Marine bacterial communities show strong spatial and seasonal patterns, often characterized by changes at high taxonomic levels. The Pelagibacteraceae are common members of bacterial communities, with well-documented biogeography at the subclade level. To identify patterns within the subclades, the abundance and diversity of Pelagibacteraceae were analyzed over a two-year period at four stations across an estuarine gradient. Pelagibacteraceae was the most abundant bacterial family, averaging 27% of the community, but varying from 1% to 57% in any one sample. Highest abundances were detected in autumn and winter. Pelagibacteraceae richness was lowest at the most inshore site, and highest in autumn and winter at all sites. Shannon diversity decreased in winter, when a few OTUs dominated the community. Dissolved oxygen, dissolved silicate and prokaryote abundance explained most of the variability in the Pelagibacteraceae communities, with salinity differentiating low salinity communities. The 10 most abundant OTUs included OTUs that varied across sites, with little seasonality as well as those with small site effects, but strong seasonal patterns indicating differences in the niches of individual OTUs. While salinity was important in structuring low salinity communities, higher salinity communities appear to be responding to additional environmental parameters including oxygen, nutrients and other organisms.

The present study quantifies mercury (Hg) concentrations in mangrove oysters (Crassostrea rhizophorae) and assesses their potential as biomonitors of Hg contamination in the Parnaíba River Delta (PRD), located on the equatorial coast of Brazil (ECB). The highest Hg concentrations occurred in the smallest individuals’ size class (20–40 mm) from the main channel of the Parnaíba River (52.1 to 195.4 ng g⁻¹ w.w.), whereas the largest individuals’ size class (larger than 60 mm) exhibited the lowest Hg concentrations (35.2–114 ng g⁻¹ w.w.). There was a significant correlation between Hg concentrations and shell sizes, either when considering all size classes or when considering only individuals of size classes smaller than 40 mm. Oysters larger than 40 mm did not present any significant correlation between Hg concentrations and size. In addition to size, higher concentrations were observed at the freshwater–seawater transition in the main channel sites. These maximum suspended particulate zones, with bottom sediment resuspension, can favor Hg adsorption to fine particles, increasing the bioavailability of Hg. A regional comparison of Hg concentrations in mangrove oysters from the ECB suggests they are efficient biomonitors at a regional level. In contrast, the environmental dynamics of the PRD, with high variability within sites, hampered its use at the local level. Mangrove oysters from the PRD are shown to be safe for human consumption, as far as Hg exposure is concerned, and the presented risk assessment shows no excessive exposure, even at high-frequency consumption rates.