Beyond rhodoliths: Community‐level carbon metabolism and calcification in coralline algal beds

Abstract

Coastal ecosystems play a major role in global carbon cycling and storage, yet the contribution of calcifying macroalgal habitats, such as coralline algal beds (CABs; rhodolith and maerl beds), remains poorly understood. Because carbon cycling in CABs is governed by community productivity and calcification, which exert opposing effects on seawater carbonate chemistry, understanding the balance between these processes is essential for determining whether CABs function as net carbon sinks or sources. Yet, community‐level measurements remain scarce, and the environmental and biological drivers of CAB carbon balance are largely unknown. Here, we quantified net community productivity (NCP), net community calcification (NCC), and the resulting net carbon balance (NCP : NCC) in shallow warm‐temperate and subtropical Atlantic CABs with contrasting rhodolith‐associated communities, using in situ benthic chamber incubations. All CABs were net autotrophic, with NCP : NCC ratios ≥ 0.6, indicating that they acted as potentially carbon neutral and net carbon sinks. By combining our measurements with available community‐level datasets, we identified light availability as the primary environmental driver of community productivity and calcification, whereas benthic community structure influenced the balance between these processes. Together, our findings demonstrate that CAB carbon balance is governed by the interaction between environmental conditions and community structure, providing the first mechanistic insight into the drivers of ecosystem carbon balance. This pronounced context dependency highlights the need for community‐level assessments across broader environmental and biological settings to test the generality of these relationships and determine under which conditions carbon burial in CABs translates into a net contribution to coastal carbon storage.

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