CHARACTERIZATION OF THE OFFSHORE MARINE MACROALGAE FROM THE NORTHWESTERN GULF OF MEXICO HARD BANK COMMUNITIES

ABSTRACT

The upper northwestern Gulf basin (Fig. 1) is characterized by a relatively shallow but well-developed continental shelf with an extensive system of mid- and outer shelf hard banks of varying origin and composition. Observations done by our laboratory while participating in monitoring cruises to the National Marine Sanctuaries of the East and West Banks of the Texas Flower Gardens, Stetson Banks (TX) and Sonnier Banks (LA) reveal a surprisingly different floristic composition among the sites. Several range extensions, new macro-algal records for the Gulf of Mexico, and new species imply a much more complex Algal Zone community than the one proposed by Rezak et al. (1985). Our ongoing project includes the development of a modern taxonomic, phylogenetic, morphological and genetic database essential to informed management for preservation of biological diversity. It also provides a framework of seasonal algal composition against which long-term trends and anomalies in algal distribution and health of the Gulf of Mexico can be assessed at future dates.

OBJECTIVES

• To characterize macroalgal diversity at and among the Gulf's offshore hard bank communities.
• To improve taxonomic resolution of Northwestern Gulf of Mexico deep water macroalgae.
• To obtain in collaboration with other Gulf researchers, a synthetic view of community structure by comparing concurrent and historical data sets to detect significant community shifts.
• To assess potential endemism and relict community hypotheses for the marine algae of the Gulf of Mexico.
• To address evolutionary and biogeographic hypotheses with modern analytical methods.

FIELD METHODS

• Collection (SCUBA diving, ROV, and dredging) of representative specimens of all macroalgae.
• Archival of specimens for laboratory analysis (diluted 5% Formalin/seawater for morphological study; silicagel, 95% alcohol for DNA analysis).
• Standard underwater video surveying techniques to determine macroalgal biodiversity and community structure.

LABORATORY AND MOLECULAR METHODS

• Characterization of archived specimens using morphological and molecular tools.
• Creation of WW-web Algal diversity database. Extraction of total DNA and construction of a DNA library.
• PCR and DNA Sequencing (Fig. 2) of relevant Nuclear and Chloroplast genes [i.e. internal transcribed spacer regions (ITS) of ribosomal DNA; chloroplast-encoded RuBiSCo] (Fig. 3, 4) to test the extent of genetic similarity among separated macroalgal populations.
• Reconstruction of Phylogenies inferred from DNA sequence analysis to address evolutionary and biogeographic hypotheses (Swofford 1998).

RESULTS

The E. and W. Texas Flower Gardens residing on the outer continental shelf (200 m isotherm) are coral-dominated, with a species-rich algal turf community. A late summer-early fall 1999 collection revealed 47 taxa belonging to the Green (Chlorophyta), Brown (Phaeophyta) and Red (Rhodophyta) algae, comprising one new species (Martensia hickersonii), several new macroalgal records for the Gulf of Mexico, such as Hypoglossum anomalum, Nitophyllum adhaerens, Taenioma nanum, Botryocladia monoica, Wrangelia argus, Boodleopsis pusilla, and several northern range extensions for the Gulf, including Hypoglossum rhizophorum.

The mid-shelf banks, Stetson and Sonnier (60 m isotherm) are surprisingly distinct in their algal flora even though their close proximity: Stetson harbors a rich Dictyota meadow interspersed with Lobophora variegata, Peyssonnelia inamoena and Padina sanctae-crucis. These meadows comprise 80-90% cover in many areas, in marked contrast to Sonnier which lacks such elaborate meadows. A total of 41 species were recorded for Sonnier, 38 spp. for Stetson Banks (Fredericq et al. in press). New records for the Gulf include the turf algae Laurencia chondrioides, Callithamniella tingitana, Diplothamnion jolyi, Griffithsia heteromorpha, and Dictyota pfaffii. Northernmost range extensions for the Northwestern Gulf include Antithamnion antillanum.

DREDGED COLLECTIONS ~ 55-75 M (~27o53.721';91o21.532')

The biogeographic affinities of many of these algae are currently being investigated with exciting preliminary results. Offshore deepwater taxa within an Order or Family are dramatically distinct from the region's intertidal, nearshore taxa. The deepwater offshore flora obtained through dredging off Louisiana is especially rich in members belonging to the orders Halymeniales and Rhodymeniales (fig. 3), with several new distribution records and new species in the process of being described. Offshore members, such as Coelarthrum, Halymenia, Botryocladia, Gloiocladia, Agardhinula, are distinct from nearshore Rhodymenia, Grateloupia and Prionitis. The first group represents remnants of a Tethyan distribution primarily having affinities with Caribbean taxa and is also present in the offshore deep water regions of the SE United States (e.g. North Carolina, USA; Schneider & Searles 1991). Gracilaria tikvahiae (Gracilariales, Rhodophyta) (fig. 4), a nearshore Gulf species characterized as belonging to the Carolinian Province, is conspecific with specimens from the E. coast of North America down to E. central Florida, W. Central and NW Gulf, to Las Coloradas, Yucatan Peninsula), and typically absent from the offshore hard banks.

The seaweeds of the NW Gulf of Mexico hard banks are much more species-rich and taxonomically distinct than previously reported (e.g. Bright & Pequegnat 1974, Eiseman & Blair 1982, Earle 1972, Rezak et al. 1985, Taylor 1960, Wynne 1998) with a surprisingly different floristic composition among the sites. Our empirical baseline data will be expanded with additional seasonal collections to test the extent of genetic similarity between separate macroalgal populations and the biogeographic scenarios that may account for alternative origins of the offshore flora.

ACKNOWLEDGMENTS

We thank Emma Hickerson and G. P. Schmahl (Flower Garden Banks National Marine Sanctuary), Quenton Dokken, Susan Childs, Carl Beaver (Texas A & M. University), Greg Boland and Les Dauterive, (Mineral Management Service) for inviting us to participate in the monitoring cruises and for their support and interest in this project. We thank Luis and Raul Aguilar, Darryl Felder, Max Hommersand, Lance Horn (NOAA/NURC), Catalina Mendoza, Luz Elena Mateo-Cid, Harlan Miller, and Jim Norris for their interest or vouchers used in this study. We also thank the crew of the M/V Fling, M/V Spree, and R/V Pelican for their assistance. This work was funded in part by U.S. department of Energy Grant DE-FG02-07ER122220, Louisiana Board of Regents Grant LEQSF (1997-1999)-RD-A-30, and NURC-NOAA grant #NA96RU-0260).

RELEVANT REFERENCES

Bright T. J. & L. H. Pequegnat (Eds.]. 1974. Biota of the West Flower Garden Bank. Gulf Publishing Co., Houston, Texas, 435 pp.

Earle S.A. 1972. Benthic Algae and Seagrasses. Pp. 15-18, Appendix III. Serial atlas of the marine environment. Am. Geographical Soc., New York.

Eiseman N.J. & S.M. Blair. 1982. New records and range extensions of deepwater algae from East Flower Garden Bank, Northwestern Gulf of Mexico. Contrib. Mar. Sci. 25:21-26.

Fredericq S., N. Phillips & B. Gavio. Observations on the macroalgae inhabiting deep-water hard bank communities in the Northwestern Gulf of Mexico. Gulf Mexico Sc. (in press).

Rezak R., Bright T. J. & D. M. McGrail. 1985. Reefs and Banks of the Northwestern Gulf of Mexico. John Wiley & Sons, NY, 259 pp.

Schneider C. & R. B. Searles. 1991. Seaweeds of the Southeastern United States. Duke Univ. Press. 554 pp.

Swofford D.L. 1998. PAUP*. Phylogenetic Analysis using Parsimony (*and other methods). Version 4.0. Sinauer, Sunderland, MA.

Taylor W. R. 1960. Marine algae of the Eastern Tropical and Subtropical Coasts of the Americas. Univ. Michigan Press, Ann Arbor, MI, 870 pp.

Wynne M. J. 1998. A checklist of benthic marine algae of the tropical and subtropical western Atlantic: first revision. Beih. Nova Hedwigia 116,155 pp.