Distribution and variability of iron input to Oregon coastal waters during the upwelling season

Abstract

We measured iron concentrations off the Oregon coast in spring (May–June) and summer (August) of 2001 as part of the Coastal Ocean Advances in Shelf Transport (COAST) program. Dissolvable and total dissolvable iron levels in surface waters were generally higher in spring (mean of 2.1 and 33.9 nmol L−1, respectively) than in summer (means of 1.4 and 15.4 nmol L−1). In spring and summer, high iron concentrations in surface waters were associated with both cold and saline, recently upwelled waters, and with fresh, relatively warm water influenced by the Columbia River. Comparison of total dissolvable iron in 0.45 μm filtered and in unfiltered samples indicated a substantial contribution from particulate iron. Iron concentrations in summer were generally lower than in spring throughout the water column, with the exception of the near‐bottom, where concentrations were generally higher in summer than spring. Optical backscatter data from moored sensors were used to infer the vertical and cross‐shelf transport of iron‐bearing particles during the upwelling season over a steep shelf. Cross‐correlation analysis showed downslope movement of particles from the deep inner shelf to the deep midshelf. There was also evidence for sinking of biogenic particles at the midshelf and inner shelf, but we found no evidence of upslope transport of benthic particles. Sufficient iron is available in this system to meet the demands of the phytoplankton, which are able to make full use of available nitrate.