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 μM 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 μM, with the river as the only source. Elevated silicic acid in the river is the dominant source, with concentrations of 60–80 μM 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.