Incorporating benthic microbial thresholds into ecological carrying capacity to sustain ecosystem services of coastal...

Oyster aquaculture provides crucial ecosystem services by mitigating coastal eutrophication. However, intensive farming frequently leads to benthic organic overloading, which threatens this bioremediation capacity. Current Ecological Carrying Capacity (ECC) assessments focus on the interaction between yield and pelagic metrics, leaving a critical management loophole regarding benthic sediment health. To address this gap, we conducted a large-scale benthic environmental and metagenomic investigation across five intensive oyster (Crassostrea gigas) farming areas in the Shandong Peninsula, China. Our results revealed that biodeposit-driven organic loading promoted total sulfur (TS) accumulation, triggering a non-linear functional regime shift in the benthic nitrogen cycle. Breakpoint analysis identified a critical threshold at a sedimentary TS concentration of 0.89 g kg^-1, beyond which the denitrification was redirected toward dissimilatory nitrate reduction to ammonium (DNRA), concurrently elevating the risk of greenhouse gas (N₂O) emissions. Crucially, a profound spatial decoupling was observed between macroscopic farming yield and benthic micro-ecological status. Shallow-water areas with low yields suffered severe benthic degradation, whereas deep-water areas sustaining highly intensive yields maintained robust eutrophication mitigation functions. This paradox underscores the decisive role of the ecosystem's assimilative capacity over absolute farming load. These findings challenge the traditional yield-focused Ecological Carrying Capacity (ECC) assessments. We therefore advocate for incorporating thresholds of microbial-driven biogeochemical potentials into the ECC management framework to ensure the holistic sustainability of coastal aquaculture.