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.