Atoll Research Bulletin No* 271 Cay Sal Bank, Bahamas: A

ATOLL RESEARCH BULLETIN NO* 271 CAY SAL BANK, BAHAMAS: A BIOLOGICALLY IMPOVERISHED, PHYSICALLY TROLLED ENVIRONMENT SEPTEMBER 198 3 CAY SAL BANK, BAHAMAS: A BIOLOGICALLY IMPOVERISHED PHYSICALLY CONTROLLED ENVIRONMENT 1 by Walter M. Goldberg ABSTRACT Cay Sal Bank is a shallow water, oceanic platform whose lagoonal surface is 9-16m below sea level. Benthic communities on the bank are divided into four major zones based upon biotic composition and substrate. These are (1) rubble- Lobophora 12) Sargassm-gorgonian-sponge (3) Halodule and associates aKd ( ) a patchy Thalassia zone. Sediment is largely restricted to the western portion of the lagoon, especially in zone 3. Although occasional patch reefs are noted, the bank lagoon is virtually devoid of coral reefs and coral communities, The submerged, leeward bases of islands and cays represent the principal substrate for the development of scleractinian corals, but these do not appear to form a reef framework, Scleractinian development on the windward sides of the islands is negligible, Patch reefs and spur and groove systems are usually poorly developed on the outer bank slope to depths approaching 30m, The best developed coral communities occur below this depth with the exception of the southwestern sector where reefs are smothered in sediment, The biological structure of the bank lagoon is interpreted as being controlled largely by wind and periodic storm forces that result in frequent resuspension and eventual offbank transport of sediment, especially to the southwest, Gay SaX is not a drowned bank; it is a submerged platform on which biological impoverishment appears to be maintained through disturbance phenomena engendered by an open and poorly developed rim. These stresses prevent establishment of more complex coral reefs or coral communities, and promote domination by fleshy algae and other more tolerant organisms. l~e~artmentof Biological Sciences, Florida International University, Miami, Florida 33199, U.S.A. (Manuscript received March 1983 - Eds.) INTRODUCTION Cay Sal Bank is a shallow, detached carbonate bank lying roughly 100 km south of the Florida Keys, 54 km north of Cuba and 50 km west of the Great Bahama Bank (Fig. 1). It is 105 km by 66 km in its greatest dimensi ns, and constitutes an overall area of nearly 4000 km 9 . The rim is lined with a scattering of islands and rocks especially along the northern and eastern margins. Over 99 percent of the bank including the southern flank and the entire center are completely submerged, primarily in depths ranging from 9-16m. Few studies have been made of this region in spite of its size and proximity to U,S, coastal waters. Agassiz (1894) visited the bank, briefly describing some of its geomorphic features; a description of the bathymetry of the bank slope (Malloy ti Hurley, 1970) and stratigraphic studies from a well drilled to 5766111 by Standard Oil Company (Myerhoff & Hatten, 1974) have made more recent contributions to our knowledge of the area, However, descriptions of the marine communities of Cay Sal Bank have apparently not been published, This paper describes the nature and distribution of the shallow-water communities of Cay SaP Bank by aerial and satellite photographk., coupled with ship-based bottom truthing by SCUBA observation and dredge sampling. Physical factors including storm tracks and frequency, wind and wave data and sediment depths on the bank lagoon are analyzed and related to community structure. Materials and Methods A total of 18 days were spent on Gay Sal Ban during the sumxer months of 1980-1982, Biologics tions were made at 97 stations scattered across the shallow Lagoon and outer margin to a depth of 60 m (Fig, 2). These consisted of simple visual surveys to confirm composition of the benthos, collections assisted minute Gapeto redge tows, Samples o materials hav en deposited in the FIU except as noted ere identified by the author. Sediment depths were recorded at 27 stations (1982 only) using a steel probe, Where sediments exceeded a depth of one meter, a 2 1/2 HP water pump was employed to force sea- water through a 5 meter length of PVC pipe. Sediment fluidized in this manner enabled the pipe to quickly penetrate to bedrock. The names of emergent islands used in this paper correspond to those given in British Admiralty Chart No. 1217 (Fig. 2). Coordinate locations have been modified according to more recent data (see appendix). RESULTS a. The Lagoonal Environment The lagoonal surface of the bank can be divided into four major biotic zones (Fig. 3). The northwestern sector with its included sand ridges is characterized by a sparse but consistent cover of seagrasses and algae including Halodule wrightii, Caulerpa spp., Syringodiurn filiforme and various species of calcareous chlorophytes (Fig. 4A). Members of this community (hereafter referred to as the Halodule community) are summarized in Table 1. A denser Halodule community appears to be responsible for several of the large dark patches noted in satellite imagery of this zone, The sediment cover of this quadrant ranges from 2 cm to 1 m. The deeper sand accumulations occur behind and south of Double Headed Shot Cays, (hereafter referred to as DHS Cays) particularly in association with the intersecting sets of bedforrns prominent in the western part of the zone, and those trending NE to SW 20 km to the east, The Halodule zone grades into a sediment-bare, rubble bottom east of Muertos Cays. This northeastern sector is covered by a mixed Sargassum-gorgonian-sponge community. The rubble characteristic of the northeast bank lagoon continues throughout the southeast sector but without as much vegetation, The bottom is covered primarily by a scattering of the brown alga Lobophora variegata, with an occasional cluster of sponges and gorgonians as above, Along the southern and eastern borders of this zone the rubble is interspersed with aggregations of corals, especially Diploria spp,, Agaricia agaricites and Montastrea annularis, together with sponges and gorgoniaas (Fig, For the most part these do not form patch reefs fi. no appreci relief above bottom), However, two reef areas are encountered along the western edge of this zone as noted in figure 3. One of these near 23'34' and 79°48~onsistsof numerous but small (1-2mZ) Diploria, Porites and Montastrea patches rising lm above the bottom (depth = 7-8m). The most significant patch reef area, several km2 in extent, is located 10 krn southwest of Damas Cays. This subzone was characterized by thickets of Acro ora cervicornis rising to a depth of 6m from a 10m b0tt.h4C). Scattered colonies of Montastrea annularis- , ~gariciaagaricites and Colpophyllia natans contribute to the framework, It may be significant that much of the rubble substrate in this zone consists of what appears to be recent Acropora cervicornis. There is little evidence of recolonization. The last zone consists of a sigmoidally shaped area extending across the bank from Cay Sal to the Damas Cays. This area is characterized by patches of Thalassia testudinum which become somewhat more extensive toward the southwestern (leeward) side. Turtle grass becomes particularly well developed in the lee of larger islands on the bank, especially the Double-Headed Shot Cays and the Anguilla Cays (Fig. 4E). Farther behind the latter group, Thalassia grades into a Halodule-calcareous green algae subzone (Fig. 3). At the edges of this area Thalassia intermingles with other communities. This is particularly notable west of Damas Cays where zonation overlap is extensive (Fig. 3)- The Damas Cays area is also notable for the existence of several "blue holes" (Capt, Bob Mbein, pers, corn,). One of these located at 23O49@8% and 70°47'28, 2. the islands, was briefly investigated. It measures approximately lOOm across and lO0m deep, beginning on the bank at a depth of 10-b3m (surface to bottom = 110-ll3m). The surrounding area is a Thalassia-gorgonian-sponge bottom, grading into colonies of Montastrea cavernosa and Agaricia agaricites at the edge of the blue hole, The vertical walls are covered wit variegata to a depth of l8m, below which Halimeda goreauii dominates, Submersible observations at the bottom reveal numerous Strornbus qigas shells apparently the result of the animal becoming entrapped while migrating through the surrounding turtle grass (E, Shinn, pers, comm.). A wider and slightly deeper blue hole is located a few km farther west (Fig* 4F), The sediment cover of the Thalassia zone is highly variable, Through most of this region the range is 2-5cm, increasing to 20-60cm under Thalassia itself, Just north of Gay Sad Island in an area covered by a mixed community of scattered angiosperms and cabcareous green algae, a sand body 4-4-5 meters deep was encountered (Pig, 4@), The area covered by this accumulation was not several sediment stations taken around the periphery of Cay Sal indicates that it is highly localized, b. The Bank Rim and Slope The only areas characterized by substantial coral development are the bank slope below 30m and the submerged portions of the islands. Most of the larger cays have been examined and appear to be uniform with regard to their flora and fauna from the littoral zone to the base of the island (Table 1). The sublittoral portions are characterized by an intertidal bench cemented with an unidentified encrusting coralline alga. Immediately below this surge zone, dropping vertically to a depth of 7-8 m, is a coral community consisting of yellow and purple sea fans (Gorgonia ventalina and G. flabellurn) plus a variety of other gorgonians, corals-and hydrocorals, especially Millepora complanata (Fig. 5A). In general the amount of scleractinian development is greater along the northern border of islands, but only along the lagoonal side where Montastrea annularis, Diploria spp. Siderastrea siderea, Agaricia agaricites and Porites porites become especially numerous (Table 1, Fig.

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