Sedimentology and temporal distribution of microbial mats from Brejo do Espinho, Rio de Janeiro, Brazil

Abstract

This study covered an annual cycle of the growth of microbial mats in Brejo do Espinho, which is a small lagoon in the State of Rio de Janeiro located near Lagoa Vermelha. Two developmental stages of the microbial mats were observed throughout the study period. These stages presented laminations with colours that varied between yellow, green and reddish brown hues. Their microfabric comprised peloids and micritic lumps, with a predominance of magnesian calcite and evidence of dolomite. The mats also contained other minerals, such as gypsum and halite. Energy dispersive X-ray spectrometer analysis also indicated the presence of a silicate associated with the organic matter and sometimes with carbonates. The first stage was characterised by a gelatinous texture, low mineral content and sparse populations of cyanobacteria that were located only in permanently flooded areas of the Brejo do Espinho. This structure occurred during the drought period and the beginning of the flood period. The second stage exhibited peloids and larger micritic lumps that sometimes formed thin carbonate layers and occurred in the infra- and mesocoastal regions of the lagoon. These structures were observed only during the lagoon drought period. Quantitatively, most represented cyanobacteria were Aphanothece Nägeli 1849 and Chroococccus Nägeli 1849 in both stages. The microbial mats of Brejo do Espinho developed only during the dry season until the start of the lagoon flooding period, which is characterised by shallow water, low turbidity, high salinity, stable substrate and virtually no terrigenous input conditions. In the flooding season, microbial mats were replaced by an intense expansion of Charophyceae algae. Thus, there is a seasonal development of microbial mats linked to extreme hydrologic conditions that limit the growth of other algae during certain periods at Brejo do Espinho.