A warm jet in a cold ocean MacKinnon, Jennifer A; Simmons, Harper L; Hargrove, John ...
Nature communications,
04/2021, Letnik:
12, Številka:
1
Journal Article
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Odprti dostop
Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn ...cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.
The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine ...tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth‐integrated N2 fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO3− into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N2 fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N2 fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 × 1012 mol N by Trichodesmium N2 fixation alone. This input can account for a substantial fraction of the N2 fixation in the North Atlantic inferred by several of the geochemical approaches.
Dinitrogen (N2) fixing microorganisms (termed diazotrophs) exert important control on the ocean carbon cycle. However, despite increased awareness on the roles of these microorganisms in ocean ...biogeochemistry and ecology, the processes controlling variability in diazotroph distributions, abundances, and activities remain largely unknown. In this study, we examine 3 years (2004–2007) of approximately monthly measurements of upper ocean diazotroph community structure and rates of N2 fixation at Station ALOHA (22°45′N, 158°W), the field site for the Hawaii Ocean Time‐series program in the central North Pacific subtropical gyre (NPSG). The structure of the N2‐fixing microorganism assemblage varied widely in time with unicellular N2‐fixing microorganisms frequently dominating diazotroph abundances in the late winter and early spring, while filamentous microorganisms (specifically various heterocyst‐forming cyanobacteria and Trichodesmium spp.) fluctuated episodically during the summer. On average, a large fraction (∼80%) of the daily N2 fixation was partitioned into the biomass of <10 μm microorganisms. Rates of N2 fixation were variable in time, with peak N2 fixation frequently coinciding with periods when heterocystous N2‐fixing cyanobacteria were abundant. During the summer months when sea surface temperatures exceeded 25.2°C and concentrations of nitrate plus nitrite were at their annual minimum, rates of N2 fixation often increased during periods of positive sea surface height anomalies, as reflected in satellite altimetry. Our results suggest mesoscale physical forcing may comprise an important control on variability in N2 fixation and diazotroph community structure in the NPSG.
Utilisation of dissolved organic phosphorus (DOP) by marine microbes as an alternative phosphorus (P) source when phosphate is scarce can help sustain non-Redfieldian carbon:nitrogen:phosphorus ...ratios and efficient ocean carbon export. However, global spatial patterns and rates of microbial DOP utilisation are poorly investigated. Alkaline phosphatase (AP) is an important enzyme group that facilitates the remineralisation of DOP to phosphate and thus its activity is a good proxy for DOP-utilisation, particularly in P-stressed regions. We present a Global Alkaline Phosphatase Activity Dataset (GAPAD) with 4083 measurements collected from 79 published manuscripts and one database. Measurements are organised into four groups based on substrate and further subdivided into seven size fractions based on filtration pore size. The dataset is globally distributed and covers major oceanic regions, with most measurements collected in the upper 20 m of low-latitude oceanic regions during summer since 1997. This dataset can help support future studies assessing global ocean P supply from DOP utilisation and provide a useful data reference for both field investigations and modelling activities.
Australia's tropical waters represent predicted 'hotspots' for nitrogen (N2) fixation based on empirical and modelled data. However, the identity, activity and ecology of diazotrophs within this ...region are virtually unknown. By coupling DNA and cDNA sequencing of nitrogenase genes (nifH) with size-fractionated N2 fixation rate measurements, we elucidated diazotroph dynamics across the shelf region of the Arafura and Timor Seas (ATS) and oceanic Coral Sea during Austral spring and winter. During spring, Trichodesmium dominated ATS assemblages, comprising 60% of nifH DNA sequences, while Candidatus Atelocyanobacterium thalassa (UCYN-A) comprised 42% in the Coral Sea. In contrast, during winter the relative abundance of heterotrophic unicellular diazotrophs (δ-proteobacteria and γ-24774A11) increased in both regions, concomitant with a marked decline in UCYN-A sequences, whereby this clade effectively disappeared in the Coral Sea. Conservative estimates of N2 fixation rates ranged from <1 to 91 nmol l(-1) day(-1), and size fractionation indicated that unicellular organisms dominated N2 fixation during both spring and winter, but average unicellular rates were up to 10-fold higher in winter than in spring. Relative abundances of UCYN-A1 and γ-24774A11 nifH transcripts negatively correlated to silicate and phosphate, suggesting an affinity for oligotrophy. Our results indicate that Australia's tropical waters are indeed hotspots for N2 fixation and that regional physicochemical characteristics drive differential contributions of cyanobacterial and heterotrophic phylotypes to N2 fixation.
A carbon budget for the northwest European continental shelf seas (NWES) was synthesised using available estimates for coastal, pelagic and benthic carbon stocks and flows. Key uncertainties were ...identified and the effect of future impacts on the carbon budget were assessed. The water of the shelf seas contains between 210 and 230 Tmol of carbon and absorbs between 1.3 and 3.3 Tmol from the atmosphere annually. Off-shelf transport and burial in the sediments account for 60-100% and 0-40% of carbon outputs from the NWES, respectively. Both of these fluxes remain poorly constrained by observations and resolving their magnitudes and relative importance is a key research priority. Pelagic and benthic carbon stocks are dominated by inorganic carbon. Shelf sediments contain the largest stock of carbon, with between 520 and 1600 Tmol stored in the top 0.1 m of the sea bed. Coastal habitats such as salt marshes and mud flats contain large amounts of carbon per unit area but their total carbon stocks are small compared to pelagic and benthic stocks due to their smaller spatial extent. The large pelagic stock of carbon will continue to increase due to the rising concentration of atmospheric CO2, with associated pH decrease. Pelagic carbon stocks and flows are also likely to be significantly affected by increasing acidity and temperature, and circulation changes but the net impact is uncertain. Benthic carbon stocks will be affected by increasing temperature and acidity, and decreasing oxygen concentrations, although the net impact of these interrelated changes on carbon stocks is uncertain and a major knowledge gap. The impact of bottom trawling on benthic carbon stocks is unique amongst the impacts we consider in that it is widespread and also directly manageable, although its net effect on the carbon budget is uncertain. Coastal habitats are vulnerable to sea level rise and are strongly impacted by management decisions. Local, national and regional actions have the potential to protect or enhance carbon storage, but ultimately global governance, via controls on emissions, has the greatest potential to influence the long-term fate of carbon stocks in the northwestern European continental shelf.
Phosphorus is an essential nutrient for all life on earth. In the ocean, the most bioavailable form of phosphorus is inorganic phosphate, but in the extensive subtropical gyres, phosphate ...concentrations can be chronically low and limit primary productivity and nitrogen fixation. In these regions, organisms produce hydrolytic enzymes, such as alkaline phosphatase (AP), that enable them to utilize the more replete dissolved organic phosphorus (DOP) pool to meet their cellular phosphorus demands. In this study, we synthesized data from 14 published studies and present our own findings from two research cruises (D326 and D361) in the eastern subtropical Atlantic to explore the relationship between AP activity (APA) and nutrients, Saharan dust and trace metals. We found that below a threshold phosphate concentration of ~ 30 nM, APA increased with an inverse hyperbolic relationship with phosphate concentration. Meanwhile, DOP concentrations decreased with enhanced APA, indicating utilization of the DOP pool. We found APA rates were significantly higher in the subtropical Atlantic compared to the subtropical Pacific Ocean, even over the same low phosphate concentration range (0 to 50 nM). While the phosphate concentration may have a first order control on the APA rates, we speculate that other factors influence this basin scale contrast. Using bioassay experiments, we show that the addition of Saharan dust and zinc significantly increased the rate of APA. To our knowledge, our results are the first direct field-based evidence that APA is limited by zinc in the subtropical ocean. Further work is required to explore the relationship between trace metals such as iron and zinc, which are co-factors of phosphohydrolytic enzymes, specifically PhoX and PhoA, respectively, and APA in the ocean.
Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to ...primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.
Knowledge of species trophic position (TP) is an essential component of ecosystem management. Determining TP from stable nitrogen isotopes (δ15N) in predators requires understanding how these tracers ...vary across environments and how they relate to predator isotope composition. We used two seal species as a model for determining TP across large spatial scales in the Arctic. δ15N in seawater nitrate (δ15NNO3) and seal muscle amino acids (δ15NAA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ15NNO3 and δ15NAA. Our results show that the spatial variation in δ15NAA in seals reflects the δ15NNO3 end members in Pacific vs. Atlantic waters. This study provides a reference for best practice on accurate comparison of TP in predators and as such, provides a framework to assess the impact of environmental and human‐induced changes on ecosystems at pan‐Arctic scales.
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