Clam harvesting decreases the sedimentary carbon stock of a Zostera marina meadow

Seagrass meadows provide ecosystem services that contribute to climate mitigation and ecosystem resilience in coastal environments, being recognised among the most effective carbon sink ecosystems on Earth. Although seagrass meadows are declining worldwide at alarming rates, direct measurements of the consequences of habitat degradation on the sedimentary carbon stock remains still scarce. The aim of this study is to investigate the effects of the physical disturbance caused by clam harvesting on the capacity of a subtidal Zostera marina meadow to accumulate organic matter and sequester carbon. Biotic (seagrass cover, density and biomass) and abiotic factors (water depth and sediment grain size distribution) of the studied meadow were characterized and carbon stocks in control and impacted areas of vegetated and bare sand zones were compared. The physical disturbance resulted in a significant reduction in shoot density (63%) and biomass (64%) in the impacted area with respect to the adjacent zone not affected by the disturbance, whereas the sedimentary carbon stock was reduced by 50%, reaching similar levels to those recorded in un-vegetated areas. The meadow exposed to the harvesting activity showed a decreasing capacity to sequester carbon by reducing the seagrass standing stock and carbon preservation in the associated sediments. Thus, clam harvesting activity not only eroded the historical carbon stock accumulated over decades but also endangers further potential accumulation. Therefore, sustainable management of the exploited area should take into account not only the durability of clam stocks but also the resilience of the seagrass meadow and its capacity to provide critical ecosystem services.