ORIGINAL PAPER Demography of Halimeda incrassata (Bryopsidales, Chlorophyta) in a Caribbean reef lagoon Brigitta I. van Tussenbroek • M. Guadalupe Barba Santos Received: 6 October 2010 / Accepted: 25 February 2011 / Published online: 15 March 2011 Ó Springer-Verlag 2011 Abstract The fate of 100 marked recruits of the rhizo- phytic alga Halimeda incrassata was followed in Puerto Morelos reef lagoon, Mexican Caribbean (20°52 0 N, 86°51 0 W), until death of the last individual from April 2005–2007. Juvenile mortality was relatively high (19%), the half-life of adult thalli was 13 months and maximal lifespan was 2 years. First age of sexual reproduction was 10 months, but only 6% of the marked thalli reproduced sexually. A subsequent static life-table approach (Febru- ary–April 2008) at two sites indicated low spatial variation in transition probabilities between the life stages. Recruits were found throughout the year (density 2–11 thalli m -2 ) and were mostly of clonal origin. In a disturbed area, the density of sexual recruits was *0.01 thalli m 2 . Temporal fluctuations in population size depended on mortality rates, which increased slightly after hurricanes Emily (July 2005) and Wilma (October 2005), but might otherwise be regu- lated by density-dependent processes. Introduction The sandy bottoms of tropical reef lagoons in the Caribbean are commonly colonized by lush vegetations of rooted (rhizophytic) calcified algae associated with the seagrasses. The common genera are Halimeda, Penicillus, Rhipocephalus and Udotea belonging to the order Bry- opsidales (UNESCO 1998; Collado-Vides et al. 2005; Cruz-Palacios and Van Tussenbroek 2005). Occupation of sandy bottoms confers some peculiar characteristics to these rhizophytic algae in comparison with the majority of the macroalgae attached to hard substrata. Although the rooting structures of the rhizophytic algae are not true roots (such as those of the angiosperms) but consist of fibrous rhizoids, they can likely take up nutrients from the sedi- ments (McGlathery et al. 1992). The rhizophytic algae function as early colonizers facilitating later establishment of the seagrasses (succession through facilitation, Williams 1990). But, when a tropical seagrass community advances towards climax, these algae continue to co-exist with the seagrasses, which provide a relatively stable environment, in terms of sediment stabilization and nutrient recycling (Williams 1990; Van Tussenbroek et al. 2006a). In these tropical communities, the rhizophytic calcified algae are important primary producers and producers of calcareous sand (Bach 1979; Hillis 1997; Harney and Fletcher 2003; Neuman and Land 1975; Van Tussenbroek and Van Dijk 2007). Debris of their calcareous remains accumulate within the shallow reef systems or are exported to deeper waters (Neuman and Land 1975). These algae provide shelter and habitat to a diverse fauna of crustaceans, polychaetes, echinoderms and gastropods (Stoner 1985; Naim 1988; Hay 1997), but they do not constitute a pri- mary food source, because the CaCO 3 and their secondary metabolites deter most herbivores (Paul and Van Alstyne 1988; Schupp and Paul 1994; Hay 1997). Despite their ecological importance, surprisingly little is known about the population dynamics of these calcareous algae. It is known, however, that the abundance (cover or biomass) of the rhizophytic calcified algae show large temporal variations (Multer 1988; Payri 1988; Ballesteros Communicated by P. Ralph. B. I. van Tussenbroek (&) Á M. G. Barba Santos Unidad Acade ´mica de Sistemas Arrecifales/Puerto Morelos, Instituto de Ciencias del Mar y Limnologı ´a, Universidad Nacional Auto ´noma de Me ´xico, Apdo. Postal 1152, Cancu ´n, Quintana Roo, Mexico e-mail: vantuss@cmarl.unam.mx; bvantussenbroek@hotmail.com 123 Mar Biol (2011) 158:1461–1471 DOI 10.1007/s00227-011-1662-2