David Alfaro Siqueiros Beltrones

Structure of diatom assemblages living on prop roots of the red mangrove (Rhizophora mangle) from the west coast of Baja California Sur, México

ABSTRACT Samples of epiphytic diatom assemblages found on prop roots of red mangrove (Rhizophora mangle) from four sites were collected at two tidal levels of exposure on two different dates from the cool season (autumn, spring). The overall floristic analysis yielded 171 diatom taxa, including 16 new records for the Baja California peninsula. Diversity estimates were among the highest ever measured for benthic diatoms using Shannons (mean H′ = 4.89) and Simpsons (0.945) indices. Many species scored high on the Biological Value Index (BVI), thus reflecting their numerical importance within the assemblages. According to diversity values and overall species composition, all sampling sites represented a similar habitat consisting of three types of substrates: prop roots, epiphytic macroalgae, and sediments. These values are considered to represent stability in the diatom assemblages living on red mangrove prop roots. Thus, the conditions determined by periodic tidal exposure are not to be considered extreme. However, principal component analysis and similarity measurements indicated that the November and April assemblages could be discriminated on the basis of taxa distribution.

Recent observations supporting the hypothesis of the colonization of thrombolite platforms by mangroves

Background. Thrombolites are sedimentary structures formed by precipitation, consolidation, and lithification of sediments generated by the evolution of cyanophyte mats that are responsible for coastal ground formation. Mangroves, however, are opportunist that colonize said substratum, in this case thrombolite platforms. Goals. To provide recent evidence of mangrove colonization of thrombolite platforms. Methods. Photographic images were taken of specimens of mangroves found fixed on the first thrombolite platform described in 2005. The platform was examined and samples were collected to analyze their mineralogical composition. Results. Evidence was gathered of two mangrove species colonizing a thrombolite platform. Specimens that had reached significant heights, indicating healthy juveniles, were found fixed on the first thrombolite platform. Collected platform structures were identified as rhizoliths belonging to mangrove trees. Conclusions. Evidence is presented that, besides this colonizing event, an earlier colonization on a thrombolite platform occurred at an inferior level. A secondary hypothesis that micrite would be a significant component of the rhizolith structure was proven through the mineralogical analysis of rhizolith fragments.