Influence of a buoyant river plume on phytoplankton nutrient dynamics: What controls standing stocks and productivity?

The influence of the Columbia River plume on phytoplankton rates and biomass accumulation was examined using multiday deckboard incubations as part of the coastal ocean processes River Influences on Shelf Ecosystems field program in August 2005. At a set of five stations encompassing the near-field plume (three stations) on the Oregon and Washington coasts, treatments consisting of control, added nitrate, and added nitrate with 0.45 μm filtered plume water were used to assess the phytoplankton community response to macronutrients and micronutrients. For a subset of these stations, nutrient (nitrate, ammonium, and ammonium inhibition of nitrate) kinetics were obtained, as well as carbon-based estimates of productivity. For all experiments, nitrogen (nitrate) was clearly controlling both biomass accumulation and growth rates. Despite the apparent poleward trend toward increasing biomass in this region, there were no obvious differences in phytoplankton physiological capacity, nor were there any symptoms of iron limitation in the short term. We conclude that phytoplankton in this region are predominantly nitrogen limited but that upon release from this limiting factor, phosphorous and/or silicic acid (in waters not influenced by the Columbia River plume) would quickly become limiting. Evidence suggests that the mesoscale differences in phytoplankton biomass between the Oregon and Washington coasts result from a combination of enhanced grazing downstream and the physically retentive and dispersive effects of the plume itself.