Abstract

Coastal waters play a crucial role as a pathway for the transport of terrestrial chemical constituents to marine environments. Carbon cycling in coastal areas is essential for understanding climate change. Here, we used a three-dimensional hydrodynamic-ecosystem model to investigate CO2 dynamics and its controlling factors in Lake Obuchi, a brackish lake in Aomori Prefecture, Japan. The model successfully reproduced the seasonal variations in water temperature, salinity, nutrients, chlorophyll-a, and carbonate system components in the lake. The simulation results show that the partial pressure of CO2 (pCO2) in Lake Obuchi remained below atmospheric levels throughout the year, suggesting that the lake functions as a CO2 sink. The pCO2 variability is dominated by spatially uniform seasonal patterns (90% of variance) driven by temperature changes, while summer periods exhibit distinct spatial patterns (6.0% of variance) that vary with river discharge magnitude. In Lake Obuchi, pCO2 is maintained at low levels because of the carbonate system components of water masses formed by mixing river water and seawater. Additionally, primary production within these water masses further reduces pCO2. This primary production is sustained by nitrogen-rich river water, combined with seasonal phosphorus supply from sediments during summer and autumn and from the sea during winter and spring. Our findings reveal that the seasonal combination of these nutrient sources maintains year-round primary production, which in turn contributes to the reduction in pCO2 in Lake Obuchi.