Estuarine ecosystems play a pivotal role in the global carbon cycle, but face mounting pressures from eutrophication and acidification. The high-resolution scientific assessment of the carbon storage capacity and sequestration efficiency in the Guangdong-Hong Kong-Macao Greater Bay Area (Greater Bay Area) has significant practical value.

Recently, a research team led by YAO Hongming and ZHU Ming from the Shenzhen Institute of Advanced Technology of the Chinese Academy of Sciences revealed the spatiotemporal distribution of carbon "source-sink" patterns and their biogeochemical driving mechanisms in the Greater Bay Area (Pearl River Estuary). The findings were published in Water Research and Limnology and Oceanography Letters, respectively.

Through high-resolution underway observations and multi-parameter biogeochemical sampling from the Greater Bay Area Blue Carbon Monitoring Platform, researchers systematically characterized carbon flux dynamics the Lingdingyang Estuary at the inner area of Pearl River Estuary. They elucidated microscopic mechanisms underlying carbon cycle "leakage" in the Pearl River Estuary, providing the first empirical evidence of synergistic "acidification-nitrification" coupling driven by eutrophication.

Researchers revealed that unlike conventional assumptions of uniform carbon source or sink behavior, estuary functioned as a "spatially partitioned carbon processor" sculpted by river-ocean interactions. The upstream river-influenced zone served as a substantial CO₂ source with peak efflux rates reaching 291.6 mmol C·m^-2·d^-1 during flood periods, while the downstream ocean-influenced zone transitioned to a CO₂ sink where net autotrophic processes reduced fluxes to -48.6 mmol C·m^-2·d^-1.

This source-to-sink transition manifested at a critical spatial threshold approximately 40 km downstream from the river mouth, reflecting the integrated regulatory impact of hydrological mixing dynamics and nutrient utilization efficiency. Moreover, the blue carbon system exhibited pronounced seasonal variability: flood season carbon fluxes substantially exceeded those during the dry season with enhanced terrestrial inputs serving as the primary regulatory mechanism for carbon release.

Despite substantial carbon fixation through phytoplankton photosynthesis, researchers found that 40-60% of the fixed carbon was rapidly remineralized to the atmosphere via heterotrophic respiration. This "production-consumption coupling" created a carbon recycling loop that significantly diminished the net carbon sequestration capacity of the estuarine ecosystem. These findings suggest that carbon sink assessments relying solely on chlorophyll concentrations or satellite-derived proxies may systematically overestimate actual sequestration rates.

Under chronic eutrophic conditions, the Pearl River Estuary has experienced acidification rates exceeding those of open ocean waters. Researchers identified a pronounced acidification-nitrification coupling hotspot in the mid-Lingdingyang region, spatially coincident with the 40 km source-sink transition zone. In the hotspot, ammonia-oxidizing microorganisms metabolized ammonium as substrate during nitrification, releasing protons that depleted alkalinity and depressed pH, thereby generating a localized acidification center.

This study provides insight into the declining carbon sequestration efficiency in waters of the Greater Bay Area, and elucidates a novel framework for developing eutrophication mitigation strategies and enhancing regional carbon sink capacity.