Proceedings of the 11 th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7-11 July 2008 Session number 26 Preliminary Results On Marine Algae Of Madagascar Reef, Yucatan, México: a Functional Group approach. I. Ortegon-Aznar 1 , H. Leon-Tejera 2 , M. Gold-Morgan 3 and N. Ramírez-Miss 1 1) Marine Biology, UADY, Merida, Mexico 2) Herbario de la Facultad de Ciencias, UNAM, D. F. México 3) Laboratorio de Ficología, Facultad de Ciencias, UNAM, D. F. México Abstract. The abundance and distribution of marine algae was recorded for the first time in Madagascar Reef, a site that may be part of the connection between the Caribbean and the Gulf of Mexico algal communities. The objective of this study is to generate the base-line information on macroalgal functional forms (MFGs) distribution and abundances that can serve to develop a monitoring program for this community. We used the Steneck and Dethier (1994) functional-form system modified. MFGs were examined on 4 sites with three different substrates. The abundance of MFGs and taxa was recorded using a modification of the Braun-Blanquet method. Community differences were analyzed with regard to MFGs and algal taxa distribution. The macroalgal assemblages were found to be variable at the MFG level and widely distributed, except for some of the more complex ones, which tended to be absent in the more continuous rocky substrate. While community analysis at both, MFG and taxa levels revealed similar trends in relation to the type of substrate, we were able to detect important differences in the number of genera that constitute these MFGs. The most diverse site concerning taxa and MFGs composition is probably associated with a more heterogeneous substrate. Key words: Macroalgae, Functional Forms, Gulf of México. Introduction The functional form model is generally used to understand ecological patterns in the diversity of the macroalgal flora. This model is based on the idea that a form-function relationship can be used to interpret distributional patterns in relation to different environmental factors (Littler and Littler 1984). Different models of functional-form groups have been proposed (Littler and Littler 1980; Steneck and Watling 1982; Steneck and Dethier 1994). For this study we used the Steneck and Dethier (1994) system but separated the green calcareous from the coenocytic, as proposed by Collado-Vides et al. (2005) and Phillips et al. (1997), to distinguish conspicuous algae such as Caulerpa, Codium from Halimeda and other cenocytic algae, taking into consideration the local flora composition and morphology. Madagascar Reef is located between the more structurally complex communities of both the Caribbean and the Gulf of México, therefore the study of this site could help answer questions about connectivity in the flora of the two biogeographical regions. The objective of this study is to generate the base- line information on taxa and macroalgal functional forms distribution and abundances that can serve to develop a monitoring program for this peculiar community. Material and Methods Study Area Madagascar Reef is located at the Northwest of the Yucatan Península, north of the port of Sisal (Fig.1). It forms part of the Inner Campeche Bank Reefs. It is an underwater extension of the Yucatan Peninsula that slopes gradually northwards for about 200 km and descends abruptly into the Sigsby Deep. Four sites were selected. Site 1 was located at 21°26´26.1´´N and 90°16´55.2´´W, had 24ºC, was 15-17.5m deep and had a sandy substrate; Site 2 was located at 21°26´19.2´´N and 90°16´56.5´´W, had 26ºC, was 7-11m deep and had a mixed rocky-sandy substrate; Site 3 was located at 21°26´19.2´´ N and 90°16´42.3´´W, had 27 ºC, was 7-12m deep and had a rocky substrate; Site 4 was located at 21°26´29.6´´N and 90°17´36´´W, had 24ºC, was 15- 17.5m deep and had a rocky substrate. Field techniques Between August and September of 2007 four sites were collected along the reef. Two 10m transects marked at 2m intervals were used at each site, with a 25 x 25 cm quadrant placed every two meters (to the 1373