16 pages, 11 figures.-- This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, Because of the difficulties in tracking a water mass over time and conducting synoptic surveys over large spatial scales, measurements of biological variables in the ocean are often restricted to isolated Eulerian observations. Here a proof-of-concept of a pseudo-Lagrangian transformation was applied to a time series of chlorophyll-a profiles acquired at a single fixed station to diagnose the shape of a phytoplankton patch, and its physical-biological rates of transformation. During August 2013, a 27-hr time series of observations of horizontal currents, hydrographic properties, turbulent mixing and chlorophyll-a was acquired at a central station of the Ría de Vigo (NW Iberian Peninsula). A chlorophyll-a patch, tilted upward toward the inner part of the Ría, was observed moving back and forth past the sampling station. Its shape and position during the sampling period were modulated by the semi-diurnal (M2) tidal currents and the vertically sheared upwelling circulation. The pseudo-Lagrangian transformation allowed estimation of chlorophyll-a net rates of change. These rates were positive between 12 and 14 m depth, and negative elsewhere, with a mean value of −0.001 ± 0.449 days−1 within the patch maximum. After accounting for the effects of diffusion and sinking, the mean net biological production rate in the upper 15 m (0.53 ± 0.25 days−1) was comparable with in vitro estimates of the balance between phytoplankton growth and mortality obtained from dilution experiments carried out under similar conditions (0.2 ± 0.7 days−1). This pseudo-Lagrangian transformation is complementary to traditional analyses for the quantification of ecological processes and biogeochemical budgets, This research was funded by projects: STRAMIX (CTM2012-35155) to Miguel Gilcoto and REMEDIOS (CTM2016-75451- C2-1-R) to B. Mouriño-Carballido from the Spanish Ministry of Economy and Competitiveness. M. Villamaña acknowledges the receipt of a FPU fellowship (FPU014/05,385) from the Spanish Ministry of Education, Culture and Sports. B. Fernández Castro was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 834330 (SO-CUP). Funding for open access charge: Universidade de Vigo/CISUG, Peer reviewed

17 pages, 7 figures, 1 table.-- This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY), Previous studies focused on understanding the role of physical drivers on phytoplankton bloom formation mainly used indirect estimates of turbulent mixing. Here we use weekly observations of microstructure turbulence, dissolved inorganic nutrients, chlorophyll a concentration and primary production carried out in the Ría de Vigo (NW Iberian upwelling system) between March 2017 and May 2018 to investigate the relationship between turbulent mixing and phytoplankton growth at different temporal scales. In order to interpret our results, we used the theoretical framework described by the Critical Turbulent Hypothesis (CTH). According to this conceptual model if turbulence is low enough, the depth of the layer where mixing is active can be shallower than the mixed-layer depth, and phytoplankton may receive enough light to bloom. Our results showed that the coupling between turbulent mixing and phytoplankton growth in this system occurs at seasonal, but also at shorter time scales. In agreement with the CTH, higher phytoplankton growth rates were observed when mixing was low during spring-summer transitional and upwelling periods, whereas low values were described during periods of high mixing (fall-winter transitional and downwelling). However, low mixing conditions were not enough to ensure phytoplankton growth, as low phytoplankton growth was also found under these circumstances. Wavelet spectral analysis revealed that turbulent mixing and phytoplankton growth were also related at shorter time scales. The higher coherence between both variables was found in spring-summer at the ~16–30 d period and in fall-winter at the ~16–90 d period. These results suggest that mixing could act as a control factor on phytoplankton growth over the seasonal cycle, and could be also involved in the formation of occasional short-lived phytoplankton blooms, This research was supported by grant CTM2016-75451-C2-1-R from the Spanish Ministry of Economy and Competitiveness to BM-C. AC was supported by a predoctoral fellowship for the doctoral formation (BES-2017-080935) from the Spanish Ministry of Economy and Competitiveness, Peer reviewed

6 pages, 4 figures, Small-scale turbulent mixing plays a pivotal role in shaping ocean circulation and a broad range of physical and biogeochemical processes. Despite advances in our understanding of the geophysical processes responsible for this mixing, the nature and importance of biomixing—turbulent mixing caused by marine biota—are still debated. A major source of uncertainty pertains to the efficiency of biomixing (the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically), which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in an area of coastal upwelling. We show that turbulent dissipation is elevated 10- to 100-fold (reaching 10−6–10−5 W kg−1) every night of the survey due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently with dissipation, and occurs with an efficiency comparable to that of geophysical turbulence. Our results demonstrate that biologically driven turbulence can be a highly effective mixing agent, and call for a re-examination of its impacts on productive upper ocean regions, Funding for this work was provided by the Spanish Ministry of Economy and Innovation under the research project REMEDIOS (grant number CTM2016-75451-C2-1-R) to B.M.-C. B.F.C. was supported by the Spanish Ministry of Economy and Innovation through a Juan de La Cierva-Formación postdoctoral fellowship (grant number FJCI-641 2015-25712) and by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 834330 (SO-CUP). E.B. was supported by a postgraduate fellowship (grant number ED481A-2019/288) from Xunta de Galicia, co-funded by FSE Galicia. A.C. was supported by a postgraduate fellowship FPI (grant number BES-2017-080935) from the Spanish Ministry of Economy and Competitiveness. A.C.N.G. acknowledges the support of the Royal Society and the Wolfson Foundation, Peer reviewed

Despite theoretical expectations, marine microeukaryote population are often highly structured and the mechanisms behind such patterns remain to be elucidated. These organisms display huge census population sizes, yet genotyping usually requires clonal strains originating from single cells, hindering proper population sampling. Estimating allelic frequency directly from population wide samples, without any isolation step, offers an interesting alternative. Here, we validate the use of meta-transcriptome environmental samples to determine the population genetic structure of the dinoflagellate Alexandrium minutum. Strain and meta-transcriptome based results both indicated a strong genetic structure for A. minutum in Western Europe, to the level expected between cryptic species. The presence of numerous private alleles, and even fixed polymorphism, would indicate ancient divergence and absence of gene flow between populations. Single nucleotide polymorphisms (SNPs) displaying strong allele frequency differences were distributed throughout the genome, which might indicate pervasive selection from standing genetic variation (soft selective sweeps). However, a few genomic regions displayed extremely low diversity that could result from the fixation of adaptive de novo mutations (hard selective sweeps) within the populations., The study was funded by PRIMROSE (EC Interreg Atlantic Area EAPA182/2016) and the Brittany Region as part of the Paleoecology of Alexandrium minutum dans la Rade de Brest–Marché n°2017-90292 project PALMIRA. The authors thank all the participants in the crew of the RV Ramón Margalef from the Remedios cruise (Research project—grant number CTM2016-75451-C2-1-R), particularly B. Mourino-Carballido for their support to the sample collection., Peer reviewed

13 pages, 8 figures, Diazotrophs are a diverse group of microorganisms that can fertilize the ocean through biological nitrogen fixation (BNF). Due to the high energetic cost of this process, diazotrophy in nitrogen-replete regions remains enigmatic. We use multidisciplinary observations to propose a novel framework for the ecological niche construction of nitrogen fixers in the upwelling region off NW Iberia—one of the most productive coastal regions in Europe—characterized by weak and intermittent wind-driven upwelling and the presence of bays. The main diazotroph detected (UCYN-A2) was more abundant and active during summer and early autumn, coinciding with relatively high temperatures (>16°C), low nitrogen:phosphorus ratios (N:P < 7.2), and a large contribution of ammonium (>75%) to the total dissolved inorganic nitrogen available. Furthermore, nutrient amendment experiments showed that BNF is detectable when phytoplankton biomass and productivity are nitrogen limited. Seasonally recurrent biogeochemical processes driven by hydrography create an ecological niche for nitrogen fixers in this system. During the spring–summer upwelling, nondiazotroph autotrophs consume nitrate and produce organic matter inside the bays. Thereafter, the combined effect of intense remineralization on the shelf and sustained positive circulation within the bays in late summer–early autumn, conveys enhanced ammonium content and excess phosphate into the warm surface layer. The low N:P ratio confers a competitive advantage to diazotrophs since they are not restricted by nitrogen supply. The new nitrogen supply mediated by BNF could extend the productivity period, and may be a key reason why upwelling bays are more productive than upwelled offshore waters, M.F. was funded by Juan de La Cierva Formación (FJC2019-038970-I, Ministerio de Ciencia e Innovación, Spanish Government). Additional funds for molecular analysis were coming from Axencia Galega de Innovacion (agreement GAIN-IEO) and GRC-EPB Plankton Ecology and Biogeochemistry Research Group (ref. IN607A2018/2). This research was funded by project REMEDIOS (CTM2016-75451-C2-1-R) to B.M.-C. from the Spanish Ministry of Economy and Competitiveness. A.F.C. was supported by Xunta de Galicia 2013 postdoctoral fellowship POS-A/2013/142 and Beatriz Galindo fellowship BG22-00067. DFR was supported by a FPU predoctoral fellowship (FPU2021/04749) from the Spanish Ministerio de Universidades, Peer reviewed

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 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 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, This research was funded by projects REMEDIOS (CTM2016-75451-C2-1-R), MAGIC (PID2024-156623OB-C21) and Blue-AI (AIA2025-163560-C43) awarded to BM-C from the Spanish Ministry of Economy and Competitiveness. DFR was supported by a FPU predoctoral fellowship (FPU2021/04749) from the Spanish Ministerio de Universidades. A.F.C. was supported by Xunta de Galicia 2013 postdoctoral fellowship POS-A/2013/142 and Beatriz Galindo fellowship BG22-00067. Contribution by RAF is supported by the Swedish Research Council (Vetenskapsrådet), Grant 2022-03319. MF was funded by Juan de La Cierva Formación (FJC2019-038970-I, Ministerio de Ciencia e Innovación, Spanish Government) and by PTA2022-021307-I, MCIN/AEI/10.13039/501100011033 and by FSE+. The authors acknowledge the financial support provided by the University of Vigo and CISUG for the article processing charge (APC), Peer reviewed

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG, Because of the difficulties in tracking a water mass over time and conducting synoptic surveys over large spatial scales, measurements of biological variables in the ocean are often restricted to isolated Eulerian observations. Here a proof-of-concept of a pseudo-Lagrangian transformation was applied to a time series of chlorophyll-a profiles acquired at a single fixed station to diagnose the shape of a phytoplankton patch, and its physical-biological rates of transformation. During August 2013, a 27-hr time series of observations of horizontal currents, hydrographic properties, turbulent mixing and chlorophyll-a was acquired at a central station of the Ría de Vigo (NW Iberian Peninsula). A chlorophyll-a patch, tilted upward toward the inner part of the Ría, was observed moving back and forth past the sampling station. Its shape and position during the sampling period were modulated by the semi-diurnal (M2) tidal currents and the vertically sheared upwelling circulation. The pseudo-Lagrangian transformation allowed estimation of chlorophyll-a net rates of change. These rates were positive between 12 and 14 m depth, and negative elsewhere, with a mean value of −0.001 ± 0.449 days−1 within the patch maximum. After accounting for the effects of diffusion and sinking, the mean net biological production rate in the upper 15 m (0.53 ± 0.25 days−1) was comparable with in vitro estimates of the balance between phytoplankton growth and mortality obtained from dilution experiments carried out under similar conditions (0.2 ± 0.7 days−1). This pseudo-Lagrangian transformation is complementary to traditional analyses for the quantification of ecological processes and biogeochemical budgets., Ministerio de Economía y Competitividad | Ref. CTM2012-35155, Ministerio de Economía, Industria y Competitividad | Ref. CTM2016-75451- C2-1-R, Ministerio de Educación, Cultura y Deporte | Ref. FPU014/05385

Bays are often ecological hotspots within highly-productive eastern boundary upwelling systems. Though the physics of such bays are well understood, there is no consensus about the factors underlying their high productivity. Three weeks of high-temporal-resolution observations in two long, narrow bays (Rías Baixas, NW-Iberia), showed that during an upwelling pulse, deep, nutrient-rich isopycnals rose into the euphotic zone inside the rías in a few hours. The response of the isopycnals to changes in wind forcing is approximately three times faster inside the rías than the Ekman spin-up time, triggering rapid nutrient uptake and subsequent formation of a subsurface chlorophyll and production maximum. The tight coupling and rapid response of phytoplankton growth to wind forcing could explain the higher productivity of the rías, and also be at play in other upwelling bays with similar morphologies and orientations. Resolving short-term variability of physical–biological coupling is crucial to discern the future evolution of upwelling bays., Xunta de Galicia | Ref. ED481A‐2019/288, Agencia Estatal de Investigación | Ref. CTM2016-75451-C2-1-R