Carbon and nutrient dynamics during coastal upwelling off Cape Blanco, Oregon

Abstract

The partial pressure of carbon dioxide (PCO 2) and concentrations of the nutrients phosphate (P) and silicate (Si) in coastal surface waters within a 100 km×300 km area centered on Cape Blanco, Oregon, were mapped at high resolution during August 17–27, 1995. Alkalinity and the concentration of total CO 2 were determined on a subset of stored samples. Over the 9–18°C range in sea-surface temperatures encountered during the cruise, PCO 2, and P and Si concentrations varied between 150–690 μatm, <0.1–1.8, and <1–33 μmol/kg, respectively. Spatial variations in the intensity of coastal upwelling set the wide range in surface water properties. Acoustic Doppler current profiler data collected throughout the cruise indicate the advective nature of many chemical features. PCO 2 data also indicate the presence of an intense phytoplankton bloom in continental shelf waters along the coast north of Cape Blanco, with little evidence of biological drawdown to the south. The available data do not provide an unambiguous explanation for this contrast. Surf-zone water was sampled from shore at 17 locations along the cruise area on August 25 and 26, 1995, concurrently with the shipboard measurements. PCO 2, and P and Si concentrations, varied in the range 250–640 μatm, 0.3–2.0, and 1.1–42 μmol/kg, respectively. North of Cape Blanco, the chemical expression of upwelling was considerably stronger in the surf-zone than at all locations sampled on board ship because the phytoplankton bloom did not extend to the coast. South of Cape Blanco, concentrations of upwelling tracers measured on board ship within a distance of 5–10 km from the coast and in the surf-zone were comparable. Vertical nutrient profiles across the continental shelf show that the composition of surf-zone water is consistent with conservative advection of nutrient-enriched bottom water from the edge of the continental shelf to the surf-zone.