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