Iron and Nitrate Supply to Oregon Coastal Waters: Physical Mechanisms and Biological Response

摘要

Iron and nitrate concentrations were measured at high-resolution underway in surface waters and in vertical profiles off the Oregon coast in July, 1999. Surface Fe and N (nitrate + nitrite) concentrations measured by flow injection analysis ranged from < 0.3 to 20 nmol 1(exp -1) and < 0.1 to 30 mu-mol 1(exp -1), respectively. Total dissolvable Fe concentrations, measured in unfiltered, acidified samples in surface waters and in vertical profiles, ranged from < 0.3 to 300 nmol 1(exp -1). Our observations indicate two dominant sources of Fe to Oregon coastal waters: Slope or shelf sediments, and the Columbia River. Sedimentary iron, largely in the particulate form, appears to be added to surface waters through wind-induced vertical mixing during strong winds; through thickening of the bottom mixed layer during relaxation or downwelling-favorable wind conditions; and by outcropping of shelf bottom-waters during upwelling events. The existence of multiple iron sources and the generally high (total) iron concentrations may explain why the distribution of phytoplankton - measured both remotely (by SeaWiFS) - and underway (by in-vivo fluorescence) - appeared to be driven primarily by physical dynamics, and was not obviously linked to the distribution of iron. Nevertheless, at some offshore stations where underway Fe was < 0.3 nmol 1(exp -1), underway measurements of the physiological state of phytoplankton by Fast Repetition Rate fluorometry were consistent with mild iron-stress.