Decadal Variation in Net Carbonate Accretion and Early Successional Community Dynamics on Settlement Tiles in Two Abrolhos Reefs, Southwestern Atlantic

Marginal Southwestern Atlantic reefs, such as the Abrolhos reefs (Brazil), occur under conditions including high sedimentation, turbid waters, and elevated nutrient inputs, while also being exposed to local and climate-related stressors. Long-term standardized assessments of net calcium carbonate (CaCO3) accretion on settlement substrates remain scarce for marginal reefs. Here, we assessed annual net CaCO3 accretion and early successional community dynamics from 2012 to 2023 using standardized Calcification Accretion Unit (CAU) tiles deployed at two reef sites in the Abrolhos Bank, Brazil: Pedra de Leste (PL), located in the nearshore reef arc, and Parcel dos Abrolhos (PA), located in the offshore reef arc. Mean net CaCO3 accretion was 329 ± 94 g m−2 yr−1, reaching a maximum in 2019−2020 (433 ± 82 g m−2 yr−1) and declining in 2022-2023 (255 ± 75 g m−2 yr−1). At PA, net CaCO3 accretion in 2022−2023 was 52% lower than the decadal mean. The relative cover of calcifying and non-calcifying functional groups also varied through time at both study sites. At PL, non-calcifying cover reached 36% in 2017−2018 and 50% in 2022−2023, driven mainly by algal turfs and ascidians, while CCA cover declined. In 2022−2023, this shift coincided with a marked reduction in net carbonate accretion. In contrast, crustose coralline algae (CCA) dominated CAU tile communities at PL in 2019−2020 and coincided with the highest net carbonate accretion observed at that site. These patterns indicate that variation in early successional community composition, particularly in the relative abundance of CCA and turf algae, was associated with temporal variation in net CaCO3 accretion on settlement tiles. The recent decline observed in PA highlights the value of continued long-term monitoring to determine whether similar changes are reflected in established reef communities.