Factors influencing the chemistry of the near-field Columbia River plume: Nitrate, silicic acid, dissolved Fe, and dissolved Mn

Factors influencing concentrations of nitrate, silicic acid, dissolved Fe, and dissolved Mn in the near-field Columbia River plume were examined during late spring and summer from 2004 to 2006 as part of the River Influences on Shelf Ecosystems program. Under upwelling-active phases, cold, high-nitrate coastal seawater was entrained in the plume, and nitrate concentrations of 16-19 mu M were observed with as much as 90% from a coastal seawater origin. Under downwelling-relaxation phases, warm, nutrient-depleted coastal seawater was entrained forming a near-field plume with nitrate concentrations of 2.5-6 mu M, with the river as the only source. Elevated silicic acid in the river is the dominant source, with concentrations of 60-80 mu M in the near-field plume. During upwelling-active phases, high concentrations of dissolved Fe (as high as 40 nM) in the cold, low-oxygen, nutrient-rich coastal seawater were entrained to form a near-field plume with 15-20 nM dissolved Fe. During downwelling-relaxation phases, dissolved Fe in the intruding underlying warm coastal seawater was 1-3 nM, producing plume concentrations of 2-13 nM, with higher concentrations during the high river flow of May 2006. Dissolved Mn in the near-field plume covaried markedly as a function of increased tidal flushing in the estuary. The use of CORIE (pilot environmental observation and forecasting system for the Columbia River) time series conductivity-temperature-depth data within the estuary, along with data presented in this study, allows extrapolation of the near-field plume chemistry throughout the spring and summer seasons to provide insight into this important source of nutrients to the coastal waters in this region.