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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]