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Rhodolith growth and CaCO 3 production remain poorly quantified along the SW Atlantic Ocean, and it is difficult to relate the available measurements with biomass estimates. Suboptimal conditions may clarify how harsh environments influence nodule growth and abundance, elucidating their relationship. Off the energetic South American equatorial coast a rhodolith bed (~65 km2), formed mainly by Mesophyllum sp. and Lithophyllum sp., alters the regional sedimentary pattern and sustains a diverse biota. Its nodules present fast growth rates (2.8 mm year−1), but small biomasses (18 nodules∙m−2 covering 26 ± 3% of the substrate), resulting in a CaCO 3 production of 163.33 g m−2∙year−1. Despite the small biomass, the bed seems stable, with living and dead nodules both on and inside the substrate. And the suboptimal environment apparently affects growth and abundance independently. Therefore, fast growth rates and relevant structural roles are not necessarily associated with dense rhodolith assemblages, and ecological assessments of rhodolith beds should consider the dynamics of both individual nodules and the whole population. • A rhodolith bed under harsh equatorial environmental conditions is investigated. • The bed has a small rhodolith density but exhibits very fast nodule growth rates. • Rhodoliths sustain a rich biota and affect sediment budgets even at low densities. • Stable and productive rhodolith beds may form even under such suboptimal conditions. [ABSTRACT FROM AUTHOR]

Multiple human stressors affect tropical intertidal sandstone reefs, but little is known about their biodiversity and the environmental impacts of these stressors. In the present study, multiple anthropogenic pressures were integrated using the relative environmental pressure index (REPI) and related to benthic community structure across an intertidal gradient in five sandstone reefs in the tropical South Atlantic coast. Greater species richness and diversity were noted in the low intertidal zones. There was a negative relationship between REPI and species richness, suggesting that increasing anthropogenic pressure has decreased benthic richness. The factors associated with the loss of richness were jetties built to control erosion, urban areas, beachfront kiosks and restaurants, fish markets, and storm sewers with illegal sewage connections. Our results highlight the need for better infrastructure planning and rigorous monitoring of coastal urban areas, since the large influence of multiple human pressures in these reefs leads to biodiversity losses. [ABSTRACT FROM AUTHOR]

Coral reefs in turbid waters have been hypothesized to be a refuge from climate change. These naturally occurring communities were brought into the spotlight because some of their species exhibited record levels of resistance to marine heatwaves (MHWs) by disturbance-tolerant corals. However, long-term monitoring data on the drivers of coral bleaching in these extreme reef habitats are scarce. Here, we describe the population structure and bleaching rates of a widespread and resilient coral (Siderastrea stellata). We examine the links between environmental factors, namely, rainfall, wind speed, turbidity, solar irradiance, sea surface temperature, MHWs, and coral bleaching status under the worst recorded drought cycle in the Tropical South Atlantic (2013–2015). We examined 2880 colonies, most of which (∼93%) fit in the size group of 2–10 cm, with a small number (∼1%) of larger and older colonies (>20 cm). The results indicated the absence of MHWs and normal sea surface temperature variations (between 26.6 °C and 29.3 °C), however, we detected an extreme rainfall deficit (30–40% less annual volume precipitation). In general, a high proportion (44–84%) of bleached colonies was found throughout the months when turbidity decreased. Siderastrea is the only reef-building coral that comprises this seascape with encrusting and low-relief colonies. During drought periods, cloudiness is reduced, turbidity and wind speed are reduced, and solar irradiance increase, driving coral bleaching in turbid reefs. However, episodic rainfall and higher wind speeds increase turbidity and decrease coral bleaching. Our hypothesis is that turbidity decreases during drought periods which increases bleaching risk to corals even without thermal stress. Our results suggest that turbidity may have related to wind and rainfall to provoke the coral bleaching phenomenon. • Turbid reefs have been hypothesized to be refuge from climate change. • Absence of heatwaves and normal sea surface temperature variations. • Extreme rainfall deficit (30–40% less annual volume precipitation). • High proportion of bleached colonies was found when turbidity decreased. • Extreme rainfall and higher wind speeds increase turbidity and decrease bleaching. [ABSTRACT FROM AUTHOR]