ATOLL RESEARCH BULLETIN NO. 532 GUIDE TO THE DOMINANT MACROALGAE OF THE STROMATOLITE FRINGING REEF COMPLEX, HIGHBORNE CAY, BAHAMAS BY DIANE S. LITTLER, MARK M. LITTLER, IAN G. MACINTYRE, EMILY BOWLIN, MIRIAM S. ANDRES, AND R. PAMELA REID ISSUED BY NATIONAL MUSEUM OF NATURAL HISTORY SMITHSONIAN INSTITUTION WASHINGTON, D.C., U.S.A. NOVEMBER 2005 FIGURE 1. Location of Highborne Cay, Bahamas showing the fringing reef and stromato- lite areas prominent along the eastern shore. “T’s” mark the ten established research tran- sects across the Highborne Cay stromatolite fringing reef complex. GUIDE TO THE DOMINANT MACROALGAE OF THE STROMATOLITE FRINGING REEF COMPLEX, HIGHBORNE CAY, BAHAMAS BY DIANE S. LITTLER,1, 2 MARK M. LITTLER,1 IAN G. MACINTYRE,3 EMILY BOWLIN,4 MIRIAM S. ANDRES4, AND R. PAMELA REID4 ABSTRACT This is the first description of macroalgae documented from a stromatolite reef complex. Because of the sediment stress conditions that promote the development of stromatolites in this open marine environment, macroalgae are generally limited in species diversity and are sparse except for the crustose coralline alga Neogoniolithon strictum, which forms the elevated reef crest behind which the sediments and stromatolites accumulate. At the 10 sites sampled, a total of 22 species were encountered, 10 Rhodophyta, 9 Chlorophyta, 2 Phaeophyta and 1 Cyanobacteria. INTRODUCTION Stromatolites, which are laminated microbial deposits, have a unique geologic history that dates back to more than three billion years and extends right up to the present day. Dravis (1983) was the first to report modern marine stromatolites on the northeastern margins of Exuma Sound. Following this initial discovery, marine stromatolites were reported at numerous locations throughout the Exuma Cays, on the western margin of Exuma Sound (Dill et al., 1986; Dill, 1991; Reid and Brown, 1991; Reid et al., 1995). These stromatolites all occur in tidal channels with the exception of those off Stocking Island, Darbay Island and Highborne Cay (Fig. 1) where they form part of a fringing reef that has a reef crest formed by the sediment-tolerant crustose coralline algal 1 Department of Botany, Box 37012, National Museum of Natural History, MCR#166, Smithsonian Institution, Washington D.C. 20013. 2Division of Marine Science, Harbor Branch Oceanographic Institution, 5600 US 1 N, Fort Pierce, Florida 34946 3 Department of Paleobiology, Box 37012, National Museum of Natural History, Smithsonian Institution, Washington D.C. 20013. 4 Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy, Miami, FL 33149. Manuscript received 29 July 2005; revised 15 September2005. 4 For modern stromatolites to exist, the environmental conditions have to be under severe stress to eliminate most other benthic organisms that can out-compete and destroy these mi- crobially laminated sedimentary structures. In the marine environment this stress is usually hyper-saline conditions, or as found off Highborne Cay, stress associated with the constant movement and inundation of sediments (Andres and Reid 2005). As macroalgae are one of the competitors with stromatolite growth, little attention has been paid to the limited numbers of macroalgae species that can survive these severe conditions of sediment stress. This is the first description of the macroalgae that are found in a stromatolite-reef complex. A complete description of this complex ecosystem is found in Reid et al. 1999 with ample photographs of the area and thorough documentation of the internal and external structures of the microbial buildup and surface mats characteristic to this unique habitat. SETTING Off the east-facing beach of Highborne Cay (76°49’W; 24°43’N), an island in the north- ern Exuma Cays, a fringing mixed microbial-coralline-algal reef extends along this windward coast for about 2.5 km (Fig. 1). The shelf, with average water depths of 10–20 m, extends sea- ward for 1–2 km before dropping off into the Exuma Sound with depths of up to 20 km. FIGURE 2. Schematic profile of the near shore reef at Highborne Cay, Bahamas. The above zones are commonly referenced in the distribution section of the species descriptions. (modified from Reid et al. 1999, Fig. 3) 5 B A FIGURE 3. A. Thrombolites forming in shallow, heavily sand inundated areas. B. Surface view of thrombolites (formed by irregular internal microstructures) showing their irregular shape and often flattened top. B A FIGURE 4. A. Stromatolites (formed by laminated internal microstructure) generally occuring in deeper water than the thrombolites. B. Close-up of a stromatolite showing the distinctly deferentiated layers that create the structure. 6 Oceanographic conditions are characterized by normal salinity, approximately 35 ‰, with sea-surface temperatures ranging from 20–28° C in winter and summer, respectively. The wa- ter is saturated with respect to both aragonite and calcite. Tides are diurnal with a range of ~1 m. The climate is dominated by the trade winds, blowing on average 10–15 knots from a southeast to easterly direction. Although the reef system extends along the entire length of the beach, it is most extensively developed in the southern to central sections, and in the very northern part. For research pur- poses this part of the reef system is subdivided into sites, numbered Site 1 in the south to Site 10 in the north (Fig. 1). The reef tract is divided into distinct morphological zones (Fig. 2): beach sand, thrombolite (irregular internal microstructure) zone (Fig. 3), stromatolite (laminated internal microstruc- ture) zone (Fig. 4), reef flat (Fig. 5), reef crest (Neogoniolithon zone, Fig. 6) and fore reef. Whereas the most of these zones are found throughout all transects, the thrombolite zone oc- curs only at a few sites. Furthermore, the individual width and lateral extension of these zones is highly variable. In overview, this reef system is widest in the southern part of the eastern beach, extending over 20–50 m with a maximum width of 54 m within Site 2 (Fig. 1). The Neogoniolithon zone is more shoreward at Site 3, resulting in deceased width to 25–35 m at Sites 3 and 4, narrowing further to 10 m and less at Sites 5–8. Although interrupted by cuts and channels, the outer reef edge is continuously traceable from Sites 1–6, whereas Sites 7 and 8 comprise a series of discontinuous narrow reefs. Sites 9 and 10 are isolated crescent-shaped reefs with extensive open sand areas in-between (Fig. 1). METHODS From 30 January to 2 February 2005, Emily Bowlin and Ian G. Macintyre collected the dominant macroalgae from ten established research transects across the Highborne Cay stro- matolite fringing reef complex. Samples were collected from the Neogoniolithon strictum zone, the reef flat, the stromatolite zone, and finally the thrombolite zone, which did not occur in some transects (Fig. 1). Samples were then taken to the laboratory in the R/V Walton Smith where they were photo- graphed and then pressed and dried. Both photographs and pressed material were transferred to the Smithsonian Institution’s National Museum of Natural History where the macroalgae were studied, identified and described by Diane S. Littler and Mark M. Littler. Highborne Cay pho- tographs were sharpened and cropped in Adobe Photoshop but otherwise remain untouched. In air photographs of the Highborne Cay specimens were taken by Emily Bowlin with the as- sistance of Ian G. Macintyre. Drawings of internal structures and in situ photographs are from “Caribbean Reef Plants”, used with the permission of the publisher and are not specifically from Highborne Cay or Highborne Cay material. However, they provide a representation of comparable in situ plants that occurred in the transects. 7 A C B FIGURE 5. A. Surface view of the reef flat at low tide. B. Underwater view of the reef flat showing the sparce algal and seagrass cover presumably due to the turbulant sand scouring and intermittent sand inundation. C. Closer view of reef flat habitat. FIGURE 6. The reef crest zone dominated by large heads of the sediment tolerant crustose coralline algal Neogoniolithon strictum. 8 RESULTS AND DISCUSSION The marine flora of the Bahamas has been extensively studied over a long time period beginning at the turn of the 19th century when M.A. Howe began focusing his efforts on col- lecting there (e.g., Howe 1904, 1905a, 1907, 1909, 1920, 1924). The myriad of algal habitats have not been intensively investigated in terms of coverage. Historical treatments in the past 75 years are limited to mostly ecologcial works and more limited floras and monographs (e.g., Jensen 1985; Littler et al. 1985, 1986, 1988, 1991; Littler and Littler 1990a, 1990b, 1991, 1992, 2000, 2004; Lapointe et al. 1992, 2004, in press a, in press b). Bahamian specimens and distributional records were also used to produce field guides such as Littler et al. (1989) and Littler and Littler (2000). Consequently, the present work adds a floristic dimension to a unique Bahamian ecosystem. Twenty two marine macroalgae commonly occurred on the stromatolite reefs of Highborne Cay. Of these, 10 were Rhodophyta (red algae), with 9 Chlorophyta (green algae), 2 Pha- eophyta (brown algae) and 1 Cyanobacteria (blue-green algae). The often overlooked brown algal species Padina haitiensis was present. This species is commonly mistaken for P. sanctae- crucis and, consequently, has been rarely reported. The reef crest was the most diverse site with 13 species present (Fig 7). The reef flat had 12 species while the stromatolite zone maintained 11 species. The thrombolite zone was the most depauperate in terms of species richness with only 5 species present. The latter paucity is presumably due to the constant and turbulent sand inundation in the shallow subtidal adjacent to the beach. FIGURE 7. Histogram showing the number of species for the four zones.