Esther Velasco-Senovilla

Diazotrophs, microorganisms capable of fertilizing the ocean through biological nitrogen fixation (BNF), are widespread in the oceans, including upwelling bays. However, their ability to respond to short‐lived upwelling pulses, lasting only a few days, remains poorly understood. To investigate diazotrophy across temporal scales, we analyzed 18 months of multidisciplinary observations from two dynamic upwelling bays in North West Iberia, combining weekly year‐round sampling with daily measurements during a 2‐week intensive summer cruise. BNF exhibited pronounced seasonal variability, with the highest rates (1.5 nmol N ) coinciding with increased abundance of the UCYN‐A2 nitroplast during late summer and early autumn. Time–frequency analysis indicated that diazotrophic abundance variability was driven not only by seasonal forcing but also by rapid environmental fluctuations, a pattern corroborated by the daily summer data set. BNF rates peaked during relaxation–downwelling periods, when they contribute with episodic yet substantial inputs of new nitrogen to the euphotic zone. In contrast, both BNF and UCYN‐A2 abundance declined sharply during nutrient‐rich upwelling pulses. Elevated BNF coincided with high abundances by microscopy of Hemiaulus spp. and Rhizosolenia spp., along with Richelia spp. detection by qPCR, presenting the first direct‐detection for diatom‐diazotroph associations in this region. These demonstrate that BNF can episodically become a main source of the new nitrogen supply during upwelling‐relaxation phases, acting as a transient “bridge” that helps sustain productivity between upwelling events. Our study underscores the need to re‐assess the role of diazotrophy in coastal upwelling systems, where its contribution may have been historically underestimated due to coarse temporal sampling.

Diazotrophs, microorganisms capable of fertilizing the ocean through biological nitrogen fixation, are widespread in the oceans, including upwelling bays. However, their ability to respond to short-lived upwelling pulses, lasting only a few days, remains poorly understood. To investigate diazotrophy across temporal scales, we analyzed 18 months of multidisciplinary observations from two dynamic upwelling bays in NW Iberia, combining weekly year-round sampling with daily measurements during a two-week intensive summer cruise. BNF exhibited pronounced seasonal variability, with the highest rates (1.5 nmol N L-1 d-1) coinciding with increased abundance of the UCYN-A2 nitroplast during late summer and early autumn. Time–frequency analysis indicated that diazotrophic abundance variability was driven not only by seasonal forcing but also by rapid environmental fluctuations, a pattern corroborated by the daily summer dataset. BNF rates peaked during relaxation–downwelling periods, when they contribute with episodic yet substantial inputs of new nitrogen to the euphotic zone. In contrast, both BNF and UCYN-A2 abundance declined sharply during nutrient-rich upwelling pulses. Elevated BNF coincided with high abundances by microscopy of Hemiaulus spp. and Rhizosolenia spp., along with Richelia spp. detection by qPCR, presenting the first evidence for diatom-diazotroph associations in this region. These demonstrate that BNF can episodically become a main source of the new nitrogen supply during upwelling-relaxation phases, acting as a transient “bridge” that helps sustain productivity between upwelling events. Our study underscores the need to re-assess the role of diazotrophy in coastal upwelling systems, where its contribution may have been historically underestimated due to coarse temporal sampling.