Role of Pro-Thrombolites in the Geomorphology of a Coastal Lagoon.

v. ill. 23 cm.QuarterlyThrombolites are lithified biosedimentary structures generated by entrapment, precipitation, and binding of sediments promoted by growth and metabolic activity of cyanobacteria. Beaches of the coastal lagoon known as Ensenada de La Paz in Baja California Sur, Me´xico, are bordered by sedimentary formations of cyanobacterial origin identified as pro-thrombolites (incipient thrombolites) that represent a first record for the region and Me´xico. Observed thrombolithic structures show grains of varied sizes embedded within a fine-grain micritic matrix, which may be surrounded by medium-grain cementing micrite. Different degrees of consolidation occur—some crumble easily, whereas others require some manual force to break. These pro-thrombolites consist of platforms >20 cm thick and/or fragments of assorted sizes and forms. In some cases the structures have lithified, forming rocky plates (thrombolites). The extension and wide distribution of pro-thrombolites around the La Paz lagoon suggests that these structures could have determined its evolution from an original (primitive) cove into a lagoon. That is, the formation of prothrombolites through the entrapment and binding of sediments may have eventually altered water circulation, promoting sand sedimentation causing the formation of the El Mogote sand bar. Likewise, pro-thrombolites may have formed large extensions of headlands through accretion. Thus, several square kilometers of populated land around the La Paz lagoon may have thrombolithic origin

Epiphytic diatoms associated with red mangrove (Rhizophora mangle) prop roots in Bahía Magdalena, Baja California Sur, Mexico

The first floristic inventory of benthic diatoms is provided for the Bahía Magdalena- Bahía Almejas lagoon system. Samplings were carried out during November of 1999. The oxydized samples were mounted permanently. Eighty six diatom taxa were identified, out of which 59 are new records for the Bahía Magdalena area, and 12 taxa are new for the Baja California peninsula. Taxa recorded previously as rare in other substrata are common or abundant on the epiphytic macroalgae of mangrove prop roots. Other species are mainly epipelic forms, while 24 are commonly found as tychoplankton in the area. Certain taxa appear to be characteristic of mangrove systems in general. Rev. Biol. Trop. 54(2): 287-297. Epub 2006 Jun 01.Se presenta el primer inventario florístico de diatomeas bénticas para el Sistema Lagunar Bahía Magdalena-Bahía Almejas. Se hicieron muestreos en noviembre de 1999. En las muestras oxidadas y montadas permanentemente, se identificaron 86 táxones, de los cuales 59 son nuevos registros para el área de Bahía Magdalena, y 12 son nuevos para la península de Baja California. Táxones previamente registrados como escasos en otros sustratos son comunes o abundantes sobre macroalgas epífitas de raíces primarias de mangle. Otras species son principalmente formas epipélicas, mientras que 24 son constituyentes comunes del ticoplancton del área. Ciertos táxones parecen ser característicos de sistemas de manglar en general

Variations in phytoplankton assemblages during 1988-1989 in a subtropical lagoon sytem on the west cost of mexico

The total phytoplankton abundance of the Magdalena-Almejas lagoon system from 1988 to 1989 was found to be similar in surface and subsurface water samples. The pattern of sea surface temperature in the system, typical of a temperate lagoon, shows a well-defined warm season from July to October and a cool one from November to May. The maximum average values for total phytoplankton abundance and microplankton diversity (H') were recorded during the cool season, whereas the warm periods are characterized by low mean values. Nanoplankton (cells 2-20 μm) may contribute most to the total phytoplankton abundance throughout the year and dominate at the two sampling levels. Microplankton (cells > 20 μm) is abundant mainly during the cool season and reaches its largest abundances around May, when temperature increases. The study area is located in a subtropical transition zone which is influenced by tropical and temperate waters, which determines the presence of tropical and temperate species during the corresponding period. The 1988- 1989 period may be considered a normal year for the lagoon system, but the phytoplankton dynamics described in terms of our observations can not be considered typical of the system, where interannual variations in the structure of phytoplankton assemblages are determined by a complex interaction of hydrological factors. This variation is further affected by El Niño/La Niña events, which should be considered common factors determining the changes observed in the phytoplankton assemblages in this area, not anoma lies