Anthropogenic destabilization of sediment carbon pools threatens the long-term climate mitigation potential of temperate seagrass meadows

Abstract

Anthropogenic pressures, including nutrient overloading and physical disturbances, are major drivers of persistent seagrass loss and threaten the blue carbon sink function of seagrass ecosystems. While the role of seagrass meadows in alleviating climate change is widely recognized, the vulnerability of sediment organic carbon (Corg) pools remains poorly understood. Here, we investigate the influence of sediment nutrient enrichment and physical sediment disturbances (sediment reworking, e.g., digging and clamming) on litter decomposition, sediment Corg stocks, microbial extracellular enzyme activities, and bacterial community structure in a temperate Zostera marina meadow. Sediment nutrient enrichment substantially accelerated Z. marina litter decomposition, resulting in 17.3-30.7 % reductions in litter mass, 17.5-30.1 % declines in litter-derived Corg accumulation, and 15.0-17.4 % losses in sediment Corg stocks compared to the control plot (natural condition) after one year. These changes are primarily driven by elevated microbial extracellular enzyme activities and a shift toward copiotrophic bacterial assemblages. Physical sediment disturbances exposed buried Z. marina litter to oxic conditions at the sediment-water interface, promoting its decomposition by 5.8-28.9 % and reducing litter-derived Corg accumulation by 34.6-45.2 %, ultimately intensifying below-ground Corg losses. However, no significant decline in sediment Corg stocks was observed in physically disturbed sediments, likely due to the limited frequency and intensity of the disturbances. Nevertheless, these results still highlight a latent risk of long-term sediment carbon destabilization under chronic or large-scale physical disturbances. Our findings reveal that anthropogenic disturbances substantially weaken the long-term blue carbon sink capacity of seagrass meadows, emphasizing the need to protect sediment carbon pools under growing human and climate pressures to support future climate mitigation efforts.