Biogeochemical Cycling Of Carbon Monoxide In The St. Lawrence Estuarine System (Canada)

Carbon monoxide (CO) plays an important role in the marine organic carbon cycling. CO in the surface ocean is produced primarily from photolysis of chromophoric dissolved organic matter (CDOM) and is lost by microbial consumption and air-sea outgassing. CO photoproduction fluxes are reasonably constrained in open-ocean waters, but remain obscure in estuary and coastal areas. Compared to photoproduction, CO microbial consumption is less studied; thermal (dark) production of CO, another potentially important source of marine CO, has drawn little attention. This study systematically investigated the photoproduction, dark production, and microbial consumption of CO in the St. Lawrence estuarine system (Canada). A budget of CO biogeochemical cycling in the estuarine system was made. Based on the empirical equation developed from the estuary, annual CO dark production in global oceans and steady- state [CO] in deep open ocean were estimated. Furthermore, annual CO microbial consumption in global oceans was derived.For CO photoproduction, effects of water temperature and the origin and light history of CDOM on the apparent quantum yields of CO (Φco) was examined. The solar insolation-weighted mean apparent quantum yield of CO (Φco) decreased as much as fourfold with increasing salinity and showed a strong positive correlation with the dissolved organic carbon-normalized absorption coefficient at 254 nm. This suggests that terrestrial CDOM is more efficient at photochemically producing CO than is CDOM of marine origin. CDOM photobleaching, mainly at the very early stage, dramatically decreasedΦco (by up to 6.4 times) for low salinity samples, but had little effect on the most marine sample. For a 20oC increase in temperature,Φco increased by ~70% for low-salinity samples and 30-40% for saline samples. This study demonstrates that water temperature, as well as the CDOM's origin and light history, strongly affect the efficiency of CO photoproduction. These factors should be taken into account in modeling the photochemical fluxes of CO and other related CDOM photoproducts on varying spatiotemporal scales. An empirical equation was derived for predictiong the CO photoproduction efficiency in the St. Lawrence estuarine system. Annual CO photoproduction in the St. Lawrence estuarine system was estimated as 26.2 Gg CO-C a-1.The dark production rate of CO, Qco, in the water column of the St. Lawrence estuarine system decreased seaward horizontally and downward vertically. Qco exhibited a positive, linear correlation with the abundance of CDOM. As for photoproduction, Terrestrial DOM was also more efficient in the CO dark production than marine DOM. The temperature dependence of Qco can be characterized by the Arrhenius equation with the activation energies of freshwater samples being higher than those of salty samples. Qco remained relatively constant between pH 4-6, ascended slowly between pH 6-8 and then rapidly with further increasing pH. Ionic strength and iron chemistry had little influence on Qco. An empirical equation, describing Qco as a function of CDOM abundance, temperature, pH and salinity, was established. Dark production in the St. Lawrence estuarine system was estimated as 4.06 Gg CO-C a-1.The investigations of CO microbial consumption in the St. Lawrence estuarine system were conducted aboardship during the cruises of July 2004, October and December 2005, and May 2007. Kco showed an increasing trend along a salinity- gradient transect from the Gulf of St. Lawrence to Quebec City and to the organic-rich Saguenay River. Kco for surface waters ranged from 0.053 to 1.01 (median: 0.14) h-1 in spring, 0.081 to 0.60 (median: 0.27) h-1 in July, 0.035 to 0.71 (median: 0.13) h-1 in October, and 0.024 to 0.38 (mean: 0.032) h-1 in December. The major intra- and inter-seasonal variances of Kco can be accounted for by variations in water temperature and bacterial abundance. Kco displayed a moderate temperature dependence, increasing by ca. 40 to 80% (mean: 53%) per 10oC of increase in temperature. The activation energy was estimated to be 22-38 (mean: 28) kJ mol-1. Microbial organisms in freshwater zones showed less temperature-dependence in relation to CO consumption than organisms in saltwater zones. CO consumption approximately followed Wright-Hobbie kinetics up to 15 nmol L-1 [CO] and transformed to inhibition kinetics at higher [CO]. The maximum CO consumption rate (Vmax) in the Wright-Hobbie kinetic equation ranged from 0.15 to 4.3 (mean: 0.72) nmol L-1 h-1 and roughly anti-correlated with salinity. The half-saturation concentration (Km) was in the range 1.7 to 6.5 (mean: 4.1) nmol L-1 but exhibited no consistent relationships with other measured parameters. An empirical equation describing Kco as a function of water temperature, salinity, and total bacterial density was established. Microbial consumption of CO in the surface mixed layer of St. Lawrence estuarine system was estimated as 24.7 Gg CO-C a-1.Total CO production in the St. Lawrence estuarine system was 30.3 Gg CO-C a-1, 87% of which came from photoproduction, hence being a major source and leaving dark production a minor source. In the surface mixed layer, microbial CO consumption was ~81% of total production, acting as the major CO sink. Taking into account of air-sea flux, the total CO sink is ~ 91% of the total CO source, a reasonable match. The empirical equation of CO dark production was extrapolated to global scales. The total CO dark production in global oceans was estimated to be 17.10 Tg CO-C a-1 with 91% from blue waters and 9% from coastal waters, or 54% from the surface mixed layer and 46% from the sub-surface. The total dark source is significant compared to the best available estimate of the total marine CO photoproduction (50 Tg CO-C a-1) while the mixed-layer dark source contributes 15.7% to the oceanic CO flux to the atmosphere. Steady-state deep-water CO concentrations inferred from Qco and microbial CO uptake rates (0.05-0.1 nmol L-1)is comparable to the modern analytical method's blank. The global microbial CO consumption rate is estimated to be ~63.58 Tg CO-C a-1, based on the calculated dark production in this study and photoproduction and air-sea flux in the published papers.