Winter CO2 temporal variations in a northern temperate bay: role of biological processes.

Accurate estimates of coastal air-sea CO2 fluxes require explicitly accounting for the physical and biological controls on sea surface CO2 partial pressure (pCO2). In many mid-latitude nearshore systems, active winter biological activities can strongly influence pCO2 temporal variations, but its role remains poorly constrained. Using two winter cruises in January and February 2024 in the Jiaozhou Bay (North China), we show that mean surface pCO2 rose by ∼50 μatm from January to February. This increase was mainly associated with reduced biological production, as indicated by declines in chlorophyll a and rises in apparent oxygen utilization. A one-dimensional diagnostic model partitioned the drivers of the pCO2 increase and attributed 43 % of the total absolute contributions to biology and 21 % to atmospheric CO2 uptake, while mixing and temperature lowered the total absolute contributions by ∼14 % and ∼18 %, respectively. The increased surface pCO2 in February reduced CO2 uptake, although a higher mean wind speed in February enhanced gas transfer velocity. Comparison with four earlier January-February surveys reveals substantial interannual differences mainly driven by winter varying biological activities, e.g., temperature-normalized pCO2 was 306 μatm on 16 January 2024 versus 398 μatm on 8 January 2012, with chlorophyll a of 1.76 and 0.64 μg L-1, respectively. This work demonstrates that rapid winter variations in biological activities can therefore markedly alter surface pCO2 and introduce major uncertainty in winter coastal CO2 flux estimates. Thus, future high-frequency winter sampling and coupled biogeochemical-hydrodynamic modeling will be vital to reduce uncertainty in regional carbon flux assessments.