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The middle and outer continental shelves of eastern equatorial South America (ESA) are characterized by intense production of carbonate sediments. Qualitative analyses of sediment deposits suggest that the calcareous green alga Halimeda incrassata is among the top CaCO 3 producers. Nevertheless, no study so far has quantified its real contributions. To better understand the sediment dynamic in this area, we measured biomass, growth rates and calcium carbonate production by this alga. The species exhibited high growth rates (3.38 segments.individual −1 .day −1 ), coverage (174 individuals.m −2 ) and biomass (214.02 g.m −2 ). Substitution of segments may allow a sedimentation rate of 1.53 mm.yr −1 and a complete turnover of the population every 60.2 days. The rapid growth indicates that this alga can produce as much CaCO 3 (1.19 kg CaCO 3 .m −2 .year −1 ) as other tropical organisms, such as corals and rhodoliths. In a conservative estimate, 773.500 tonnes of CaCO 3 are produced per year in a 5000 km 2 area off the northern coast of Brazil. Sedimentation rate seems to be higher than that promoted by continental inputs in middle and outer continental shelf. On the other hand, population turnover is twice as slow as in other H. incrassata assemblages, suggesting that South American populations are sensible to physical disturbances. New studies are necessary to accurately estimate H. incrassata coverage along the Brazilian coast and to integrate data on other CaCO 3 producers, such as foraminifera and coralline algae. This would allow a better understanding of the role of South American continental shelf on the global carbon budget. Furthermore, analysis on the health of these organisms is urgent, since a decline in their populations could negatively affect ecosystems functioning and services. [ABSTRACT FROM AUTHOR]

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]