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 (<2 μm) were the main autotrophs. However, peak values of biomass and metabolism were recorded in summer stratified periods during upwelling season, with elongated and the medium sized diatom species Leptocylindrus danicus dominating. The autumn period characterized by the presence of the Iberian Poleward Current featured warm, nutrient‐poor waters, enhanced heterotrophy, occurrence of red‐tide forming organisms, and presence of mixotrophic dinoflagellates, suggesting metabolic adaptability as a response to nutrient limitation. Seasonal shifts in size structure and metabolism reveals a dynamic microbial community tightly linked to physical forcing. Additionally, by comparing our shelf station with previous studies in the inner bays (Rías) and the shelf break, we define a land‐sea gradient in microbial structure and metabolic regime: from consistently autotrophic, microplankton‐dominated communities inside the Rías to smaller, more heterotrophic assemblages offshore. These findings provide a conceptual framework for understanding how microbial productivity may shift under future changes in upwelling regimes.