Does the Columbia River plume influence phytoplankton community structure along the Washington and Oregon coasts?

As part of the River Influences on Shelf Ecosystems (RISE) program, we examined the influence of the Columbia River plume on the composition and biomass of phytoplankton communities on the Washington and Oregon coasts. We determined the taxonomic composition, size structure, and biomass of phytoplankton assemblages in near‐surface shelf waters during four 3‐week cruises in spring and/or summer of 2004–2006. As the Columbia plume is very dynamic, the influence of the plume was examined in three ways. Two comparisons of the entire data set were made: (1) a geographical comparison of the Washington, Oregon, and Columbia River mouth regions based on latitude and (2) a “plume” versus “nonplume” comparison based on salinity. A third comparison focused on samples taken in and outside of three specific plumes under different upwelling/downwelling conditions. In whole data set comparisons, there were no significant differences in chlorophyll, carbon biomass, or diatom community structure between regions or between plume and nonplume samples. However, within some cruises there were regional and plume differences in chlorophyll and biomass. Diatom community composition differed between cruises, but within a cruise it was similar across regions and in plume/nonplume samples, indicating there was no unique plume community. On finer time and space scales, differences in community structure as well as biomass were evident between samples in and outside of specific plumes. Over broader scales, the Columbia plume acts to make coastal phytoplankton communities more homogeneous. Specific impacts will depend on the history of upwelling and nutrient status of the coastal waters which the plume encounters.