Size and distribution of rhodolith beds in the Loreto Marine Park: their role in coastal processes

Abstract

Size, species, growth form, and rhodolith-bed distribution in the Loreto Marine Park were evaluated, based on sampling undertaken in June and December 2009 and 2010. Extensive surveys on nine major beds were performed using global positioning system and scuba methodologies with the goals of determining (1) the size and distribution of rhodolith beds along the central Loreto Marine Park area, (2) the physical structure of the beds, and (3) the species and growth-form composition. We found a total of 732.8 ha of the seafloor covered by rhodolith beds, of which several islands (Coronado and Carmen) and some coastal areas (between Ensenada Blanca and Puerto Escondido) represented the most extensive accumulations. Most of the studied beds were composed by living material, but in the bed located off the south end of Carmen Island a ratio of 50:50 living-to-dead rhodoliths was discovered. Three species were found commonly to compose the beds (Lithophyllum margaritae, Neogoniolithon trichotomum, and Lithothamnion muellerii), whereas only two growth forms were present (fruticose and incrusting) dominating the beds. Branch density, rhodolith density, and sphericity were found to be similar in all sampling areas. This suggests that the various beds are related to main currents within the park. However, incrusting growth forms and densely branched fruticose plants were only present in the bed from the Danzante area. This suggests that the presence of a rocky bottom is relevant to the composition of growth forms. Results of this study strongly suggest that rhodolith beds contribute to coastal processes by constantly producing sand derived from broken material. In addition, the extensive cover by rhodoliths is significantly high for the shallow areas surveyed. Future surveys into deeper waters and other areas to more fully understand the role of rhodoliths and their taphonomy in coastal areas are needed.