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Under-Ice Phytoplankton Blooms: Shedding Light on the “Invisible” Part of Arctic Primary Production

The growth of phytoplankton at high latitudes was generally thought to begin in open waters of the marginal ice zone once the highly reflective sea ice retreats in spring, solar elevation increases, and surface waters become stratified by the addition of sea-ice melt water. In fact, virtually all recent large-scale estimates of primary production in the Arctic Ocean (AO) assume that phytoplankton production in the water column under sea ice is negligible. However, over the past two decades, an emerging literature showing significant under-ice phytoplankton production on a pan-Arctic scale has challenged our paradigms of Arctic phytoplankton ecology and phenology. This evidence, which builds on previous, but scarce reports, requires the Arctic scientific community to change its perception of traditional AO phenology and urgently revise it. In particular, it is essential to better comprehend, on small and large scales, the changing and variable icescapes, the under-ice light field and biogeochemical cycles during the transition from sea-ice covered to ice-free Arctic waters. Here, we provide a baseline of our current knowledge of under-ice blooms (UIBs), by defining their ecology and their environmental setting, but also their regional peculiarities (in terms of occurrence, magnitude, and assemblages), which is shaped by a complex AO. To this end, a multidisciplinary approach, i.e., combining expeditions and modern autonomous technologies, satellite, and modeling analyses, has been used to provide an overview of this pan-Arctic phenological feature, which will become increasingly important in future marine Arctic biogeochemical cycles

Green Edge Ice Camp Campaigns: Understanding the Processes Controlling the Under-Ice Arctic Phytoplankton Spring Bloom

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797∘ N, 63.7895∘ W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea-ice cover from the surface to the bottom (at 360 m depth) to better understand the factors driving the PSB. Key variables, such as conservative temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured at the ice camp. Meteorological and snow-relevant variables were also monitored. Here, we present the results of a joint effort to tidy and standardize the collected datasets, which will facilitate their reuse in other Arctic studies. The dataset is available at https://doi.org/10.17882/59892 (Massicotte et al., 2019a)

Les communautés phytoplanctoniques dans un océan arctique en mutation : iogéographie, phénologie, productivité

L’océan Arctique subit actuellement des modifications majeures et abruptes dans ces compartiments atmosphériques et océaniques reliées au changement climatique. Les premières conséquences écologiques à la perte de la glace de mer sont indéniables, telles qu’une augmentation globale de la production primaire (PP) annuelle à travers l’Arctique. Cependant, dans certaines régions, des études suggèrent plutôt une perte de productivité en réponse à une intensification locale de la stratification verticale. La réponse des communautés arctiques phytoplanctoniques au changement climatique reste toujours une question d’actualité complexe et incertaine, ayant de potentiels impacts sur l’ensemble des niveaux trophiques des écosystèmes marins. L’objectif premier de cette thèse se concentre sur cette problématique, avec une emphase particulière sur la biogéographie, la phénologie (c.-à-d., l’étude des cycles biologiques récurrents annuels) et la productivité des communautés phytoplanctoniques arctiques. Plus spécifiquement, cette étude s’appuie sur deux approches de recherche complémentaires : (1) la compilation et l’analyse de bases de données historiques traitant de la répartition verticale et spatiale, de la productivité et de l’écologie du phytoplancton arctique, et de (2) l’utilisation de la télédétection pour décrire la biogéographie, la phénologie et les changements en cours au niveau des communautés phytoplanctoniques arctiques. Basée sur une compilation inédite de profils verticaux de chlorophylle a (chl a; c.-à-d., 5206 stations), nous avons documenté la variabilité spatio-temporelle de la répartition verticale du phytoplancton et des différents régimes de productivité (de régions oligotrophes à eutrophes) pour l’ensemble de l’océan Arctique. Un modèle empirique a également pu être développé prédisant la répartition verticale de la chl a basée sur les valeurs de chl a de surface en fonction des saisons et des provinces biogéographiques de l’océan Arctique. Ce modèle nous a permis d’améliorer les estimations satellitales de la PP, mais aussi de mieux comprendre l’écologie et la phénologie des communautés phytoplanctoniques. Un intérêt particulier s’est porté sur les mécanismes de formation et de maintien des maxima de chlorophylle de subsurface (MCS) et de leur contribution à la PP annuelle. Par leur position verticale dictée par les profondeurs combinées de la nitracline et des masses d’eaux atlantiques (dans l’Arctique de l’Est) et pacifiques (dans l’Arctique de l’Ouest), ces MCSs contribuent significativement à la PP principalement dans les régions oligotrophes et lors des périodes de post-floraison. Dans un second temps, l’utilisation de la télédétection nous a révélé une conséquence inattendue du recul de la glace de mer en Arctique au niveau de la phénologie du phytoplancton arctique. Les régions qui avaient une seule floraison annuelle il y a seulement une dizaine d’années, développent maintenant une deuxième floraison à l’automne. Cette nouvelle floraison, qui coïncide avec le retard de la prise des glaces et à l’exposition accrue de la surface de l’océan aux vents automnaux, implique que l’océan Arctique pourrait passer d’un mode polaire à un mode boréal. Des scénarios biogéographiques du devenir de la PP annuelle, étroitement liés à la phénologie, peuvent ainsi être définis en réponse au retrait du couvert de glace de mer. Ces prédictions nous aideront à mieux appréhender de possibles changements au niveau des communautés phytoplanctoniques, pouvant engendrer par la suite des répercussions sur le cycle du carbone et les écosystèmes marins arctiques.The Arctic Ocean is currently experiencing major and abrupt changes in its atmospheric and oceanic compartments due to climate change. The first emerging ecological consequences to the loss of sea ice are undeniable, such as increasing annual primary production (PP) globally in the Arctic Ocean. However, in some areas, studies suggest a decrease in productivity in response to a local intensification of the vertical stratification of the upper water column. The response of phytoplankton communities to climate change remains complex and difficult to predict, with potential dramatic impacts extending through all trophic levels of marine ecosystems. The primary objective of this thesis explores this fundamental question, with a particular emphasis on biogeography, phenology (i.e., the study of annual recurring biological cycles) and productivity of Arctic phytoplankton communities. More specifically, this study is based on two complementary research approaches: (1) the compilation and analysis of historical databases covering the vertical and spatial distribution, productivity and ecology of Arctic phytoplankton, and (2) the use of remote sensing data describing the biogeography, phenology and ongoing changes in Arctic phytoplankton communities. Based on a unique compilation of vertical profiles of chlorophyll a (chl a; i.e., 5206 stations), we documented the spatio-temporal variability of the vertical distribution of phytoplankton and the range of productivity regimes (from oligotrophic to eutrophic regions) across the Arctic Ocean. An empirical model has also been developed to predict the vertical distribution of chl a based on surface chl a values depending on season and the province of the Arctic Ocean. The benefits of this model allow us to improve satellite-derived PP estimates and improve our understanding of the ecology and phenology of phytoplankton communities. Particular attention has been focused on the mechanisms of formation and maintenance of subsurface chlorophyll maximum (SCM) and their contribution to annual PP. On account of their vertical position, dictated by the depths of both the nitracline and Atlantic (in the Eastern Arctic) and Pacific (in the Western Arctic) waters, these SCMs appear important to PP, particularly in oligotrophic regions and during post-bloom periods. In a second step, the use of remote sensing could reveal an unexpected consequence of Arctic ice loss on Arctic phytoplankton. Regions that experienced a single annual bloom only a decade ago now develop a second bloom in the fall. This new bloom, which coincides with delayed freeze-up and increasing exposure of the sea surface to winds in the fall, implies that the Arctic Ocean may be shifting from a polar to a temperate mode. Biogeographic scenarios for the future of the annual PP, which is closely related to phenology, can thus be defined in response to the current receding sea-ice cover. These predictions will allow us to better anticipate the possible changes in phytoplankton productivity and community structure and the potential cascading repercussions on the carbon cycle and marine Arctic ecosystems

Influence des facteurs du milieu sur la structure et le fonctionnement des communautés phytoplanctoniques du Haut-Arctique canadien : distinction des régions oligotrophes et eutrophes

RÉSUMÉ: Une étude biogéographique a été menée dans le Haut-Arctique canadien, afm d'examiner la dynamique phytoplanctonique en relation avec les facteurs du milieu. Lors de radiales de 3500 km couvrant la mer de Beaufort, l'archipel canadien et la baie de Baffm, un ensemble de variables environnementales (i.e. la structure hydrographique, et les conditions atmosphériques et celles du couvert de glace de mer) et biologiques ont été mesurées dans la zone euphotique pendant la fin de l'été en 2005, le début de l'automne en 2006 et l' automne en 2007. La production et la biomasse (chlorophylle (chI) a) phytoplanctonique ont été mesurées à sept profondeurs optiques et à la profondeur du maximum de fluorescence chlorophyllienne (ZDCM). De plus, la composition taxinomique, l'abondance et la structure de taille du phytoplancton ont été caractérisées au Z DCM. À l'aide d'analyses multidimensionnelles (i.e. analyses de redondance et analyses multidimensionnelles non métriques), les interactions entre la composition phytoplanctonique et les variables environnementales et biologiques ont été examinées. Deux régimes phytoplanctoniques distincts de par la production, la biomasse, l'abondance et la structure de taille des communautés, ont ainsi pu être mis en évidence: un système basé sur les flagellés situé au sud-ouest de la mer de Beaufort, dans le golfe Amundsen et la partie centrale de l'archipel canadien et un système basé sur les diatomées caractérisant la baie de Baffin, le détroit de Lancaster et le centre du golfe Amundsen. Les régions oligotrophes sont, d'une part, caractérisées par une faible production et une faible biomasse de cellules phytoplanctoniques de grande taille (> 5 /lm) et, d' autre part, par une abondance relative élevée de picophytoplancton eucaryote « 2 /lm) et de nanoflagellés non identifiés (2 - 20 /lm). En contraste, les régions eutrophes sont caractérisées par une forte production et une forte biomasse de cellules phytoplanctoniques de grande taille, en majeure partie des diatomées centrales surtout représentées par le genre Chaetoceros. Les différences entre ces deux régimes phytoplanctoniques sont expliquées, en grande partie par une faible stratification de la colonne d' eau et de fortes concentrations de nitrates au niveau du ZDCM dans les régions eutrophes versus oligotrophes. Cette étude démontre le rôle clé du mélange vertical et des apports en nutriments sur le type de régime et de communautés phytoplanctoniques, soulignant la diversité de régions biogéographiques pouvant être significativement perturbées par les changements en cours dans l'Arctique. -- ABSTRACT: A large-scale biogeographic study was conducted to assess phytoplankton dynamics and its environmental control across the Canadian High Arctic. Repeated 3500 km transects across the Beaufort Sea, the Canadian Arctic Archipelago and Baffm Bay provided the opportunity to measure environmental (i.e. hydrographic structure, atrnospheric and sea ice conditions) and biological variables measured in the upper water column during late summer 2005, early faU 2006 and faH 2007. Phytoplankton production and chlorophyH (chi) a biomass were measured at seven optical depths and at the depth of maximum chi a fluorescence (ZDCM). In addition, phytoplankton taxonomic composition, abundance and size structure were determined at the Z DCM. Redundancy analyses and non-metric multidimensional scaling were used to assess relationships between phytoplankton composition in relation to biological and environmental variables. Two distinct phytoplankton regimes were documented based on the production, biomass, abundance and size structure of phytoplankton communities: (1) a flageHate-based system extending over the eastem Beaufort Sea, the Amundsen Gulf and the central part of the Canadian Arctic Archipelago, and (2) a diatom-based system centered in Baffm Bay, Lancaster Sound and in central Amundsen Gulf. The oligotrophic regions were characterized by low production and biomass of large phytoplankton cells (> 5 f!m) and hlgh relative abundance of eukaryotic picophytoplankton « 2 f!m) and unidentified nanoflageHates (2 - 20 f!m). The eutrophic regions were characterized by high production and biomass of large ceHs and high relative abundance of centric diatoms, mainly Chaetoceros species. The differences between the two phytoplankton regimes were explained, in part, by differences' in stratification of the water column and nitrate concentrations at the Z DCM. This study demonstrates the key role of water column mixing and nutrient input on phytoplankton communities and regimes in the Canadian High Arctic, underpinning a diversity of biogeographic regions that may be significantly altered by ongoing Arctic changes

Phytoplankton dynamics in a changing Arctic Ocean

International audienceChanges in the Arctic atmosphere, cryosphere and Ocean are drastically altering the dynamics of phytoplankton, the base of marine ecosystems. This Review addresses four major complementary questions of ongoing Arctic Ocean changes and associated impacts on phytoplankton productivity, phenology and assemblage composition. We highlight trends in primary production over the last two decades while considering how multiple environmental drivers shape Arctic biogeography. Further, we consider changes to Arctic phenology by borealization and hidden under-ice blooms, and how the diversity of phytoplankton assemblages might evolve in a novel Arctic `biogeochemical landscape'. It is critical to understand these aspects of changing Arctic phytoplankton dynamics as they exert pressure on marine Arctic ecosystems in addition to direct effects from rapid environmental changes

Plankton assemblage estimated with BGC-Argo floats in the Southern Ocean

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Influence of seabird colonies and other environmental variables on benthic community structure, Lancaster Sound Region, Canadian Arctic

International audienceThe Canadian Arctic shelters millions of seabirds each year during the breeding season. By the excretion of important quantities of guano, seabirds locally concentrate nutrient-rich organic matter in the marine areas surrounding colonies. Seabirds, acting as biological vectors of nutrients, can markedly affect terrestrial ecosystems, but their influence on the structure of marine benthic communities is still under-studied. Sessile and long-lived megabenthic species can integrate environmental variation into marine food webs over long time frames. The objectives of this study were (1) to characterize the epifaunal and infaunal communities of the Lancaster Sound Region (LSR) and (2) to test the influence of the presence of seabird colonies and other environmental parameters on the structure of those benthic communities. Our prediction was that benthic diversity, number of taxa, total biomass of infauna and total density of epifauna and infauna, would be higher in areas with colonies present. Photos of the seafloor (data on epifauna) and grab samples (data on infauna) were taken at three control areas and at five areas near seabird colonies, within a depth range of 122 to 442 m. A database of 26 environmental parameters was built to study the environment-benthos relationships. Infauna, which was relatively uniform across the LSR, was numerically dominated by Annelida. Epifauna was much patchier, with each study area having unique epibenthic assemblages. Brittle stars were highly abundant in epifaunal communities, reaching 600 individuals per square meter. The presence of seabird colonies was not a major driver of benthic community structure in the LSR at the depths studied. Negative effects of colonies were detected on the density and number of taxa of infauna, perhaps due to top-down effects transmitted by the seabirds which feed in the water column and can directly reduce the quantity of food reaching the seabed. Sediment concentration of pigment, percent cover of gravel and boulders, depth, temperature and duration of open water explained a substantial part of the observed variation across the LSR. Food availability, as expressed by sediment pigment concentration, is a factor driving benthic communities, even if potential pathways through seabirds did not broadly affect the benthos at the point source

Environmental context of all stations from the Tara Oceans Expedition (2009-2013), about the annual cycle of key parameters estimated daily from remote sensing products at a spatial resolution of 100km

The Tara Oceans Expedition (2009-2013) sampled the world oceans on board a 36 m long schooner, collecting environmental data and organisms from viruses to planktonic metazoans for later analyses using modern sequencing and state-of-the-art imaging technologies. Tara Oceans Data are particularly suited to study the genetic, morphological and functional diversity of plankton. The present data set includes text files, one for each station, that contain the annual cycle of key parameters estimated daily from remote sensing products at a spatial resolution of 100km centered on the sampling location. File names correspond to the station identifier (TARA_station#). A description of all parameters is provided in a README file