Abstract
Abstract. It is generally known that the interplay between the carbon and nutrients supplied from subsurface waters via biological metabolism determines the CO2 fluxes in upwelling systems. However, quantificational assessment of such interplay is difficult because of the dynamic nature of both upwelling circulation and the associated biogeochemistry. We recently proposed a new framework, the Ocean-dominated Margin (OceMar), for semi-quantitatively diagnosing the CO2 source/sink nature of an ocean margin over a given period of time, highlighting that the relative consumption between carbon and nutrients determines if carbon is in excess (i.e., CO2 source) or in deficit (i.e., CO2 sink) in the upper waters of ocean margins relative to their off-site inputs from the adjacent open ocean. In the present study, such a diagnostic approach based upon both couplings of physics–biogeochemistry and carbon–nutrients was applied to resolve the CO2 fluxes in the well-known upwelling system off Oregon and northern California of the US west coast, using data collected along three cross-shelf transects from the inner shelf to the open basin in spring/early summer 2007. Through examining the biological consumption on top of the water mass mixing revealed by the total alkalinity–salinity relationship, we successfully predicted and semi-analytically resolved the CO2 fluxes showing strong uptake from the atmosphere beyond the nearshore regions. This CO2 sink nature primarily resulted from the higher utilization of nutrients relative to dissolved inorganic carbon (DIC) based on their concurrent inputs from the depth. On the other hand, the biological responses to intensified upwelling were minor in nearshore waters off the Oregon–California coast, where significant CO2 outgassing was observed during the sampling period and resolving CO2 fluxes could be simplified without considering DIC/nutrient consumption, i.e., decoupling between upwelling and biological consumption. We reasoned that coupling physics and biogeochemistry in the OceMar model would assume a steady state with balanced DIC and nutrients via both physical transport and biological alterations in comparable timescales.
Highlights
The contemporary coastal ocean, characterized by high primary productivity due primarily to the abundant nutrient inputs from both river plumes and coastal upwelling, is generally seen as a significant CO2 sink at the global scale (Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Laruelle et al, 2010; Borges, 2011; Cai, 2011; Dai et al, 2013)
Instead of accounting for all of the water masses contributing to the California Current (CC) system, the mixing scheme in the upper waters along the three transects was examined via the total alkalinity–salinity (TAlk–Sal) relationship obtained during the sampling period so as to quantify the conservative portion of dissolved inorganic carbon (DIC) and nitrate (NO3)
The semi-analytical diagnostic approach of mass balance that couples physical transport and biogeochemical alterations was well applied to the CO2 sink zones off Oregon and northern California in spring/early summer 2007, extending from the outer shelf to the open basin
Summary
The contemporary coastal ocean, characterized by high primary productivity due primarily to the abundant nutrient inputs from both river plumes and coastal upwelling, is generally seen as a significant CO2 sink at the global scale (Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Laruelle et al, 2010; Borges, 2011; Cai, 2011; Dai et al, 2013). On the other hand, Evans et al (2011) suggest that the spring/early summer undersaturated pCO2 conditions in some offshore areas result from non-local productivity associated with the Columbia River (CR) plume, which transports ∼ 77 % of the total runoff from western North America to the Pacific Ocean (Hickey, 1989) In this context, the Oregon–California shelf in the upwelling season could be a potential OceMar-type system, with the majority of DIC and nutrients in the upper layer originating from the non-local deep waters in the subtropical gyre of the eastern North Pacific (eNP), though riverine inputs might complicate the application of the OceMar framework. The upper waters in offshore areas beyond the upwelling circulation on the Oregon–California shelf would be largely fed by on-site deep waters via vertical mixing, with minor influence of the CR plume
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