Maerl Beds as Key Drivers of the Silicon Cycle in Coastal Ecosystems: A Case Study of the Bay of BrestBiogeochemical Cycling

María López-Acosta^1,2, Aude Leynaert², Jacques Grall²

¹Instituto de Investigaciones Marinas (IIM), CSIC, Spain - ²Institut Universitaire Européen de la Mer, France

Over the past decade, significant progress has been made in understanding the role of benthic silicifiers —particularly sponges and microphytobenthos— in the Bay of Brest (France), a coastal ecosystem dominated by maerl beds. These habitats provide substrate for a diverse array of benthic organisms, including benthic silicifiers, which are essential for the functioning of the silicon cycle and broader nutrient and ecological dynamics in the region. Maerl beds support the highest sponge abundance in the area (up to 92 sponges m-2; in average, 25 sponges m-2), contributing to a substantial silica stock. Additionally, microphytobenthos —microscopic photosynthetic organisms that colonize the maerl pieces— enhance silica remineralization, playing a key role in the silica pump that drives the community of planktonic diatoms. Thus, maerl beds promote the benthic-pelagic coupling of silicon, a key nutrient for marine primary producers, and helps mitigating nutrient imbalances that can lead to harmful algal blooms (HABs), which have caused severe ecological and economical issues in the area.

Recent research has also underscored the vulnerability of these ecosystems to anthropogenic disturbances. For example, the impacts of dredging activities in the bay cause significant disruptions to both sponge and microphytobenthos biodiversity, along with silicon cycling. Such impacts could exacerbate nutrient disequilibrium and increase the risk of HABs events. Our work emphasize the critical role of benthic silicifiers in maintaining ecosystem functioning and highlights the importance of effective conservation strategies to protect maerl beds from anthropogenic pressures.