Coral Reefs of the Southern Gulf of Mexico

Chapter 7. Reef Algae

Reef Algae Roy L. Lehman Coral reef algal communities include the zooxanthellae, symbiotic algae in the gastrodermis of the hermatypic (reef-building) corals, free-living, encrusting coralline algae, phytoplankton, mat-forming and boring micro-filamentous algae, and calcified, fleshy, and turf macroalgae. Coral reefs should probably be called tropical reefs, biotic reefs, or even algal reefs. Corals cannot build a reef single-handedly; algae contribute greatly to reef productivity and growth. Nutrients supplied to hermatypic corals by zooxanthellae allow them to grow and reproduce quickly enough to form reefs. Coralline algae contribute greatly to reef biomass, may deposit more calcium carbonate than the corals themselves (Littler and Littler 1984) and are the cement that holds the reef together. Reefs are formed as much by the accumulation of calcareous sediment as by the growth of corals. The spaces formed by coral fragments and large rubble fill with fine and coarse carbonate sediment. Encrusting calcareous algae grow over the sediment, cementing it into place. These algal reef builders also prevent erosion of the reef by waves by consolidating and cementing the substrate together. The stony substrate that is formed is tough enough to withstand waves that can destroy even the hardiest of corals. The most significant of the attached primary producers on shallow reefs, like those in the southern Gulf of Mexico, are the red (division Rhodophyta), brown (division Ochrophyta, class Phaeophyceae), and green fleshy algae (division Chlorophyta). Many genera (e.g., Sargassum) bear a superficial likeness to higher plants but are distinguished by their lack of vascular tissues; they do not have true leaves, stems, and roots. Coral reef phytoplankton, the microscopic plants that float within the photic layer of the ocean, include golden algae (division Ochrophyta, class Chrysophyceae) and cyanobacteria ( blue-green algae, kingdom Monera, phylum Cyanophycota). Although the primary productivity of phytoplankton around reefs is small compared to that of the zooxanthellae and macroalgae, when the smallest phytoplankton, the picoplankton and nanoplankton , are included, abundance and productivity might be higher than earlier thought (Dawes 1998). Cyanobacteria are also important because they are nitrogen fixers and provide a substantial source of nutrients within the coral reef environment (Adey and Goertemiller 1987). They form surface mats or may bore into certain shells and skeletons of corals. Most notable is a free-living filamentous alga, Calothrix (Berner 1990). There is also evidence that some corals may have symbionts that can fix nitrogen, providing nutrients for the zooxanthellae. There have been few studies of the algae associated with coral reefs in the southern Gulf of Mexico. A revised and expanded checklist of the fleshy macroalgae was assembled from a survey of literature that covers the coral reefs of the 7 Algal Biodiversity on Southern Gulf of Mexico Coral Reefs A4339.indb 87 A4339.indb 87 7/6/07 11:19:16 AM 7/6/07 11:19:16 AM 88 Lehman southern Gulf of Mexico (www.gulfbase.org) and contains 331 total species, including 165 species of red algae (division Rhodophyta), 50 species of brown algae (division Ochrophyta, class Phaeophyceae), and 116 species of green algae (division Chlorophyta). The list contains major taxonomic rearrangements, name changes, and author revisions and conforms primarily to the taxonomic arrangement and names used by Wynne (1998) and Littler and Littler (2000). Table 7.1 compares the number and percentage of algal species by higher taxonomic groups found on reefs of the southern Gulf of Mexico. Veracruz Reef System Huerta M. and Barrientos (1965) compared the marine algae of Tuxpan with those found at Blanquilla and Isla de Lobos reefs. They indicated that these reefs are similar in regard to the flora, as they are both offshore coral reefs with shallow, protected waters. They reported a total of 31 species of macroalgae at Blanquilla and 43 species at Isla de Lobos. Isla de Enmedio Reef was studied by Huerta M. et al. (1977), Lehman and Tunnell (1992a, 1992b), and Lehman (1993). Together, they report the highest species richness (148) for any reef of the southwestern Gulf of Mexico. Lehman and Tunnell (1992b) and Lehman (1993) evaluated and collected samples from Isla de Enmedio Reef and adjacent reefs each June over a 10-year period. They reported a total of 91 species of macroalgae. Of these, 34 (including four forms) belong to the division Chlorophyta, 11 to Ochrophyta, and 46 to Rhodophyta . A comparison of species richness with the earlier list of Huerta M. et al. (1977) showed a slight difference in the total number of species identified (97...