et al., 1982) and a virulent disease (Peters et al. 1983). Reefs are constructed principally by the massive scleractinian coral species. Most of the corals that are found associated with reefs in the western Atlantic — Caribbean occur at Dry Tortugas (Jaap, et al., 1989). The following table identifies the stony corals (Milleporina, Scleractinia) reported from Dry Tortugas.
Milleporina and Scleractinia reported from Dry Tortugas based on literature and personal observations. Phyllum Cnidaria Class Hydrozoa, Owen 1843 Order Milleporina Hickson 1901 Family Milleporidae Fleming 1828 Millepora alcicornis Linnè 1758 (Figure 15) Millepora complanata Lamarck 1816 Class Anthozoa Ehrenberg 1834 Order Scleractinia Bourne 1900 Family Astrocoeniidae Koby 1890 Stephanocenia michelinii Milne Edwards and Haime 1848 Family Pocillopridae Gray 1842 Madracis decactis (Lyman 1859) Madracis pharensis (Heller, 1868) Madracis mirabilis (sensu Wells 1973 Madracis formosa Wells 1973 Family Acroporidae Verrill 1902 Acropora cervicornis (Lamarck 1816) Acropora palmata (Lamarck 1816) Acropora prolifera (Lamarck 1816) Family Agariciidae Gray 1847 Agaricia agaricites (Linnè 1758) Forma agaricites (Linnè 1758) Forma purpurea (LeSeuer 1821) Forma humilis Verrill 1901 Forma carinata Wells 1973 Agaricia lamarcki Milne Edwards and Haime 1851 Agaricia fragilis (Dana 1846) Leptoseris cucullata ((Ellis and Solander 1786) Family Siderastreidae Vaughan and Wells 1943 Siderastrea radians (Pallas, 1766) Siderastrea siderea (Ellis and Solander 1786) Family Poritidae Gray 1842 Porites astreoides Lamarck 1816 Porites branneri Rathbun 1887 Porites porites (Pallas 1766) Forma porites (Pallas 1766) Forma clavaria Lamarck 1816 Forma furcata Lamarck 1816 Forma divaricata LeSueur 1821 Family Faviidae Gregory 1900 Favia fragum (Esper 1795) Favia gravida Verrill 1868 Diploria labyrithiformis (Linnè 1758) Diploria clivosa (Ellis and Solander 1786) Diploria strigosa (Dana 1846) Manicina areolata (Linnè 1758) Forma areolata (Linnè 1758) Forma mayori Wells 1936 Colpophyllia natans (Houttuyn 1772) Cladocora arbuscula (LeSueur, 1821) Montastraea annularis (Ellis and Solander 1786) Forma annularis (Ellis and Solander 1786) Forma faveolata (Ellis and Solander 1786) Forma franksi (Gregory 1895) Montastraea cavernosa (Linnè 1767) Solenastrea hyades (Dana 1846) Solenastrea bournoni (Milne Edwards and Haime 1849 Family Rhizangiidae D’Orbigny 1851 Astrangia soliteria (LeSueur 1817) Astrangia poculata (Milne Edwards and Haime 1848) Phyllangia americana Milne Edwards 1850) Family Oculinidae Gray 1847 Oculina diffusa Lamarck 1816 Oculina robusta Pourtales 1871 Family Meandrinidae Meandrina meandrites(Linnè 1758) Forma meandrites(Linnè 1758) Forma danai Milne Edwards and Haime 1848 Dichocoenia stokesii Milne Edwards and Haime 1848 Dendrogyra cylindrus Ehrenberg 1834 (Figure 8) Family Mussidae Ortmann 1890 Mussa angulosa (Pallas 1766) Scolymia lacera (Pallas 1766) Scolymia cubensis (Milne Edwards and Haime 1849) Isophyllia sinuosa (Ellis and Solander 1786) Isophyllastrea rigida (Dana 1846) Mycetophyllia lamarckiana (Milne Edwards and Haime 1849) Mycetophyllia danaana (Milne Edwards and Haime 1849) Mycetophyllia ferox Wells 1973 Mycetophyllia aliciae Wells 1973 Family Caryophylliidae Eusmilia fastigiata (Pallas 1766) ? ? Bank Reefs The bank reef habitat occurs in an arc along the northeastern to southern margins of DTNP. This habitat includes spur and groove structures and large isolated formations with up to three meters of relief. Bird Key Reef (Figure 2) in the southern portion of the park is a good example of this reef type. Shinn et al (1977) examined the reef by coring (Figure 18), and reported 8 to 13 m of reef growth since the reef’s inception and a growth rate estimated at 1.91 to 2.22 meters per 1000 years. The reef is estimated to be 5,883± 224 to 6,017± 90 YBP from carbon dating (Shinn et al. 1977). The community structure of Bird Key Reef stony coral fauna was examined by Jaap et al (1989). Three species of coral (Montastraea annularis, M. cavernosa, and Siderastrea siderea) were the principal frame work builders on this reef. Coral diversity, cover, and the habitat complexity increased with depth. Coral cover (as determined by linear measurement) was highest in depths between 9 and 13 m. Octocorals exhibited their greatest species richness in depths less than 8 m. Thirty-three species of stony corals were inventoried at Bird Key Reef in1975-1976. The following is a short characterization of the Bird Key Reef Florida Marine research Institute monitoring site. BIRD KEY REEF: 24°36.689’N, 82°52.226’W (Figure 2) Bird Key Reef is approximately 2 km south of Garden Key, Ft. Jefferson. The reef originates at "Five foot" channel and extends 2.2 km in a SSW direction. The reef crest is emergent at low tide. The distance from the crest to the seaward terminus of the reef is 360 to 400 m. The area seaward of the crest was originally an elkhorn coral (Acropora palmata) community (Agassiz 1882, Wells 1932). Octocorals are currently the most abundant and conspicuous organisms in this area. Topography becomes diverse and rugged about 200 m (at a 120° bearing) from the reef crest, where water depths exceed 9 m. The reef structure is spur and groove with individual spurs that vary in length and width. We identified four biological communities on Bird Key Reef : rubble and fleshy algae (from intertidal to approximately 8 m depth); octocoral dominated hardgrounds (from 8 to 10 m depth); a transition or ecotone of octocoral hardgrounds (10 to about 12 m depth); and high relief spur and groove structures with massive corals (9.4 to 25 m depth). We have monitored the abundance of stony corals at Bird Key Reef since 1989. The most abundant stony coral species were Montastraea annularis, M. cavernosa, Siderastrea siderea, and Agaricia agaricites. The mean number of coral taxa seen in the 2.56m2 quadrat (N=5 samples for each time period) was 7.00± 1.41 to 8.80± 2.04 and the mean number of colonies per quadrat ranged from 21.80± 9.45 to 37.00± 18.28 from 1989 to 1997. Bird Key exhibited intermediate and relatively stable stony coral species richness (Figure 26) and species diversity and evenness indexes also document relative stability (Figure 27). The topographic complexity provides excellent refuge for sessile and mobile organisms. Sponges, octocorals, and stony corals are conspicuous on the structures. The grooves between the structures contain sediments. Night diving at the site indicates that the sediments between the reef structures are import as refuge for polycheates and crustaceans that are hidden in the sediments during the daylight hours, but are found in the waters above the reef at night.
Patch Reefs Patch reefs are isolated accumulations of massive corals that are often surrounded by seagrass and sediments. At DTNP patch reefs lie inside the bank reef formations in the northeast to southeast, to the south and east of Loggerhead Key, and west of Garden Key (Figure 10). The highest concentration of the patch reefs is a large area southwest of Loggerhead Key (on the charts as Loggerhead Reef). These formations are isolated or in loose clusters. Well developed patch reefs have massive colonies of Montastraea annularis that are several meters in diameter. A good example of this type of formation is the area due west of Loggerhead Key, commonly referred to as, "Little Africa" (Figure 5). Isolated patch reefs off the edge of Loggerhead Key, in 15 m depths, have a circular to irregular outline and come to within 8 m of the surface. The surrounding area is seagrass, rubble and sediments. The massive corals are typically eroded around the bases with small to moderate openings that lead to the interior of the reef. The galleries provide refuge for invertegrates such as lobsters and crabs. The dead areas on the massive corals are often occupied by algae (Halimeda and Dictyota), sponges, octocorals, and other stony corals (Porites porites, Mycetophyllia spp). An example of a deep-water patch reef is Texas Rock (Figure 2). We installed monitoring stations on Texas Rock in 1989 and have monitored the stony corals annually since that date. The reef is within central basin. The following is a short description of Texas Rock. TEXAS ROCK: 24°40.810’N, 82°53.180’W. (Figure 2) Texas Rock is approximately 5.4 km NNW of Garden Key. It is a patch reef, approximately 2 km in diameter. Surrounding depths are 20 m and the top of the reef platform is about 8 m deep. Massive buttresses, pinnacles, and canyons define the western margin of the reef (Figure 14). Large colonies of Colpophyllia natans, Montastraea annularis and, M. cavernosa are conspicuous on the pinnacles. The most abundant species in the monitoring stations are Montastraea annularis, Siderastrea siderea, and Colpophyllia natans. The mean number of taxa ranged from 11.20± 1.72 to 13.60± 1.85 and the mean number of colonies has ranged from 58.40± 14.83 to 80.80± 15.87 between 1989 and 1997. The species richness and the number of colonies of stony corals at Texas Rock was higher than that of Bird Key and Pulaski Shoal (Figure 26). The species diversity index values for stony corals was similar to Bird Key Reef, but the variability was less (Figure 27). The evenness index documented a stable pattern, but the values tended to be lower than those for Bird Key and Pulaski Shoal (Figure 27). The patch reefs are important refuge habitat for fish and invertebrates. We have observed large fish such as red grouper, jewfish, and amberjack at Texas Rock. Their residence may be transitory or permanent. The small galleries are often occupied by lobster, crabs, and small fish. Staghorn Coral Reefs Staghorn reefs are constructed by two species of staghorn corals (Acropora cervicornis and Acropora prolifera), that are able to rapidly monopolize a large area. Their success is partially the result of broken fragments surviving and growing into new colonies. These species have the highest growth rate of any scleractinian corals in Florida. Vaughan (1916) reported 4 cm per year, Shinn (1966) reported a rate of 10.9 cm a year and Jaap (1974) reported a growth rate of 11.5 cm per year. The large thickets of staghorn coral up to two meters high have virtually no other coral species associated with them (Figure 6). In the period prior to January 1977, staghorn reefs were the most commonly occurring reef in Dry Tortugas. In an area west of Loggerhead Key, huge fields of staghorn coral were typical (Davis 1977). Davis (1982) estimated staghorn reefs comprised 478 hectares of the seafloor (55.3 percent of all reef habitat). The staghorn reef community is very susceptible to perturbation from meteorological phenomena. For example, the passage of a winter cold front, January, 1977, eliminated up to 95 percent of the extant staghorn reefs (Walker 1981, Davis 1982, Porter et al. 1982, Roberts et al. 1982). The Mavro Vetranic ship grounding near Pulaski Shoal (Tilmant et al. 1989) exposed a deep cross section of reef strata composed of alternating layers of staghorn corals and star and brain corals. Over centuries, staghorn coral reefs have been dynamic: proliferating and waning in time and space. In 1989, we installed permanent monitoring sites east and west of Loggerhead Key. These areas had extensive staghorn coral thickets in 1975-77. As reported above, these thickets were severely impacted by hypothermic stress during the January 1977 cold front passage. We sampled these areas by a quadrat census from 1989-1991 and recorded that recovery of staghorn corals was not occurring west of Loggerhead Key. There was evidence of recruitment and growth at White Shoal (east of Loggerhead Key), particularly on the north end. We have subsequently returned to these sites (between 1991 and 1997) and examined them qualitatively. The area west of Loggerhead Key is still characterized as staghorn coral rubble covered with Dictyota, Lobophora, and Halimeda algae (Figure 11). The White Shoal area has extensive thickets of Acropora cervicornis that have occupy the northeastern portions of the bank. Other areas within the park have moderately large staghorn coral reefs.
Elkhorn Coral Reefs The extant elkorn (Acropora palmata, Figure 7) assemblage at Dry Tortugas is located in front of Garden Key. It is a a remnant population that has survived hurricane Georges. It occupies approximately 800 m2. This formely abundant coral now is at risk of local extinction. Deep Coral Banks. The Tortugas Banks are poorly studied; remoteness, depth and currents have restricted observations. We assume that the Tortugas Banks were created by during a lower stand of sea level and that they were unable to grow fast enough to keep pace with rising sea level following the Wisconsin glacial epoch. Growth was retarded after the water became too deep for light penetration to stimulate vigorous coral growth. Corals found on the banks have the appearance that they are light starved. As depth increases, corals respond by maximizing their surface area; rather than building a mound or hemisphere, the form is like a pancake (Figure 21). In the area that has been commonly referred to as Sherwood Forest, relief is relatively low and appears to be a foundation of karst limestone with moderate sized platy corals overlying the Swiss cheese-like limestone (Figure 21). In other areas, such as Black Coral Rock, large relief structures protrude like mountains upward from the seafloor (Figures 19, 25). Multiple structures with valleys in between adjacent structures provide a mosaic of horizontal, vertical, and angled surfaces, offering niches to a wide variety of plants and animals (Figure 22). The black corals (Antipathes spp.) which are uncommon in Florida Keys reefs, are attached along wall faces (Figure 20). Black corals are branching type of coral that has a yellow to red outer tissue layer with a solid black matrix skeleton. The skeleton has value in the manufacture of jewelry and in many areas the collection pressure has made black corals rare. The black corals are listed in the Florida Rare and Endangered biota of Florida (Deyrup and Franz, 1994) as being extripated (meaning no longer found in Florida). This is inaccurate, they are rare, but do occur in isolated places. They favor deep reef environments with moderate to strong currents. Black corals are listed as totally protected under the Convention on International Trade in Endangered Species of Wild Fauna and Flora on Endangered Species (Cites). Moderate to strong currents are common in Tortugas Banks and may be one of the reasons that black corals are moderately abundant in the area. Reef corals are abundant on the deep reefs and a principal faunal and major contractual component of the reef structures. The most common corals are Montastraea complex. Other common genera are Siderastrea, Colpophyllia, and Agaricia. The Codacean algae Halimeda is common, it occupies the areas between the corals.
Octocoral Dominated Hardbottom This was the habitat type which Davis (1982) identified as major bottom type. He reported 3,965 hectares of octocoral covered hardbottom within Dry Tortugas National Park (4.08 percent of the seafloor in the park). The most conspicuous characteristics of the octocoral hardbottom are the abundant sea whips, sea plumes, sea fans, and the topography is rather flat (Figure 12). Octocoral species density at a monitoring station at Pulaski Shoal was 15.50± 3.50 and 92.60± 31.74 colonies per m2. The area is like a jungle with the bottom virtually obscured by the octocoral canopy. The octocoral hardgrounds have a rich diversity in species. The following is a list of species that are reported from Dry Tortugas. These data are based on the literature and Jennifer Wheaton’s field notes. Octocoral species observed from Dry Tortugas Phylum Cnidaria Subclass Octocorallia Haeckel 1866 Order Alcyonacea Lamouroux 1816 Family Briareidae Gray 1840 Briareum asbestinium (Pallas 1766) Family Anthothelidae Iciligorgia schrammi Duchassaing 1870 Erythropodium caribaeorum Duchassaing and Michelotti 1860 Family Plexauridae Gray 1859 Plexaura homomalla Esper 1792 Plexaura flexuosa Lamouroux 1821 Eunicea succinea (Pallas 1766) Eunicea calyculata (Ellis and Solander 1786) Eunicea laxispica (Lamarck 1815) Eunicea mammosa Lamouroux, 1816 Eunicea fusca Duchassaing and Michelotti 1860 Eunicea lanciniata Duchassaing and Michelotti 1860 Eunicea tourneforti Milne Edwards and Haime 1857 Eunicea knighti Bayer 1961 Plexaurella dichotoma (Esper 1791) Plexaurella grisea Kunze 1916 Plexaurella fusifera Kunze 1916 Muricea elongata Lamouroux 1821 Muricea laxa Verrill 1864 Muricea atlantica Kükenthal 1919 Pseudoplexaura porosa (Houttuyn 1772) Pseudoplexaura flagellosa (Houttuyn 1772) Pseudoplexaura crucis Bayer 1961 Family Gorgoniidae Lamouroux 1812 Pseudopterogorgia acerosa (Pallas 1766) Pseudopterogorgia americana (Gmelin 1791) (Figure 9) Pseudopterogorgia bipinnata (Verrill 1864) Gorgonia ventalina (Linné 1758) Pterogorgia anceps (Pallas, 1766) Pterogorgia citrina (Esper 1792) Pterogorgia guadalupensis Duchassaing and Michelin 1846 ?????`???? An example of a typical octocoral habitat follows. PULASKI SHOAL: 24°41.661’N, 82°46.296’W. (Figure2) Pulaski Shoal is approximately 12.8 km NE of Garden Key and just east of a navigation marker (light tower). The habitat is low relief limestone with sedimentary channels bisecting the platform As of 1994, the community structure included 24 octocoral taxa with 559 colonies (species and subspecies) occupying the monitoring stations. In contrast, there were 12 taxa and 87 colonies of stony corals. The sampling area was 12.8 m2 (five, 2.56 m2 quadrats). The most abundant taxa were: Eunicea spp., Pseudopterogorgia acerosa, and Plexaura flexuosa. The species richness and the number of colonies of stony corals observed at Pulaski Shoal were less that those reported for Bird Key and Texas Rock (Figure 26). The species diversity index was relatively lower than Bird Key and Texas Rock (Figure 27) and the evenness index values were about the same as Bird Key; however, the variability was much greater (Figure 27).. Sedimentary Habitats. A large portion of the Dry Tortugas sea floor is composed of sediments (silt, sand, gravel); Davis (1982) estimated that sediments were contributing 10,892 hectares or 47.80% of the benthic habitat in DTNP. This is the largest component. If seagrasses are included (because seagrasses grow in sediments) the sediment benthic contribution in DTNP is 78 percent. Work in Dry Tortugas sedimentary habitats is very limited. Sedimentary habitats provide niches for virtually every marine phyla, thus the biodiversity of these habitats is relatively high. Because the organisms are living (for the most part) under the surface of the sediments, thus there is a misconception that this area is barren of life (Cahoon et al. 1990, Snelgrove 1999). We are hard pressed to provide so much as a species list of the flora and fauna that are obligate to the sedimentary habitat. Bacteria, diatoms, protozoa, molluscs, crustaceans, echinodems, ploycheates, gobies, and blenneys are examples of higher order taxonomic categories that are found in the sediments. The sediments function as a forage area for larger predators (Cox et al. 1997). They also are a pool of geo-chemical material (calcium carbonate).
Spiny Lobster (Panulirius argus) Within DTNP the spiny lobster populations have been totally protected from harvest for decades. As a result, researchers have the opportunity to compare population dynamics of the spiny lobster in DTNP with the areas in the Florida Keys that have intense fishing pressure. The lobster populations at Dry Tortugas provide a very unique asset for research (Davis 1977, Davis and Dodrill 1989, Cox et al 1996,). Hurricanes Hurricane Georges passed directly over Garden Key and Dry Tortugas National Park. There was physical damage, as of this date we have not had an opportunity to survey the area so we can only caution that many of the shoal-water reefs were probably changed by the storm. Over the centuries hurricanes have played an important role in reef, island, and shoal creation and distribution within the Dry Tortugas area. Other Controlling Factors The two most important influences are meteorology and physical oceanography. The weather has two components: winter cold fronts (hypothermia) and late summer doldrum conditions that generate hyperthemic water temperatures. In the former, temperatures that are 14° C or less stress or result in mortality of sensitive organisms. The 1977 event resulted in loss of staghorn coral populations (Walker 1981, Roberts et al. 1982). Other organisms suffered a similar fate. The other extreme in temperature (hyperthermia) results in coral bleaching events that stress the fire corals, anemones, zooanthids, corallimorphs, octocorals, and scleractinian corals. Bleaching here refers to loss of the endosymbiotic algae (Zooxanthellae) and the normally colorful organisms appear white or mottled. The late summer mean maximum temperatures for Dry Tortugas (August and September) are 29.3 and 28.8 respectively (Vaughan 1918). When temperatures exceed 31°C for a prolonged period, bleaching occurs. Higher late summer temperatures are often associated with ENSO years. Mild to moderate bleaching normally does not result in mortality; however, it has been demonstrated to affect growth and reproduction. We monitored a bleaching event at Bird Key Reef, from 31 August to 13 September, 1991. Temperatures rose from 30.4 °C, peaked at 31.5°C on 4 August and remained above 31°C until 7 to 8 September. The rise in temperature was related to a windless period. Concurrent with temperature rise, the zooxanthellate cnidarians discolored and many lost all of their color (bleached). The oceanographic influences include impacts from storm waves and the occasional influence of cold-water meanders that are forced in shallow waters (Vukovich 1988). The events last for a few days and are documented to depress water temperatures at 20 m, but not at 10 m depths (based on thermograph records). During the period of 7 to 11 August, 1988, a thermograph deployed at 20 m, Bird key Reef detected an intrusion of cold-water (22 to 23°C). The ambient temperature at the site during the cold-water intrusion was 29°C. We suspect that the cold water intrusions may be responsible for terminating reproduction-spawning that is frequent during late summer.
Figures Cover figure. Pulaski Shoal, turtle in octocoral and sponge habitat. Figure 1. Dry Tortugas National Park benthic resource map (October 1998 draft). Figure 2. Dry Tortugas monitoring sites map. Figure 3. Interior of the Carnegie Laboratory, Loggerhead Key, 1932, photo taken by John W. Wells. Figure 4. Field work, 1932, Carnegie Laboratory, 1932, photo taken by John W. Wells. Figure 5. Massive Montastraea annularis coral, Little Africa west of Loggerhead Key, photo taken by W.C. Jaap. Figure 6. Dense thicket of staghorn coral (Acropora cervicornis) west of Loggerhead Key, photo taken by W.C. Jaap. Figure 7. Elkhorn coral (Acropora palmata) and fused staghorn coral (Acropora prolifera, Five foot Channel, south of Garden Key, photo taken by W.C. Jaap. Figure 8. Pillar coral (Dendrogyra cylindrus), near Southwest Channel, photo taken by W.C. Jaap. Figure 9. The sea plume (Pseudopterogorgia americana), White Shoal, photo taken by W.C. Jaap. Figure 10. Aerial photo, west of Loggerhead Key, dark areas are reef habitat. Photo copied from U.S. Fish and Wildlife Service & Mineral Management Service document, taken in 1983. Figure 11. Staghorn coral rubble and clumps of brown algae, west of Loggerhead Key, 1991. This area was dense staghorn corals prior to the 1977 hypothermal event, photo taken by W.C. Jaap. Figure 12. Hardbottom habitat near Iowa Rock, photo taken by W.C. Jaap. Figure 13. Large barrel sponge (Xestospongia muta) and octocorals, photo taken by W.C. Jaap. Figure 14. Patch reef habitat, Texas Rock, photo taken by W.C. Jaap. Figure 15. Colony of fire coral (Millepora alcicornis), Five Foot Channel, south of Garden Key, photo taken by W.C. Jaap. Figure 16. Seagrass, west of moat wall, Garden Key, photo taken by W.C. Jaap. Figure 17. Deep reef habitat at, "Black Coral Rock" west of Loggerhead Key, photo taken by W.C. Jaap. Figure 18. Bank reef habitat, Bird Key Reef, photo taken by W.C. Jaap. Figure 19. Deep reef habitat, at, "Sherwood Forest", west of Loggerhead Key, photo taken by W.C. Jaap Figure 20. Black coral (orange plume), Black Coral Rock, west of Loggerhead Key, photo taken by W.C. Jaap. Figure 21. Deep reef habitat, at, "Sherwood Forest", west of Loggerhead Key, photo taken by W.C. Jaap. Figure 22. Coraline algae and sponge at Sherwood Forest, west of Loggerhead Key, photo taken by W.C. Jaap. Figure 23. A PVC pipe encrusted with octocorals, sponges, and other biota, Bird Key Reef. Pipe was inserted into a core hole by Shinn et al. in 1975, photo was taken in 1997 by W.C. Jaap. Figure 24. A deep reef west of Loggerhead Key with a jewfish (Epinephelus itajara), photo taken by W.C. Jaap. Figure 25. Deep reef habitat at Black Coral Rock, west of Loggerhead Key, photo taken by W.C. Jaap. Figure 26. Taxonomic richness and numbers of colonies, Bird Key, Pulaski Shoal, and Texas Rock, Quadrat census, N=5, 2.56 m2 each year. Figure 27. Species diversity (Shannon Weiner H’n) and evenness (Pielou’s J’n), Pulaski Shoal, and Texas Rock, Quadrat census, N=5, 2.56 m2 each year. Acknowledgements Support for sustaining this effort has come from the U.S. National Park Service, The John D. and Catherine D. MacArthur Fund, The Trust Fund for Critical State Concern, and general state revenues. The 1997 field work was supported by the U.S. Geological Survey, Biological Resource Division. We are grateful to the U.S. National Park Service for providing the necessary permits to work within Dry Tortugas National Park. We thank Wayne Landrum and his staff for the courtesies that they have provided to us. We thank all who have offered moral and physical support while we have conducted research at Dry Tortugas, especially: Cliff Green, Linda Vanaman, Jim Tilmant, Caroline Rogers, Bruce Rogers, Chris Rogers, Mike Eng, Monica Eng, Richard Curry, Ron Matchock, Kelly Donnelly, Ginger Garrison, Barbara Kojis, Leanne Miller, Joe Kimmel, John Ogden, John Halas, and Bob Muller.
LITERATURE CITED
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