A modelling study of temporal and spatial pCO2 variability on the biologically active and temperature-dominated Scotian Shelf

Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air-sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2, whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO(2)) on a seasonal scale. In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO(2) undergoes a strong seasonal cycle with an amplitude of similar to 200-250 mu atm. These changes are associated with both a strong biological drawdown of dissolved inorganic carbon (DIC) in spring (corresponding to a decrease in pCO(2) of 100-200 mu atm) and pronounced effects of temperature, which ranges from 0 degrees C in the winter to near 20 degrees C in the summer, resulting in an increase in pCO(2) of similar to 200-250 mu atm. Throughout the summer, events with low surface water pCO(2) occur associated with coastal upwelling. This effect of upwelling on pCO(2) is also in contrast to the general assumption that upwelling increases surface pCO(2) by delivering DIC-enriched water to the surface. Aside from these localized events, pCO(2) is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO(2), and simulates annual outgassing of CO2 from the ocean of +1.7 +/- 0.2 mol C m(-2) yr(-1) for the Scotian Shelf, net uptake of CO2 by the ocean of -0.5 +/- 0.2 mol C m(-2) yr(-1) for the Gulf of Maine, and uptake by the ocean of -1.3 +/- 0.3 mol C m(-2) yr(-1) for the Grand Banks.