The first-ever high-resolution cross-shelf sections of nutrient and PCO2 measurements, made during the upwelling season of 2001 off the Oregon Coast as part of the CoOP-sponsored COAST (Coastal Ocean Advances in Shelf Transport) program, show the effects of strong upwelling of nutrient-and CO2-rich water, followed by rapid biological uptake. Photosynthetic production draws nitrate from over 30 µM to undetectable levels; along with this nitrate uptake, PCO2 is drawn down from values of 300 µatm above (in recently upwelled waters), to 200 µatm below, atmospheric saturation. High PCO2 surface waters are confined to a narrow region near the coast; low PCO2 conditions persist seaward over areas covering most of the shelf. If these conditions are representative of other upwelling areas in the North Pacific, the total CO2 uptake by these regions approaches 1012 mol/year, a significant contribution to the total Summer-time uptake of CO2 by the entire North Pacific.
This phenomenon makes the Oregon Coast unique among upwelling regions of the world, which are typically sources of CO2 to the atmosphere. Three factors appear to contribute to this singularity: 1) Upwelled source waters have high alkalinity (ALK) and preformed nitrate relative to total CO2 (TCO2), which allows for the necessarily high ALK:TCO2 ratios implied by low PCO2; 2) Productivity limited only by available nitrate (i.e. no micronutrient limitation) such that all upwelled nitrate can be consumed; and 3) only moderate warming of upwelled waters in comparison to lower-latitude upwelling regions.
Figure: Section of CO2 partial pressure (PCO2) across Heceta Bank in August, 2001. Surface PCO2 (top panel) is consistently undersaturated with respect to the atmosphere, even though the upwelled source water (deep waters in lower panel) is greatly supersaturated with respect to the atmosphere. This deep source water has sufficient excess preformed nutrient concentrations relative to its preformed PCO2 that the rapid biological productivity in the OR coast can draw surfacePCO2 down to the levels seen in the above panel. These conditions seem to prevail over the study region for the entire upwelling season.