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

9 pages, 4 figures.-- This is an open access article under the terms of the Creative Commons Attribution License, 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, This research was funded by project REMEDIOS (CTM2016-75451-C2-1-R) to BM-C from the Spanish Ministry of Economy and Competitiveness. E.B. acknowledges a predoctoral fellowship (ED481A-2019/288) from Xunta de Galicia, co-funded by FSE Galicia (2014–2020). BFC 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 program under the Marie Skłodowska-Curie grant agreement No. 834330 (SO-CUP), 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

22 pages.-- Open Access, Bays within eastern boundary upwelling systems (EBUS) are ecological hot-spots featuring a diverse range of spatio-temporal dynamics. At the EBUSs’ poleward limit, upwelling occurs in short-lived (<1 week) pulses modulated by synoptic wind variability. The circulations in long, narrow bays can respond to these fluctuations within few hours. The short-term biological response to these pulses was investigated in two of these bays (Rias Baixas, NW-Iberia) with a two-week quasi-synoptic spatio-temporal survey in the summer 2018. A four-day-long upwelling pulse caused deep, nutrient-rich isopycnals to rise into the euphotic zone inside the bays, triggering a rapid (~1.7 days) nutrient uptake and formation of a subsurface chlorophyll maximum (~3.8 days). The phytoplankton biomass was transported rapidly toward deep, offshore waters when the winds weakened. These results suggest that high productivity in narrow bays is controlled by the transient exposure of deep, nutrient-rich waters to light during upwelling pulses, Regional Government of Galicia, Ministry of Economy and Competitiveness, H2020 Marie Skłodowska-Curie Actions, No

The dataset includes: Microstructure profiles collected with a MSS Sea&Sun profiler during the three intensive samplings of the cruise (I01, I02, I03), Ocean currents measured with a bottom moored RD Instruments acoustic Doppler profiler (ADCP, 300Khz) for the duration of the cruise, Acoustic backscatter from a ship-borne echosounder Simrad EK80 for the frequencies 18, 38, 70, 120 and 200 KHz and the three intensive samplings of the cruise (I01, I02, I03), European anchovy (Engraulis encrasicolus) egg counts from plankton hauls samplings, This dataset includes data collected during the cruise REMEDIOS-TL in the Ría de Pontevedra (NW Iberia) at station P2 (42.357°N, 8.773°W) from 29 June to 18 July 2018 onboard of the Research Vessel Ramón Margalef belonging to the Spanish Institude of Oceanography.
The archived data are described in a manuscript entitled "Intense upper ocean mixing due to large aggregations of spawning fish" by Fernández Castro et al. published in Nature Geoscience: Fernández Castro, B., Peña, M., Nogueira, E. et al. Intense upper ocean mixing due to large aggregations of spawning fish. Nat. Geosci. 15, 287–292 (2022). https://doi.org/10.1038/s41561-022-00916-3. The manuscript presents evidence that night-time aggregations of anchovies produce intense ocean turbulence and mixing. All the data needed to support the conclusions of the article are included in this dataset, The REMEDIOS project is funded by the Spanish Ministry of Economy and Innovation under the research project REMEDIOS (CTM2016-75451-C2-1-R), acoustic_backscatter_EK80_REMEDIOS_TL_I01.nc.-- acoustic_backscatter_EK80_REMEDIOS_TL_I02.nc.-- acoustic_backscatter_EK80_REMEDIOS_TL_I03.nc.-- bottom_moored_ADCP_REMEDIOS_TL.nc.-- egg_counts.xls.-- MSS_microstructure_REMEDIOS_TL_I01.nc.-- MSS_microstructure_REMEDIOS_TL_I02.nc.-- MSS_microstructure_REMEDIOS_TL_I03.nc, 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

VIII Encuentro de la Oceanografía Física 2024, Valencia, 10-12 julio 2024, Upwelling bays harvest outstanding biological production, which is frequently jeopardized by harmful algae blooms (HAB). The mechanisms responsible for this high productivity and the specific role of Thin Layers of Phytoplankton (TLP) in toxicity events are not completely understood. The REMEDIOS project combined field observations, analysis of time-series monitoring data, and numerical modeling in the Galician Rías: 1) to describe the temporal variability of phytoplankton growth and its connection with turbulent mixing, and 2) to discern the role of TLP in HAB toxicity hotspots. The analysis of a twentyfour years dataset showed that seasonality was the most important component of chlorophyll-a variability (~50%), followed by a shorter-term mode (~30%) and a long-term trend (~20%). Whilst a relationship between turbulence mixing and phytoplankton at seasonal and longer time scales could not be established, our results suggest that mixing could be involved in the formation of short-lived phytoplankton blooms. The chlorophyll-a long-term trend, which mimics the temporal evolution of the winter North Atlantic Oscillation, was negatively correlated with the average duration of upwelling events. Consistently, high-temporal-resolution cruise observations showed that the rapid response of phytoplankton to synoptic wind forcing inside the Rías, three times faster than in the shelf, could explain their higher productivity. The monitoring dataset analysis showed more frequent TLP in R a de Pontevedra, characterized by longer toxicity events caused by Dinophysis species. Numerical simulations and observations indicate that weaker rates of turbulent energy production in Ría de Pontevedra could foster stable conditions, leading to higher TLP and HAB persistence. TLP maintenance may be supported by nutrients flushing via night-time biological mixing due to frenetic activity of spawning fish aggregations. These results emphasize that understanding short-term variability of physical-biological coupling is crucial for discerning the drivers of phytoplankton production and toxicity of upwelling bays, and their evolution under global change, 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, Peer reviewed

Poster.-- VI International Symposium on Marine Sciences, Vigo (Spain), 20-22th June 2018, The Galician Rías, located in the NW of the Iberian Peninsula, are a highly productive ecosystem under the influence of the Coastal upwelling. This region sustains an important shellfish activity, which every year is threatened by episodes of harmful phytoplankton. A particular case of microalgae bloom, named thin layers of phytoplankton (TLP), occurs when high-cell densities are located within a narrow depth interval. These features are believed to play an important role in the sudden formation and maintenance of harmful algae blooms. Previous analysis of the dataset collected by the INTECMAR monitoring program (http://www.intecmar.org) in the Galician Rías, weekly between 2012-2015, showed that the occurrence of TLP was higher between May and July, associated with the base of shallow pycnoclines (Mouriño et al., this issue). Despite this evidence, the mechanisms responsible for the formation of TLP in the Galician Rías remains unknown. Here we investigate the environmental conditions favorable for the formation of TLP in the Galician Rías during the period of 2012-2015. For that we first built a database containing the meteorological and hydrographic information corresponding to the INTECMAR weekly sampling. Statistical procedures based on principal component and cluster analysis were used to investigate the relationship between the occurrence of TLP and the favorable environmental conditions, This is a contribution to the REMEDIOS project (CTM 2016-75451-C2-1-R) funded by Ministerio de Economía y Competitividad to Beatriz Mouriño-Carballido, Peer reviewed

V Encuentro de Oceanografía Física 2018 (EOF), Vigo (Spain), 20-22th June 2018, By removing the effects of local advection, Lagrangian studies are well suited for studying biological dynamics. However, following water masses moving in different directions at different depths is presently impossible, and biological measurements are often restricted to isolated Eulerian observations. Temporal variations in Eulerian observations at a single station result from combinations of physical and biological processes that are often difficult to separate. In coastal regions, a dominant physical forcing is the advection and straining of horizontal gradients by (sub)tidal currents; these oscillatory currents have the potential of moving water parcels past a stationary sampling station multiple times. In August 2013, a 27-hour time-series of currents and fluorescence observations was carried out in a central station in the Ría de Vigo (NW Iberian Peninsula), by using an Acoustic Doppler Profiler (ADCP) and a microstructure turbulence profiler, which includes a fluorescence sensor. The transformation of fluorescence data from an Eulerian to a pseudo-Lagrangian frame was applied in order to study the spatial structure of the fluorescence patch observed. The 3D shape of the fluorescence patch was constructed from the oscillatory Eulerian velocity field, revealing the structure of a relatively unchanging patch moving back and forth past the sampling station. Our results suggest that the bi-directional upwelling circulation and tidal currents modulate the shape and position of the patch during the sampling period. This alternative, a spatially resolved view of Eulerian biological observations, allows separation of advective and biological effects in the interpretation of local temporal variations in highly dynamic environments like the Ría de Vigo, Funding for this study was provided by the Spanish Ministry of Economy and Competitiveness under the research projects CTM2012-35155 to M. Gilcoto and CTM2016-75451-C2-1-R to B. Mouriño. M.V. acknowledges a FPU fellowship (FPU014/05385) and a mobility grant (EST16/00083) from the Spanish Ministry of Education, Culture and Sports. B.F.C. was supported by a Juan de La Cierva Formación fellowship (FJCI-641 2015-25712) from the Spanish Ministry of Economy and Competitiveness, Peer reviewed

V Encuentro de Oceanografía Física 2018 (EOF), Vigo (Spain), 20-22th June 2018, “Follow the nitrogen”, the mantra proposed by Capone et al (2006) for searching for life on Mars, summarizes the relevance that this element plays on life. By limiting primary productivity in both terrestrial and marine ecosystems, this element controls the flow of energy through the environments, and the atmospheric CO2 uptake by photosynthetic organisms. Quantifying how much nitrogen entries the euphotic zone is key to understand the role of the ocean in climate, as for the long-term this supply constraints the export of carbon to the deep ocean. Nitrate turbulent diffusion represents one of the main mechanisms by which nitrogen is fuelled to phytoplankton cells habiting the euphotic zone. However, methodological difficulties to quantify turbulence in the field have severely limited our knowledge about the magnitude of this process in the ocean. Here we used nitrate diffusive fluxes, derived from microstructure turbulence observations, and environmental variables collected in 166 stations, located in contrasting marine environments, to build multiple linear regression models to predict nitrate turbulent diffusion. A model including three predictors (sea surface temperature, the vertical gradient of nitrate, and surface chlorophyll-a) was able to explain 50% of the flux variability. This model was combined with temperature, nitrate and chlorophyll concentration gathered from the NASA (National Aeronautics and Space Administration), WOA09 (World Ocean Atlas) and NOAA (National Oceanographic and Atmospheric Administration) databases to predict the magnitude of nitrate diffusion in the global ocean. These estimates showed, in general, a good agreement with the scarce nitrate diffusion rates reported in the literature. As far as we know, this study represents the first attempt to develop a climatology of nitrate diffusion into the euphotic zone on a global scale, This is a contribution to the REMEDIOS (CTM 2016-75451-C2-1-R, Ministerio de Economía y Competitividad), CHAOS (CTM2012-30680, Ministerio de Economía y Competitividad) and NICANOR (EM2013/021, Xunta de Galicia) projects to Beatriz Mouriño-Carballido. V.M. (FPU13/01674) and M.V. (FPU014/05385) were supported by a FPU fellowship from the Spanish Ministry of Education, Culture and Sports. B.F.C. and J.L.O were supported, respectively, by a Juan de La Cierva Formación (FJCI-641 2015-25712) and a FPI fellowship (CTM2012-30680) from the Spanish Ministry of Economy and Competitiveness, Peer reviewed

V Encuentro de Oceanografía Física 2018 (EOF), Vigo (Spain), 20-22th June 2018, The surface boundary layer is a key element of the ocean dynamics, controlling the exchange of heat, organic volatiles and dissolved gases with the atmosphere, and influencing the productivity of phytoplankton. Owing to the dynamic interactions with the atmosphere, the surface boundary layer is typically turbulent. Historically, it has been defined as a “mixed layer”, i.e. a portion of the water column where the distribution of density (or any other property) is uniform. This definition does not provide information about the intensity of turbulence and the time-scale of mixing, i.e. the time required to homogenize any gradient in the properties’ distribution. The time required to mix a layer of vertical dimension h is inversely proportional to the turbulent diffusivity (Kz), τ~h2/Kz. Due to the lack of turbulence microstructure data, it has been usually assumed that mixed layers are “instantaneously” mixed by turbulence. However, recent measurements in the field revealed that the extension of the mixed and turbulent layers (defined as a subsurface layer of enhanced turbulence above a background value) might be very different (Sutherland, et al. 2014). Here, we used one-dimensional random-walk simulations (Ross and Sharples, 2004) to quantify the mixing time-scale of the mixed layer under different oceanographic conditions. The simulations were forced with turbulent diffusivity profiles derived from microstructure measurements in subtropical, temperate and polar regions. We show that, particularly in (sub)tropical regions, the mixing time-scales range from a few days to >1 month, and that effective mixed layers in the time scale of ~1 day are frequently much shallower than indicated by the classical definition. As the mixing time-scale determines the capability of phytoplankton to overcome light limitation and bloom (Fischer et al. 2014), our results have implications for phytoplankton growth, which are explored with a simple plankton model, This is a contribution to the REMEDIOS project (CTM 2016-75451-C2-1-R to B. Mouriño-Carballido) funded by Ministerio de Economía y Competitividad. B. Fernández Castro was supported by a Juan de La Cierva Formación fellowship (FJCI-641 2015-25712, Ministerio de Economía y Competitividad, Spanish Goverment). M. Villamaña and J. L. Otero-Ferrer were suported by FPU and FPI contracts of the Ministerio de Economía y Competitividad. Additional funding came from the Spanish Government projects CSD 2008-00077 to Carlos Duarte/Emilio Marañon/X. A. Álvarez-Salgado and CTM2015-69392-C3-2-R to Javier Arístegui/X. A. Álvarez-Salgado, 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

Due to the fertilization effect of the upwelling and the increase in primary production, the Rías Baixas sustain an intense shellfishing activity which, every year, faces the menace of harmful phytoplankton blooms. It is believed that the aggregation of phytoplankton in thin layers, where the biomass accumulates in a thickness less than 5 meters, might explain why traditional monitoring programs, with insufficient vertical resolution, are often unable to detect and predict toxic events. Preliminary studies indicate that the formation of these features is associated with strictly physical or biological processes, or a combination of both. Despite their relevance, nowadays, the knowledge about thin layers in this region is limited to a couple of specific observations. In order to verify the following hypotheses: H1) Pseudo-nitzschia spp. and Dinophysis spp. frequently bloom in thin layers in the Rías Baixas, and H2) Physical processes related to shear and turbulence levels control the formation of thin layers of Pseudo-nitzschia spp., this project proposes a multidisciplinary approach which combines database analysis, field observations carried out during the REMEDIOS-TLP cruise (Jun-Jul 2018), and numerical modeling techniques. Specifically, the following objectives are proposed: O1) To describe the characteristics of thin layers of phytoplankton, with special interest in Pseudo-nitzschia spp., in the Rías Baixas and adjacent shelf, and O2) To investigate the mechanisms responsible for the initiation, maintenance and dissipation of thin layers of phytoplankton., Debido á elevada produción primaria asociada ao alforamento, as Rías Baixas sosteñen unha intensa actividade marisqueira que, cada ano é ameazada pola aparición de toxinas de orixe fitoplanctónica. Crese que a agregación de fitoplancto en estruturas de capas finas, donde a maior parte da biomasa acumúlase nun espesor <5m, podería explicar que os sistemas tradicionais de mostraxe, con insuficiente resolución vertical, frecuentemente non sexan capaces de detectar e predicir os eventos de toxicidade. Estudios previos indican que a formación de estas estruturas está asociada a procesos estritamente físicos, biolóxicos ou unha combinación de ambos. A pesar da súa relevancia, polo momento o coñecemento de estas estruturas na rexión limítase a un par de observacións puntuais. Co fin de verificar as seguintes hipóteses: H1) Nas Rías Baixas Pseudo-nitzschia spp. e Dinophysis spp. frecuentemente proliferan en forma de capas finas, H2) Os procesos físicos relacionados cos niveles de cizalla e turbulencia controlan a formación de capas finas de Pseudo-nitzschia spp., este proxecto propón una aproximación multidisciplinar que combinará análise de bases de datos, observacións de campo realizadas durante a campaña REMEDIOS-TLP (Xuño-Xullo 2018), e modelado numérico. En concreto, propóñense os seguintes obxectivos: O1) Describir as características das capas finas de fitoplancto, con especial interese en Pseudo-nitzschia spp., nas Rías Baixas e plataforma adxacente, e O2) Investigar os mecanismos responsables da formación, mantemento e disipación das capas finas de fitoplancto., Debido a la elevada producción primaria asociada al afloramiento, las Rías Baixas sostienen una intensa actividad marisquera que, cada año, se ve amenazada por la aparición de toxinas de origen fitoplanctónico. Se cree que la agregación de fitoplancton en estructuras de capas finas, donde la mayor parte de la biomasa se acumula en un espesor <5 m, podría explicar que los sistemas tradicionales de muestreo, con insuficiente resolución vertical, frecuentemente no sean capaces de detectar y predecir los eventos de toxicidad. Estudios previos indican que la formación de estas estructuras está asociada a procesos estrictamente físicos, biológicos o una combinación de ambos. A pesar de su relevancia, por el momento el conocimiento de estas estructuras en la región se limita a un par de observaciones puntuales. Con el fin de verificar las siguientes hipótesis: H1) En las Rías Baixas Pseudo-nitzschia spp. y Dinophysis spp. frecuentemente proliferan en forma de capas finas, H2) Los procesos físicos relacionados con los niveles de cizalla y turbulencia controlan la formación de capas finas de Pseudo-nitzschia, este proyecto plantea una aproximación multidisciplinar que combinará análisis de bases de datos, observaciones en el campo realizadas durante la campaña REMEDIOS-TLP (Junio-Julio 2018), y modelado numérico. En concreto, se plantean los siguientes objetivos: O1) Describir las características de las capas finas de fitoplancton, con especial interés en Pseudo-nitzschia spp., en las Rías Baixas y la plataforma adyacente, y O2) Investigar los mecanismos responsables de la formación, mantenimiento y disipación de las capas finas de fitoplancton., Agencia Estatal de Investigación | Ref. CTM2016-75451-C2-1-R, Xunta de Galicia | Ref. ED481A 2019/288

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