Distinguishing between physical and biological controls on the spatial variability of pCO2: A novel approach using OMP water mass analysis (St. Lawrence, Canada)

Present-day air-sea CO2 flux estimates in the coastal ocean are subject to large uncertainties due to its heterogeneous nature and concomitant lack of data. Factors controlling the dissolved inorganic carbon (DIC) and CO2 fluxes vary within and between coastal subsystems, hampering the development of robust upscaling and modeling techniques. By applying a multi-tracer, quantitative water mass analysis, physical and biogeochemical factors can be differentiated. This study adopts an expanded version of optimum multiparameter (OMP) water mass analysis, an inverse modeling technique, to estimate the mixing fractions of predefined source water masses as well as the contribution of biological activity (photosynthesis, respiration) at a given observation point in the surface mixed layer that exchanges CO2 gas with the atmosphere. We apply the method to hydrographic, nutrient, and inorganic carbon data collected in the Estuary and Gulf of St. Lawrence, the world's largest estuarine system and an excellent analogue of the more general coastal environment. Biological activity is identified as the dominant control on mixed-layer CO2 partial pressure (pCO2) dynamics along the St. Lawrence land-ocean continuum, explaining the upstream to downstream shift from pCO2 supersaturation (net heterotrophy) to pCO2 undersaturation (net autotrophy). Although mixing of freshwater and seawater along the Estuary is the major contributor to the DIC pool, it contributes little (or negligibly) to the spatial variability of surface-water pCO2.