Ortiz Prosper, Antonio l.

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    Population dynamics of hurricane-generated fragments of elkhorn coral Acropora palmata (Lamarck, 1816)
    (2005-05) Ortiz Prosper, Antonio l.; Weil, Ernesto F.; College of Arts and Sciences - Sciences; Aponte, Nilda E.; Yoshioka, Paul M.; Bird Picó, Fernando; Aponte, Nilda E.; Department of Marine Sciences; Alfaro, Mónica
    Fragmentation and local dispersion of hurricane-generated fragments have been considered an important feature in the life history, population dynamics and genetic structure of the elkhorn coral Acropora palmata. The significance of fragmentation as a strategy of this species to maintain local populations in the time of their recent decline was assessed by studying the dynamics and size structure of Hurricane Georges (1998) generated fragments in three reefs (Media Luna Reef, Laurel Reef and San Cristóbal Reef) within La Parguera coral reef system on the southwestern coast of Puerto Rico. Two belt transects (30 m long and 2 m wide) were established across the largest patch of living A. palmata in each reef. All fragments with living tissue were counted, labeled and measured. Additional information collected included: 1) maximum length and width of living tissue on the fragment at three time intervals (one month, one year and three years after fragmentation), 2) type of reef substrate where the fragment landed (hard bottom, rubble, sand, live standing coral), and 3) condition of the fragment (loose, slightly stable or cemented to the substrate) was recorded in time. No significant difference in mean fragment densities was observed among reefs (p> 0.05; Tukey- Kramer). Fragment abundance after a year was 369 (1.02 ± 0.35 fragments/m2). No significant relationships were observed between fragment density and coral cover or with the types of reef substrates observed in each transect. However, there was a significant difference (p< 0.05, Tukey-Kramer) in mean fragment size (length, width and surface area) among reefs. The mean overall mortality within the first year was 28.24 ± 3.68 % (n= 151 fragments from San Cristóbal Reef only), after 3 years was 47.96 ± 17.71 % (n= 254 fragments from all reefs). Fragment mortality at San Cristóbal Reef was significantly lower (22.92 ± 10.42 %) than at Media Luna and Laurel Reefs (60.54 ± 6.13 %, 60.43 ± 8.43%, respectively; p< 0.025, ANOVA). Mean percentage of colonies with partial tissue mortality in the first year was significantly higher (85.92 ± 4.94 %) compared to 3 years (50.81 ± 12.53 %; p< 0.05, t-test). The mean percentage of loss tissue per fragment was 61.74 ± 9.97 % after the first year, and did not increase after 3 years (57.56 ± 6.69 %; p>0.05, t-test). After three years, loose fragments were significantly (p< 0.05, Tukey-Kramer) more abundant than cemented fragments, and the major mode of cementation was by means of coralline algae calcification (average= 29.92 %, n= 254 fragments). No significant differences were found in mean fragment survivorships across different conditions (loose, slight stable or cemented fragments). Substrates had a moderate effect on the survival rate of the fragments within the first year, but after three years, a significant (p< 0.05, Tukey-Kramer) lower survivorship was observed in those fragments that landed over sand compared to those that landed over hard substrates and standing live colonies. A positive and significant relationship (Regression, p< 0.01; R2= 0.63-0.77) was observed between fragment size (length and surface area) and mortality with a possible threshold size (20-25 cm length) above which fragments are less likely to die in the first year. While initial fragment size was an important factor determining the early survival of the ramet, the effect of continuous tissue loss by partial mortality (disease, predation, sedimentation) produced fragments with reduced size, affecting future survival. The predicted decline of populations by the fragment size-based models suggests a complex response of populations to recent environmental change and may be related to the recent demise of this species in the wider Caribbean.