Phytoplankton Community Composition Along Environmental Gradients in the North Pacific Ocean

Abstract

Prochlorococcus is a cyanobacterium smaller than 1 μm that accounts for much of the primary production in nutrient-poor areas such as the North Pacific Subtropical Gyre (NPSG). In the transition from the NPSG to more productive coastal regions, there are fronts that have sharp changes in chemical, physical, and biological properties. In more coastal, nutrient-rich conditions, larger phytoplankton are more abundant, including Synechococcus and picoeukaryotes. Data collected on cruises going north from the NPSG (the Gradients cruises) were compared to data collected on the TN398 cruise going east from the NPSG to the California coast. A SeaFlow flow cytometer measured small phytoplankton, including Prochlorococcus, Synechococcus, and picoeukaryotes. Prochlorococcus was most abundant in nutrient-poor conditions in the NPSG, reaching a concentration of 300 cells/μL, and larger phytoplankton, including Synechococcus and picoeukaryotes, were most abundant in the coastal ocean and subpolar region. The diameters of Prochlorococcus, Synechococcus, and picoeukaryotes varied on a diel cycle that was most strongly observed in the gyre. The average diameter of Prochlorococcus and Synechococcus increased by about 0.2 μm outside the NPSG, while the diameter of picoeukaryotes observed by SeaFlow decreased by about 0.4 μm. Prochlorococcus abundance was negatively correlated with nitrate and nitrite. In the future, these variables could be compared seasonally, annually, and across ocean basins to better understand how these populations are responding to climate change.