Marine heatwaves (MHWs), have doubled in frequency globally in recent decades and are becoming longer, more intense, and increasingly disruptive to marine ecosystems. However, despite their growing ecological and biogeochemical importance, major productive coastal systems remain understudied, particularly in the Southern Hemisphere. Here, we provide the first comprehensive characterization of MHWs across the Patagonian Shelf (PS), one of the most biologically productive marine regions on Earth, using 40 years of satellite-derived daily sea surface temperature (SST) data. We first assess how the choice of MHW detection method (fixed versus moving climatology) and SST-dataset selection affect MHW metrics. Then we quantify MHW frequency, intensity, duration, and long-term trends, revealing that the PS experiences on average 1.9 +/- 2 MHWsyr(-1) with a mean cumulative duration of 23-28 dyr(-1)and an average intensity of 1.36 +/- 0.3 degrees C. We show that MHW activity varies substantially across the region, with the northern sector and the outer shelf experiencing the most frequent and intense events (>2 eventsyr(-1) and >2 degrees C). A notable increase in MHW days (+5-10 d per decade) is observed in the northern PS, whereas no significant trends are observed to the south (i.e., south of 48 degrees S). These trends are consistent with background warming of the ocean in this region, suggesting a mechanistic link, whereby long-term warming enhances the likelihood of MHWs occurrence and duration. We further demonstrate that a component of MHW variability can be attributed to the El Ni & ntilde;o Southern Oscillation, which exerts a stronger influence on the intensity of thermal anomalies than on the cumulative duration of the events. Together, these findings constitute the first comprehensive assessment of MHWs on the PS and provide essential insight for anticipating their ecological and climatic impacts in one of the Southern Hemisphere's key marine ecosystems.

The phytochrome superfamily comprises photosensory proteins that enable organisms to perceive changes in light intensity and quality and is widespread across plants, fungi, algae, and microbes. In terrestrial plants, phytochromes sense red and far-red light to regulate key developmental and physiological processes. In marine environments, however, where red and far-red wavelengths penetrate only the upper few meters of water, the function of phytochromes has remained unclear. Recent work shows that diatom phytochromes exhibit photoreversible responses across a broad spectral range, extending beyond red and far-red, suggesting a role in underwater light sensing. Here, we examine the role of phytochromes in light perception and collective behavior in the marine diatom Phaeodactylum tricornutum. Comparing wild-type and phytochrome knockout strains under different light wavelengths reveals that activation of phytochromes by blue or far-red light synchronizes cell movements into a coordinated "wobbling dance." This behavior is absent in phytochrome-deficient mutants, demonstrating the essential role of phytochromes. Our results further suggest that this collective motion involves intercellular communication, potentially mediated by variable red and far-red autofluorescence. Together, these findings uncover a previously unrecognized light-driven social behavior in marine diatoms and highlight the ecological significance of phytochrome-mediated communication in microbial communities.

Seabirds' distribution is generally influenced by the ecological dynamics of marine environments. Understanding how oceanographic features shape seabird foraging behaviour remains a challenge. We combined GPS tracking locations (n = 2883) of 39 European Storm-petrels ( Hydrobates pelagicus) breeding in four West Mediterranean colonies during incubation over multiple years (2019-2021) with near-real-time remotely sensed oceanographic drivers. We model habitat selection using GPS tracking data from one colony, Benidorm Island, and use data from other three colonies for validation. We show that suitable foraging areas are strongly characterized by low sea surface temperature, high chlorophyll concentration and eddy kinetic energy. Based on this model, we predict habitat suitability maps for 2018-2022. Cross-validation using data from the other three colonies highlights that the identified suitable areas are universally applicable across other Storm-petrel colonies in the Western Mediterranean. We identified the Alboran Sea, the North African coast, the Gulf of Lion and the Ebro River Delta as the most suitable regions. These areas are characterized by high mesoscale variability, suggesting the importance of dynamic oceanographic features in determining foraging habitat. Identified main foraging areas are largely outside of Marine Protected Areas (MPAs), thus vulnerable to anthropogenic threats such as overfishing and energy infrastructure development. The critical foraging areas identified for this species underscore the need to expand the MPA network and/or adopt sustainable resource extraction in unprotected marine areas.

High-biomass microalgal blooms frequently occur in littoral environments worldwide, often causing noxious effects on aquatic ecosystems and coastal communities. Here, we combine field observations and a simple retention-dispersion model to disentangle the short-term (hours) environmental drivers shaping the nearshore dynamics of such outbreaks. Temperature, salinity, fluorescence, current velocities, and meteorological variables were measured in the nearshore waters of a coastal location in Mallorca (Balearic Islands) during the summer of 2018. Daily averages from field data were used to adjust wind and buoyancy flow variations into a one-dimensional advection-diffusion model. Results reveal that the interplay between wind forcing and cross-shore density gradients drives an alternating retention dispersion mechanism, effectively explaining the observed diel chlorophyll variability within the nearshore boundary. This simplified model captures the primary dynamics of the bloom, isolating key factors that influence its behavior and offering practical insights for coastal water quality monitoring and management.

Fluorescence in phytoplankton and other autotrophic organisms originates within the cell chloroplasts, where a fraction of the absorbed solar radiation is reemitted at longer wavelengths by photopigments. While traditionally employed as an indicator of physiological status, emerging evidence suggests that natural chlorophyll fluorescence (ChlF) may also play unanticipated functional roles in the marine environment. Here, we examine the ChlF emission fields generated by pennate planktonic diatoms, a key phytoplankton group playing a critical role in global biogeochemical cycles. Using cell micromanipulation experiments, we demonstrate that the ChlF emitted by Pseudo-nitzschia fraudulenta (Bacillariophyceae) is markedly anisotropic, a feature attributed to both the cells’ elongated morphology and the arrangement of chloroplasts within the cytoplasm. In these diatoms, fluorescence is preferentially emitted in the transapical direction, accounting for up to 35% of total emission. However, peak ChlF intensities occur at the cell apices, suggesting that the silica frustule focuses fluorescent light emission along the longitudinal axis. At elevated cell densities (~10 6 cells/L), the underwater light field is modulated by the combined effects of ChlF emission anisotropy and preferential alignment of diatom cells within the water column. Numerical simulations indicate that ChlF intensity can vary by up to 15% depending on whether cells are predominantly aligned—commonly in stratified water columns—or randomly oriented. These results suggest that diatom-driven modulation of the light field through structured ChlF emission may influence microscale optical environments, with potential consequences for processes ranging from intercellular signaling to large-scale phytoplankton dynamics, including remote sensing–based assessments of phytoplankton physiology.

Early-life stages play a key role in the dynamics of bipartite life cycle marine fish populations. Difficult to monitor, observations of these stages are often scattered in space and time. While Mediterranean coastlines have often been surveyed, no effort has been made to assemble historical observations. Here we build an exhaustive compilation of dispersal traits for coastal fish species, considering in situ observations and growth models (Di Stefano et al., 2023; https://doi.org/10.17882/91148). Our database contains over 110 000 entries collected from 1993 to 2021 in various subregions. All observations are harmonized to provide information on dates and geolocations of both spawning and settlement, along with pelagic larval durations. When applicable, missing data and associated confidence intervals are reconstructed from dynamic energy budget theory. Statistical analyses allow traits’ variability to be revisited and sampling biases to be revealed across taxa, space and time, hence providing recommendations for future studies and sampling. Comparison of observed and modelled entries provides suggestions to improve the feed of observations into models. Overall, this extensive database is a crucial step to investigate how marine fish populations respond to global changes across environmental gradients.

The phytochrome superfamily, a group of proteins that enable some organisms to detect changes in light intensity and quality, is widespread in terrestrial and marine microbes, fungi, algae, and plants. In terrestrial plants, these photosensory receptors monitor variations in the light environment by sensing red ( R ) and far-red ( FR ) regions of the spectrum and trigger important developmental, metabolic, and physiological responses. However, the role of these photosensors in marine microbes, living in environments where, due to absorption of water molecules R and FR radiation does not penetrate beyond the upper few meters, remains controversial. Here, we investigate the role of phytochromes in light perception of the marine diatom Phaeodactylum tricornutum and their involvement in light-driven collective behavior. We perform experiments comparing the social conduct of wild-type and phytochrome knock-out strains to different light wavelengths. Our results show that cell movements become synchronized in a coordinated wobbling dance upon activation of their phytochromes by blue or far-red light, therefore, demonstrating the key role of phytochrome in light-mediated diatom collective behaviour. Furthermore, our experiments suggest that the observed phytochrome-mediated concerted dance implies a form of intercell communication, proposedly mediated by variable R/FR autofluorescence emission in the frequency range of diatom wobbling movements. Our findings provide new insights into communication pathways in aquatic microorganisms and emphasize the importance of social conduct in the sea at all ecological levels. ### Competing Interest Statement The authors have declared no competing interest.

Along some Mediterranean coastal areas and other world regions, nutrient and chlorophyll concentrations often show gradient increases of up to one order of magnitude perpendicular to the coast. This nearshore stripe, extending a few hundred meters from the coast, is enriched by submarine groundwater discharges (SGD) containing elevated nutrient concentrations that may eventually sustain high biomass phytoplankton blooms. During a survey carried out in the summer of 2018, we examined the short-term (hours) variability of the phytoplankton biomass (measured as chlorophyll; Chl) in response to environmental changes associated with SGD and wind forcing in the nearshore waters of Palma Beach (Mediterranean Sea). Continuous CTD records revealed a general salinity decline indicative of SGD along the shoreline. Large and pulsed salinity fluctuations (i.e. 2-3 psu variations, 1-4 h) were observed each day that were consistent with offshore advection episodes of the lower salinity water retained in the nearshore (peak crosshore velocity 5-6 cm s-1). Chl near the shoreline was markedly higher than offshore (3.55 ± 1.29 and 0.68 ± 0.27 mg m-3 respectively) but recurrently fluctuated in the afternoon to up to >7 mg m-3. Primary production estimations showed that despite the higher production in the nearshore (50.29 ± 10.98 μmol O2 L-1 d-1, 4-fold offshore values) productivity per unit chlorophyll did not significantly vary (p > 0.01) therefore suggesting that, at this time scales, high biomass episodes in the nearshore are driven by an accumulation mechanism. Statistical analysis (CCA) demonstrates that Chl variability is largely explained (93 %) by variations in wind and current velocity. Our results provide evidence that the dynamics of this nearshore environment are modulated by the interplay between the shoreward wind-induced flow and the offshore directed density flow. This mechanism could explain the occurrence and episodic nature of high biomass blooms in the nearshore, as well as be an important factor influencing the microbial community structure at the coastal zone.

The Southwestern Atlantic Ocean (SAO), is considered as one of the most productive areas of the world, with high abundance of ecologically and economical important fish species. Yet, the biological responses of this complex region to climate variability are still uncertain. Here, using 24 years of satellite derived Chl-a datasets, we classified the SAO into coherent regions based on homogeneous temporal variability of Chl-a concentration, as revealed by the SOM (Self-Organizing Maps) analysis. These coherent biogeographical regions were the basis of our regional trend analysis in phytoplankton biomass, regional phenological indices, and environmental forcing variations. A generalized positive trend in phytoplankton concentration is observed, especially in the highly productive areas of the northern shelf-break, where phytoplankton biomass is increasing at an outstanding rate up to 0.42 ± 0.04 mg m-3 per decade associated with the sea surface temperature (SST) warming (0.11 ± 0.02 °C decade-1) and the mixed layer depth shoaling (-3.36 ± 0.13 m decade-1). In addition to the generalized increase in chlorophyll, the most sticking changes in phytoplankton dynamics observed in the SAO are related to the secondary bloom that occurs in most of the regions (15 ± 3 and 24 ± 6 days decade-1) which might be explained by the significant warming trend of SST, which would sustain the water stratification for a longer period, thus delaying the secondary bloom initialization. Consistent with previous studies, our results provided further evidences of the impact of climate change in these highly productive waters.

The coastal ocean is experiencing changes in its physical and chemical properties that strongly affect planktonic metabolism assemblages and, in some cases, favor the occurrence of harmful algal blooms (HABs). Here we analyze the variations in phytoplankton biomass, gross and net primary production (NCP) as well as community respiration (CR) at two nearshore sampling sites (P1 and P2) located at a Mediterranean beach where high biomass HABs are recurrent. At P1, the most exposed site, phytoplankton chlorophyll was generally low, whereas dinoflagellates outbreaks of the genus Gymnodinium and Alexandrium were recurrent during summer at P2 spanning for 10-20 days. During bloom episodes, NCP increased up to 10-fold (>80 mmol O2 m(-3) day(-1)). Contrastingly, variation in CR only reached an average of 1.8-fold the rates of non-bloom conditions. Remarkably, although the enhanced NCP:CR ratio suggests net autotrophic population growth, production per unit biomass at P1 and P2 was not significantly different. Our results indicate that although summer conditions favor the necessary primary production enhancement leading to HAB occurrences, the short-term dynamics driving high biomass episodes are not driven by metabolic variations but instead are governed by subtle accumulative processes of some flagellate species in the nutrient-rich nearshore environment.

We examine 20 years of monthly global ocean color data and modeling outputs of nutrients using selforganizing map (SOM) analysis to identify characteristic spatial and temporal patterns of high-nutrient lowchlorophyll (HNLC) regions and their association with different climate modes. The global nitrate-to-chlorophyll ratio threshold of NO3 : Chl > 17 (mmolNO(3) mg Chl(-1)) is estimated to be a good indicator of the distribution limit of this unproductive biome that, on average, covers 92 x 10(6) km(2) ( similar to 25% of the ocean). The trends in satellite-derived surface chlorophyll (0.6 +/- 0.4% yr 1 to 2 +/- 0.4% yr(-1)) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades, but much of this variation, particularly in the Southern Ocean, is produced by a climate-driven transition in 2009-2010. Indeed, since 2010, the extent of the HNLC zones has decreased at the poles (up to 8 %) and slightly increased at the Equator (< 0.5 %). Our study finds that chlorophyll variations in HNLC regions respond to major climate variability signals such as the El Nino-Southern Oscillation (ENSO) and Meridional Overturning Circulation (MOC) at both short (2-4 years) and long (decadal) timescales. These results suggest global coupling in the functioning of distant biogeochemical regions.

Nursery areas are essential fish habitats due to their relevance in the survival of early stages of fish populations. They are also considered as of high priority in marine conservation strategies. Here, we investigated the diet of white seabream [Diplodussargus (Linnaeus, 1758)] settlers in six nursery areas located in the shallow waters of coves in Minorca Island (Balearic Islands, Spain). Our aim was to characterize the food sources at different stages of juvenile development and their site-related variability in order to discern the importance of trophic links in cove selection during settling. The gut contents of 101 juveniles captured at different coves, three to the north of the island (N), and three in the southern coast (S), revealed a marked preference for feeding on crustaceans and, in particular, on harpacticoid copepods (>90% of gut contents). Copepods represented the main food source (80 ±4.4%; mean ±S.E.) in younger seabream individuals (10 - 15 mm length). A higher diversity in prey items was observed in the larger size-classes (s2: 16 - 23 and S3: 24 - 30 mm, respectively), which incorporated other prey items such as amphipods, isopods, foraminiferans or ostracods. Diet composition did not vary between the two surveyed locations (North vs. South of the island), but it did show significant differences among the six coves (p<0.001). Comparison between cove sediment infaunal composition and gut contents revealed that predation on sediment communities was scarce. Instead, diet was typically of phytal origin. Our results highlight the importance of the algal component of shallow coastal areas as a foraging habitat. In particular, harpacticoid copepods were key for survival during early development phases. The potential use of harpacticoid copepods to track ontogenic shifts in habitat use by juvenile fish is discussed.

Supple mentary material S1.1 SOM analysisSOM is a subtype of artificial neural network that uses an unsupervised machine learning algorithm to process and extract hidden structures in large amount of data.The learning process seeks to identify low-dimensional features in high-dimensional data according to a similarity measure while preserving the topological properties of the input space.This technique provides

We estimated pelagic primary production (PP) in the coastal (<200 m depth) Mediterranean Sea from satellite-borne data, its contribution to basin-scale carbon fixation, its variability, and long-term trends during the period 2002–2016. Annual coastal PP was estimated at 0.041 Gt C, which approximately represents 12 % of total carbon fixation in the Mediterranean Sea. About 51 % of this production occurs in the eastern basin, whereas the western and Adriatic shelves contribute with ∼25 % each of total coastal production. Strong regional variability is revealed in coastal PP, from high-production areas (>300 g C m−2) associated with major river discharges to less productive provinces (<50 g C m−2) located in the southeastern Mediterranean. PP variability in the Mediterranean Sea is dominated by interannual variations, but a notable basin-scale decline (17 %) has been observed since 2012 concurring with a period of increasing sea surface temperatures in the Mediterranean Sea and positive North Atlantic Oscillation and Mediterranean Oscillation climate indices. Long-term trends in PP reveal slight declines in most coastal areas (−0.05 to −0.1 g C m−2 per decade) except in the Adriatic where PP increases at +0.1 g C m−2 per decade. Regionalization of coastal waters based on PP seasonal patterns reveals the importance of river effluents in determining PP in coastal waters that can regionally increase up to 5-fold. Our study provides insight into the contribution of coastal waters to basin-scale carbon balances in the Mediterranean Sea while highlighting the importance of the different temporal and spatial scales of variability.

Communication between conspecific individuals is an essential part of life both in terrestrial and marine realms. Until recently, social behavior in marine phytoplankton was assumed to rely mainly on the secretion of a variety of infochemicals that allowed population-scale collective responses. Here, we demonstrate that pelagic diatoms also use Sun-stimulated fluorescence signals for synchronizing their behavior. These unicellular microorganisms, playing a key biogeochemical role in the ocean, use photoreceptor proteins and red–far-red fluorescent radiation to communicate. A characteristic beaconing signal is generated by rhythmic organelle displacement within the cell cytoplasm, triggering coordinated population behavior. These light-based communication networks could critically determine major facets of diatom ecology and fitness and regulate the dynamics of larger-scale ocean processes.

Abstract. We estimated pelagic primary production (PP) in the coastal ( 300 g C m−2) associated with major river discharges, to less productive provinces (