Revealing Seasonal Patterns and Metabolism of Microbial Plankton in a Productive Coastal Upwelling System

Abstract Increasing our knowledge about the structure and metabolism of the microbial plankton community in coastal upwelling systems is essential for efficiently managing the resources of these highly productive ecosystems. Here we explore seasonal variations of this community in the NW Iberian coastal upwelling system by measuring variables informing on community structure and metabolism. The microbial community remained mostly autotrophic year‐round, with variations imposed by hydrographic conditions. Changes in plankton biomass were driven by the rapid response of larger cells to water column shifts. During upwelling, large diatoms (>20 μm) dominated. Conversely, when downwelling prevailed, autotrophic nanoflagellates (2–20 μm) and picoflagellates (