Global mesozooplankton communities show lower connectivity in deep oceanic layers.

Mesozooplankton is a key component of the ocean, regulating global processes such as the carbon pump, and ensuring energy transfer from lower to higher trophic levels. Yet, knowledge on mesozooplankton diversity, distribution and connectivity at global scale is still fragmented. To fill this gap, we applied DNA metabarcoding to mesozooplankton samples collected during the Malaspina-2010 circumnavigation expedition across the Atlantic, Indian and Pacific oceans from the surface to bathypelagic depths. We highlight the still scarce knowledge on global mesozooplankton diversity and identify the Indian Ocean and the deep sea as the oceanic regions with the highest proportion of hidden diversity. We report no consistent alpha-diversity patterns for mesozooplankton at a global scale, neither across vertical nor horizontal gradients. However, beta-diversity analysis suggests horizontal and vertical structuring of mesozooplankton communities mostly attributed to turnover and reveals an increase in mesozooplankton beta-diversity with depth, indicating reduced connectivity at deeper layers. Additionally, we identify a water mass type-mediated structuring of mesozooplankton bathypelagic communities instead of an oceanic basin-mediated as observed at upper layers. This suggests limited dispersal at deep ocean layers, most likely due to weaker currents and lower mixing of water mass types, thus reinforcing the importance of oceanic currents and barriers to dispersal in shaping global plankton communities.

Shrinking body size of European anchovy in the Bay of Biscay.

Decreased body size is often cited as a major response to ocean warming. Available evidence, however, questions the actual emergence of shrinking trends and the prevalence of temperature-driven changes in size over alternative drivers. In marine fish, changes in food availability or fluctuations in abundance, including those due to size-selective fishing, provide compelling mechanisms to explain changes in body size. Here, based on three decades of scientific survey data (1990-2021), we report a decline in the average body size-length and weight-of anchovy, Engraulis encrasicolus L., in the Bay of Biscay. Shrinking was evident in all age classes, from juveniles to adults. Allometric adjustment indicated slightly more pronounced declines in weight than in total length, which is consistent with a change toward a slender body shape. Trends in adult weight were nonlinear, with rates accelerating to an average decline of up to 25% decade-1 during the last two decades. We found a strong association between higher anchovy abundance and reduced juvenile size. The effect of density dependence was less clear later in life, and temperature became the best predictor of declines in adult size. Theoretical analyses based on a strategic model further suggested that observed patterns are consistent with a simultaneous, opposing effect of rising temperatures on accelerating early growth and decreasing adult size as predicted by the temperature-size rule. Macroecological assessment of ecogeographical-Bergmann's and James'-rules in anchovy size suggested that the observed decline largely exceeds intraspecific variation and might be the result of selection. Limitations inherent in the observational nature of the study recommend caution and a continued assessment and exploration of alternative drivers. Additional evidence of a climate-driven regime shift in the region suggests, however, that shrinking anchovy sizes may signal a long-lasting change in the structure and functioning of the Bay of Biscay ecosystem.

Genomic landscape of geographically structured colour polymorphism in a temperate marine fish.

The study of phenotypic variation patterns among populations is fundamental to elucidate the drivers of evolutionary processes. Empirical evidence that supports ongoing genetic divergence associated with phenotypic variation remains very limited for marine species where larval dispersal is a common homogenizing force. We present a genome-wide analysis of a marine fish, Labrus bergylta, comprising 144 samples distributed from Norway to Spain, a large geographical area that harbours a gradient of phenotypic differentiation. We analysed 39,602 biallelic single nucleotide polymorphisms and found a clear latitudinal gradient of genomic differentiation strongly correlated with the variation in phenotypic morph frequencies observed across the North Atlantic. We also detected a strong association between the latitude and the number of loci that appear to be under divergent selection, which increased with differences in coloration but not with overall genetic differentiation. Our results demonstrate that strong reproductive isolation is occurring between sympatric colour morphs of L. bergylta found at the southern areas and provide important new insights into the genomic changes shaping early stages of differentiation that might precede speciation with gene flow.

Sequencing effort dictates gene discovery in marine microbial metagenomes.

Massive metagenomic sequencing combined with gene prediction methods were previously used to compile the gene catalogue of the ocean and host-associated microbes. Global expeditions conducted over the past 15 years have sampled the ocean to build a catalogue of genes from pelagic microbes. Here we undertook a large sequencing effort of a perturbed Red Sea plankton community to uncover that the rate of gene discovery increases continuously with sequencing effort, with no indication that the retrieved 2.83 million non-redundant (complete) genes predicted from the experiment represented a nearly complete inventory of the genes present in the sampled community (i.e., no evidence of saturation). The underlying reason is the Pareto-like distribution of the abundance of genes in the plankton community, resulting in a very long tail of millions of genes present at remarkably low abundances, which can only be retrieved through massive sequencing. Microbial metagenomic projects retrieve a variable number of unique genes per Tera base-pair (Tbp), with a median value of 14.7 million unique genes per Tbp sequenced across projects. The increase in the rate of gene discovery in microbial metagenomes with sequencing effort implies that there is ample room for new gene discovery in further ocean and holobiont sequencing studies.

Pan-regional marine benthic cryptobiome biodiversity patterns revealed by metabarcoding Autonomous Reef Monitoring Structures.

Autonomous Reef Monitoring Structures (ARMS) have been applied worldwide to characterize the critical yet frequently overlooked biodiversity patterns of marine benthic organisms. In order to disentangle the relevance of environmental factors in benthic patterns, here, through standardized metabarcoding protocols, we analyse sessile and mobile (<2 mm) organisms collected using ARMS deployed across six regions with different environmental conditions (3 sites × 3 replicates per region): Baltic, Western Mediterranean, Adriatic, Black and Red Seas, and the Bay of Biscay. A total of 27,473 Amplicon Sequence Variants (ASVs) were observed ranging from 1,404 in the Black Sea to 9,958 in the Red Sea. No ASVs were shared among all regions. The highest number of shared ASVs was between the Western Mediterranean and the Adriatic Sea (116) and Bay of Biscay (115). Relatively high numbers of ASVs (103), mostly associated with the genus Amphibalanus, were also shared between the lower salinity seas (Baltic and Black Seas). We found that compositional differences in spatial patterns of rocky-shore benthos are determined slightly more by dispersal limitation than environmental filtering. Dispersal limitation was similar between sessile and mobile groups, while the sessile group had a larger environmental niche breadth than the mobile group. Further, our study can provide a foundation for future evaluations of biodiversity patterns in the cryptobiome, which can contribute up to 70% of the local biodiversity.

Composition, uniqueness and connectivity across tropical coastal lagoon habitats in the Red Sea.

BACKGROUND: Tropical habitats and their associated environmental characteristics play a critical role in shaping macroinvertebrate communities. Assessing patterns of diversity over space and time and investigating the factors that control and generate those patterns is critical for conservation efforts. However, these factors are still poorly understood in sub-tropical and tropical regions. The present study applied a combination of uni- and multivariate techniques to test whether patterns of biodiversity, composition, and structure of macrobenthic assemblages change across different lagoon habitats (two mangrove sites; two seagrass meadows with varying levels of vegetation cover; and an unvegetated subtidal area) and between seasons and years. RESULTS: In total, 4771 invertebrates were identified belonging to 272 operational taxonomic units (OTUs). We observed that macrobenthic lagoon assemblages are diverse, heterogeneous and that the most evident biological pattern was spatial rather than temporal. To investigate whether macrofaunal patterns within the lagoon habitats (mangrove, seagrass, unvegetated area) changed through the time, we analysed each habitat separately. The results showed high seasonal and inter-annual variability in the macrofaunal patterns. However, the seagrass beds that are characterized by variable vegetation cover, through time, showed comparatively higher stability (with the lowest values of inter-annual variability and a high number of resident taxa). These results support the theory that seagrass habitat complexity promotes diversity and density of macrobenthic assemblages. Despite the structural and functional importance of seagrass beds documented in this study, the results also highlighted the small-scale heterogeneity of tropical habitats that may serve as biodiversity repositories. CONCLUSIONS: Comprehensive approaches at the "seascape" level are required for improved ecosystem management and to maintain connectivity patterns amongst habitats. This is particularly true along the Saudi Arabian coast of the Red Sea, which is currently experiencing rapid coastal development. Also, considering the high temporal variability (seasonal and inter-annual) of tropical shallow-water habitats, monitoring and management plans must include temporal scales.

Plastic debris in the open ocean.

There is a rising concern regarding the accumulation of floating plastic debris in the open ocean. However, the magnitude and the fate of this pollution are still open questions. Using data from the Malaspina 2010 circumnavigation, regional surveys, and previously published reports, we show a worldwide distribution of plastic on the surface of the open ocean, mostly accumulating in the convergence zones of each of the five subtropical gyres with comparable density. However, the global load of plastic on the open ocean surface was estimated to be on the order of tens of thousands of tons, far less than expected. Our observations of the size distribution of floating plastic debris point at important size-selective sinks removing millimeter-sized fragments of floating plastic on a large scale. This sink may involve a combination of fast nano-fragmentation of the microplastic into particles of microns or smaller, their transference to the ocean interior by food webs and ballasting processes, and processes yet to be discovered. Resolving the fate of the missing plastic debris is of fundamental importance to determine the nature and significance of the impacts of plastic pollution in the ocean.

RAD-seq derived genome-wide nuclear markers resolve the phylogeny of tunas.

Although species from the genus Thunnus include some of the most commercially important and most severely overexploited fishes, the phylogeny of this genus is still unresolved, hampering evolutionary and traceability studies that could help improve conservation and management strategies for these species. Previous attempts based on mitochondrial and nuclear markers were unsuccessful in inferring a congruent and reliable phylogeny, probably due to mitochondrial introgression events and lack of enough phylogenetically informative markers. Here we infer the first genome-wide nuclear marker-based phylogeny of tunas using restriction site associated DNA sequencing (RAD-seq) data. Our results, derived from phylogenomic inferences obtained from 128 nucleotide matrices constructed using alternative data assembly procedures, support a single Thunnus evolutionary history that challenges previous assumptions based on morphological and molecular data.

Carotenoid metabolic profiling and transcriptome-genome mining reveal functional equivalence among blue-pigmented copepods and appendicularia.

The tropical oligotrophic oceanic areas are characterized by high water transparency and annual solar radiation. Under these conditions, a large number of phylogenetically diverse mesozooplankton species living in the surface waters (neuston) are found to be blue pigmented. In the present study, we focused on understanding the metabolic and genetic basis of the observed blue phenotype functional equivalence between the blue-pigmented organisms from the phylum Arthropoda, subclass Copepoda (Acartia fossae) and the phylum Chordata, class Appendicularia (Oikopleura dioica) in the Red Sea. Previous studies have shown that carotenoid-protein complexes are responsible for blue coloration in crustaceans. Therefore, we performed carotenoid metabolic profiling using both targeted and nontargeted (high-resolution mass spectrometry) approaches in four different blue-pigmented genera of copepods and one blue-pigmented species of appendicularia. Astaxanthin was found to be the principal carotenoid in all the species. The pathway analysis showed that all the species can synthesize astaxanthin from ß-carotene, ingested from dietary sources, via 3-hydroxyechinenone, canthaxanthin, zeaxanthin, adonirubin or adonixanthin. Further, using de novo assembled transcriptome of blue A. fossae (subclass Copepoda), we identified highly expressed homologous ß-carotene hydroxylase enzymes and putative carotenoid-binding proteins responsible for astaxanthin formation and the blue phenotype. In blue O. dioica (class Appendicularia), corresponding putative genes were identified from the reference genome. Collectively, our data provide molecular evidences for the bioconversion and accumulation of blue astaxanthin-protein complexes underpinning the observed ecological functional equivalence and adaptive convergence among neustonic mesozooplankton.

Biomass changes and trophic amplification of plankton in a warmer ocean.

Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels.

Diel dynamics of dissolved organic matter and heterotrophic prokaryotes reveal enhanced growth at the ocean's mesopelagic fish layer during daytime.

Contrary to epipelagic waters, where biogeochemical processes closely follow the light and dark periods, little is known about diel cycles in the ocean's mesopelagic realm. Here, we monitored the dynamics of dissolved organic matter (DOM) and planktonic heterotrophic prokaryotes every 2 h for one day at 0 and 550 m (a depth occupied by vertically migrating fishes during light hours) in oligotrophic waters of the central Red Sea. We additionally performed predator-free seawater incubations of samples collected from the same site both at midnight and at noon. Comparable in situ variability in microbial biomass and dissolved organic carbon concentration suggests a diel supply of fresh DOM in both layers. The presence of fishes in the mesopelagic zone during daytime likely promoted a sustained, longer growth of larger prokaryotic cells. The specific growth rates were consistently higher in the noon experiments from both depths (surface: 0.34 vs. 0.18 d-1, mesopelagic: 0.16 vs. 0.09 d-1). Heterotrophic prokaryotes in the mesopelagic layer were also more efficient at converting extant DOM into new biomass. These results suggest that the ocean's twilight zone receives a consistent diurnal supply of labile DOM from the diel vertical migration of fishes, enabling an unexpectedly active community of heterotrophic prokaryotes.

Climate regime shifts and biodiversity redistribution in the Bay of Biscay.

Global ocean warming, wave extreme events, and accelerating sea-level rise are challenges that coastal communities must address to anticipate damages in coming decades. The objective of this study is to undertake a time-series analysis of climate change (CC) indicators within the Bay of Biscay, including the Basque coast. We used an integrated and flexible methodology, based on Generalized Additive Mixed Models, to detect trends on 19 indicators (including marine physics, chemistry, atmosphere, hydrology, geomorphology, biodiversity, and commercial species). The results of 87 long-term time series analysed (~512,000 observations), in the last four decades, indicate four groups of climate regime shifts: 1) A gradual shift associated with CC starting in the 1980s, with a warming of the sea surface down to 100 m depth in the bay (0.10-0.25 °C per decade), increase in air temperature and insolation. This warming may have impacted on benthic community redistribution in the Basque coast, favouring warm-water species relative to cold-water species. Weight at age for anchovy and sardine decreased in the last two decades. 2) Deepening of the winter mixed layer depth in the south-eastern bay that probably led to increases in nutrients, surface oxygen, and chlorophyll concentration. Current increases on chlorophyll and zooplankton (i.e., copepods) biomass are contrary to those expected under CC scenarios in the region. 3) Sea-level rise (1.5-3.5 cm per decade since 1990s), associated with CC. 4) Increase of extreme wave height events of 16.8 cm per decade in the south-eastern bay, probably related to stormy conditions in the last decade, with impacts on beach erosion. Estimating accurate rates of sea warming, sea-level rise, extreme events, and foreseeing the future pathways of marine productivity, are key to define the best adaptation measures to minimize negative CC impacts in the region.

The global network of ports supporting high seas fishing.

Fisheries in waters beyond national jurisdiction ("high seas") are difficult to monitor and manage. Their regulation for sustainability requires critical information on how fishing effort is distributed across fishing and landing areas, including possible border effects at the exclusive economic zone (EEZ) limits. We infer the global network linking harbors supporting fishing vessels to fishing areas in high seas from automatic identification system tracking data in 2014, observing a modular structure, with vessels departing from a given harbor fishing mostly in a single province. The top 16% of these harbors support 84% of fishing effort in high seas, with harbors in low- and middle-income countries ranked among the top supporters. Fishing effort concentrates along narrow strips attached to the boundaries of EEZs with productive fisheries, identifying a free-riding behavior that jeopardizes efforts by nations to sustainably manage their fisheries, perpetuating the tragedy of the commons affecting global fishery resources.

Subseafloor Archaea reflect 139 kyrs of paleodepositional changes in the northern Red Sea.

The vertical distribution of subseafloor archaeal communities is thought to be primarily controlled by in situ conditions in sediments such as the availability of electron acceptors and donors, although sharp community shifts have also been observed at lithological boundaries suggesting that at least a subset of vertically stratified Archaea form a long-term genetic record of coinciding environmental conditions that occurred at the time of sediment deposition. To substantiate this possibility, we performed a highly resolved 16S rRNA gene survey of vertically stratified archaeal communities paired with paleo-oceanographic proxies in a sedimentary record from the northern Red Sea spanning the last glacial-interglacial cycle (i.e., marine isotope stages 1-6; MIS1-6). Our results show a strong significant correlation between subseafloor archaeal communities and drastic paleodepositional changes associated with glacial low vs. interglacial high stands (ANOSIM; R = .73; p = .001) and only a moderately strong correlation with lithological changes. Bathyarchaeota, Lokiarchaeota, MBGA, and DHVEG-1 were the most abundant identified archaeal groups. Whether they represented ancient cell lines from the time of deposition or migrated to the specific sedimentary horizons after deposition remains speculative. However, we show that the majority of sedimentary archaeal tetraether membrane lipids were of allochthonous origin and not produced in situ. Slow post-burial growth under energy-limited conditions would explain why the downcore distribution of these dominant archaeal groups still indirectly reflect changes in the paleodepositional environment that prevailed during the analyzed marine isotope stages. In addition, archaea seeded from the overlying water column such as Thaumarchaeota and group II and III Euryarchaeota, which were likely not have been able to subsist after burial, were identified from a lower abundance of preserved sedimentary DNA signatures, and represented direct markers of paleoenvironmental changes in the Red Sea spanning the last six marine isotope stages.

The Simrad EK60 echosounder dataset from the Malaspina circumnavigation.

We provide the raw acoustic data collected from the R/V Hesperides during the global Malaspina 2010 Spanish Circumnavigation Expedition (14th December 2010, Cádiz-14th July 2011, Cartagena) using a Simrad EK60 scientific echosounder operating at 38 and 120 kHz. The cruise was divided into seven legs: leg 1 (14th December 2010, Cádiz-13th January 2011, Rio de Janeiro), leg 2 (17th January 2011, Rio de Janeiro-6th February 2011, Cape Town), leg 3 (11th February 2011, Cape Town-13th March 2011, Perth), leg 4 (17th March 2011, Perth-30th March 2011, Sydney), leg 5 (16th April 2011, Auckland-8th May 2011, Honolulu), leg 6 (13th May 2011, Honolulu-10th June 2011, Cartagena de Indias) and leg 7 (19th June 2011, Cartagena de Indias-14th July 2011, Cartagena). The echosounder was calibrated at the start of the expedition and calibration parameters were updated in the data acquisition software (ER60) i.e., the logged raw data are calibrated. We also provide a data summary of the acoustic data in the form of post-processed products.

Global biodiversity patterns of marine phytoplankton and zooplankton.

Although the oceans cover 70% of the Earth's surface, our knowledge of biodiversity patterns in marine phytoplankton and zooplankton is very limited compared to that of the biodiversity of plants and herbivores in the terrestrial world. Here, we present biodiversity data for marine plankton assemblages from different areas of the world ocean. Similar to terrestrial vegetation, marine phytoplankton diversity is a unimodal function of phytoplankton biomass, with maximum diversity at intermediate levels of phytoplankton biomass and minimum diversity during massive blooms. Contrary to expectation, we did not find a relation between phytoplankton diversity and zooplankton diversity. Zooplankton diversity is a unimodal function of zooplankton biomass. Most strikingly, these marine biodiversity patterns show a worldwide consistency, despite obvious differences in environmental conditions of the various oceanographic regions. These findings may serve as a new benchmark in the search for global biodiversity patterns of plants and herbivores.

Marine water environmental DNA metabarcoding provides a comprehensive fish diversity assessment and reveals spatial patterns in a large oceanic area.

Current methods for monitoring marine fish (including bony fishes and elasmobranchs) diversity mostly rely on trawling surveys, which are invasive, costly, and time-consuming. Moreover, these methods are selective, targeting a subset of species at the time, and can be inaccessible to certain areas. Here, we used environmental DNA (eDNA), the DNA present in the water column as part of shed cells, tissues, or mucus, to provide comprehensive information about fish diversity in a large marine area. Further, eDNA results were compared to the fish diversity obtained in pelagic trawls. A total of 44 5 L-water samples were collected onboard a wide-scale oceanographic survey covering about 120,000 square kilometers in Northeast Atlantic Ocean. A short region of the 12S rRNA gene was amplified and sequenced through metabarcoding generating almost 3.5 million quality-filtered reads. Trawl and eDNA samples resulted in the same most abundant species (European anchovy, European pilchard, Atlantic mackerel, and blue whiting), but eDNA metabarcoding resulted in more detected bony fish and elasmobranch species (116) than trawling (16). Although an overall correlation between fishes biomass and number of reads was observed, some species deviated from the common trend, which could be explained by inherent biases of each of the methods. Species distribution patterns inferred from eDNA metabarcoding data coincided with current ecological knowledge of the species, suggesting that eDNA has the potential to draw sound ecological conclusions that can contribute to fish surveillance programs. Our results support eDNA metabarcoding for broad-scale marine fish diversity monitoring in the context of Directives such as the Common Fisheries Policy or the Marine Strategy Framework Directive.

Picocyanobacteria Community and Cyanophage Infection Responses to Nutrient Enrichment in a Mesocosms Experiment in Oligotrophic Waters.

Prochlorococcus and Synechococcus are pico-sized cyanobacteria that play a fundamental role in oceanic primary production, being particularly important in warm, nutrient-poor waters. Their potential response to nutrient enrichment is expected to be contrasting and to differ from larger phytoplankton species. Here, we used a metagenomic approach to characterize the responses to nutrient enrichment in the community of picocyanobacteria and to analyze the cyanophage response during a mesocosms experiment in the oligotrophic Red Sea. Natural picoplankton community was dominated by Synechococcus clade II, with marginal presence of Prochlorococcus (0.3% bacterial reads). Increased nutrient input triggered a fast Synechococcus bloom, with clade II being the dominant, with no response of Prochlorococcus growth. The largest bloom developed in the mesocosms receiving a single initial input of nutrients, instead of daily additions. The relative abundances of cyanophage sequences in cellular metagenomes increased during the experiment from 12.6% of total virus reads up to 40% in the treatment with the largest Synechococcus bloom. The subsequent collapse of the bloom pointed to a cyanophage infection on Synechococcus that reduced its competitive capacity, and was then followed by a diatom bloom. The cyanophage attack appears to have preferentially affected the most abundant Synechococcus clade II, increasing the evenness within the host population. Our results highlight the relevance of host-phage interactions on determining population dynamics and diversity of Synechococcus populations.

Seasonal variability and vertical distribution of autotrophic and heterotrophic picoplankton in the Central Red Sea.

The Red Sea is characterized by higher temperatures and salinities than other oligotrophic tropical regions. Here, we investigated the vertical and seasonal variations in the abundance and biomass of autotrophic and heterotrophic picoplankton. Using flow cytometry, we consistently observed five groups of autotrophs (Prochlorococcus, two populations of Synechococcus separated by their relative phycoerythrin fluorescence, low (LF-Syn) and high (HF-Syn), and two differently-sized groups of picoeukaryotes, small (Speuk) and large (Lpeuk)) and two groups of heterotrophic prokaryotes of low and high nucleic acid content (LNA and HNA, respectively). Samples were collected in 15 surveys conducted from 2015 to 2017 at a 700-m depth station in the central Red Sea. Surface temperature ranged from 24.6 to 32.6 °C with a constant value of 21.7 °C below 200 m. Integrated (0-100 m) chlorophyll a concentrations were low, with maximum values in fall (24.0 ± 2.7 mg m-2) and minima in spring and summer (16.1 ± 1.9 and 1.1 mg m-2, respectively). Picoplankton abundance was generally lower than in other tropical environments. Vertical distributions differed for each group, with Synechococcus and LNA prokaryotes more abundant at the surface while Prochlorococcus, picoeukaryotes and HNA prokaryotes peaked at the deep chlorophyll maximum, located between 40 and 76 m. Surface to 100 m depth-weighted abundances exhibited clear seasonal patterns for Prochlorococcus, with maxima in summer (7.83 × 104 cells mL-1, July 2015) and minima in winter (1.39 × 104 cells mL-1, January 2015). LF-Syn (0.32 - 2.70 × 104 cells mL-1 ), HF-Syn (1.11 - 3.20 × 104 cells mL-1) and Speuk (0.99 - 4.81 × 102 cells mL-1) showed an inverse pattern to Prochlorococcus, while Lpeuk (0.16 - 7.05 × 104 cells mL-1) peaked in fall. Synechococcus unexpectedly outnumbered Prochlorococcus in winter and at the end of fall. The seasonality of heterotrophic prokaryotes (2.29 - 4.21×105 cells mL-1 ) was less noticeable than autotrophic picoplankton. The contribution of HNA cells was generally low in the upper layers, ranging from 36% in late spring and early summer to ca. 50% in winter and fall. Autotrophs dominated integrated picoplankton biomass in the upper 100 m, with 1.4-fold higher values in summer than in winter (mean 387 and 272 mg C m-2, respectively). However, when the whole water column was considered, the biomass of heterotrophic prokaryotes exceeded that of autotrophic picoplankton with an average of 411 mg C m-2. Despite being located in tropical waters, our results show that the picoplankton community seasonal differences in the central Red Sea are not fundamentally different from higher latitude regions.