This dataset includes depth profiles in the euphotic zone of nutrient (nitrate, silicate, phosphate) concentrations and profiles of silicic acid uptake rates from seawater samples collected on EXPORTS cruise DY131 during May 2021. This research focuses on the vertical export of the carbon associated with a major group of phytoplankton, the diatoms in the North Atlantic near the Porcupine Abyssal Plain. The major objective is to understand how diatom community composition and the prevailing nutrient conditions create taxonomic differences in metabolic state that combine to direct diatom taxa to different carbon export pathways. The focus is on diatoms, given their large contribution to global marine primary productivity and carbon export which translates into a significant contribution to the biogeochemical cycling of carbon (C), nitrogen (N), phosphorus (P), iron (Fe) and silicon (Si). It is hypothesized that the type and degree of diatom physiological stress are vital aspects of ecosystem state that drive export. To test this hypothesis, combined investigator expertise in phytoplankton physiology, genomics, and trace element chemistry is used to assess the rates of nutrient use and the genetic composition and response of diatom communities, with measurements of silicon and iron stress to evaluate stress as a predictor of the path of diatom carbon export. The EXPORTS field campaign in the North Atlantic sampled a retentive eddy over nearly a month. At the beginning of the cruise, nitrate was abundant while silicic acid was nearly undetectable. Such low dissolved Si concentrations significantly limit diatom silicification resulting in diatoms with reduced mineral ballast and low Si:C and Si:N ratios that would reduce sinking rates and competition for Si can alter diatom taxonomic composition. Both factors can the path cells follow through the food web ultimately altering diatom carbon export. Within each ecosystem state examined in the EXPORTS program, nutrient biogeochemistry, diatom and phytoplankton community structure, and global diatom gene expression patterns (metatranscriptomics) are characterized in the ocean. Nutrient amendment experiments with tracer addition (14C, 32Si) are used to quantify the level of Si, N, and Fe stress being experienced by the phytoplankton and to contextualize taxa-specific metatranscriptome responses for resolving gene expression profiles in the in situ communities.

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Depth profiles in the euphotic zone of nutrient (nitrate, silicate, phosphate) concentrations, profiles of silicic acid uptake rates, and assessment of limitation by Si, N, and Fe on both silicic acid uptake and carbon fixation.

See the related dataset (https://www.bco-dmo.org/dataset/893324) for the data from the nutrient limitation assessments.

Seawater samples were collected using an epoxy coated CTD-rosette mounted with Go-Flo samplers and a Sea-Bird Electronics CTD (SBE9plus). Go-Flo bottles were transferred to a trace metal clean van for subsampling into polypropylene tubes (nutrients), polypropylene bottle (biogenic silica and particulate carbon and nitrogen) or TM acid-cleaned polycarbonate incubation bottles (Si-32 & C-14 incubation experiments).

Nutrient samples were filtered through 0.2 micrometer (μm) polycarbonate filters and frozen at -20° Celsius (C). Samples for biogenic silica concentrations were size fractionated by serial filtration through 5 μm and 0.6 μm polycarbonate filters Filters were stored frozen at -20°C. Particulate organic carbon and nitrogen were measured on samples from experiments examining the effect of added Fe and Si on carbon fixation. These samples were filtered through precombusted GFF filters placed in glass scintillation vials and frozen at -20°C.

Samples for silicic acid uptake profiles were spiked with the radioisotope Si-32. Nutrient limitation assays were performed on pairs of samples where the rate of silicic acid uptake (Si-32) or carbon fixation (C-14 in paired light/dark bottles) were determined in unaltered controlled samples and in samples augmented with either silicic acid (20 μM), nitrate (20 μM) or iron chloride (1 nanomolar (nM)). All samples were incubated on deck in simulated in situ incubators cooled with flowing surface seawater for 24 hours. Profiles sampled six depths from near surface to the 1% light level. Nutrient limitation assays were performed at the 40% and 10% light levels.

Particles from incubated samples were size fractionated by serial filtration through 5 μm and 0.6 μm 25-millimeter (mm) polycarbonate filters. For C-14 incubations, total radioactivity in each sample was determined by sampling 100 μl of sample seawater prior to filtration. Filters from Si-32 incubations were placed on plastic planchettes and dried before covering with mylar film and stored for analysis ashore using low-level beta counters (Riso Inc). Filters from C-14 incubations were acidified in glass scintillation vials, scintillation cocktail (Ultima Gold XR) was added followed by liquid scintillation counting. Total radioactivity samples received 100 μL of b-phenethylamine and 5 mL of scintillation cocktail prior to analysis at sea using a Tri-Carb 3110TR scintillation counter.

Biogenic silica concentrations were determined by NaOH digestion followed by colorimetric analysis of the resulting dissolved Si. Particulate organic carbon and nitrogen samples were analyzed by Dumas combustion. Nutrient concentrations were determined by flow injection using a Lachat Instruments QuikChem 8500 Series 2 anayzer.

For more information, see the Protocol documents attached as Supplemental Files and https://msi.ucsb.edu/facilities-services/analytical-lab/services.

Silicon uptake was calculated as the product of the fraction of total Si-32 radioactivity taken up and the ambient silicic acid concertation. Rates of primary production were calculated as the product of the fraction of total C-14 radioactivity taken up and a DIC value of 2132 micromoles per kilogram (μmol kg-1) correcting for isotope discrimination (x 1.05).

Nutrient concentrations were adjusted using certified JAMSTEC CRMs.

BCO-DMO Processing:
- removed "~" as a missing data value (replaced by blanks/empty values);
- renamed fields to comply with BCO-DMO naming conventions;
- created ISO 8601 date-time field and removed original date and column columns.

Coverage: Sub-Arctic Pacific, Ocean Station Papa

NSF Award Abstract:
This project focuses on a group of microscopic single-celled photosynthetic organisms in the ocean called diatoms. Diatoms float in the surface ocean as part of a group of organisms collectively called phytoplankton. There are thousands of different species of diatoms distributed across the global ocean. A famous oceanographer Henry Bigelow once said "All fish is diatoms" reflecting the importance of diatoms as the base of the food chain that supports the world's largest fisheries. Despite their small size, diatom photosynthesis produces 20% of the oxygen on earth each year. That's more than all of the tropical rain forests on land. The major objective of the research is to understand how the metabolic differences among diatom species affects the amount of diatom organic carbon that is carried, or exported, from the surface ocean to the deep ocean. As diatoms are photo-synthesizers like green plants, their biological carbon comes from converting carbon dioxide dissolved in seawater from the atmosphere into organic forms. Diatoms also require a series of other nurtrients supplied by the ocean such as nitrogen and phosphorous and, uniquely for diatoms, the silicon used to construct their glass shells. This research will investigate how genetic and physiological differences among diatoms influence how each species react to changes in nutrient levels in the ocean and how those shifts affect the export of diatom carbon to the deep sea. The link between diatoms' physiological response and their carbon export comes about because shifts in physiology affect diatom attributes like how fast they sink and how tasty they are to predators. So if we can relate the physiological condition of different diatoms to the food-web pathways followed by different species, we can ultimately use knowledge of diatom physiological status and food web structure to predict how much diatom carbon gets to the deep sea. The research involves investigators with expertise in the physiology and genomics of diatoms and in the ocean's chemistry. The work will initially take place in the subarctic North Pacific in conjunction with the NASA Export Processes in the Ocean from RemoTe Sensing (EXPORTS) field program. The EXPORTS program is using a wide variety of methods to quantify the export and fate of photo-synthetically fixed carbon in the upper ocean. The research supports the training of undergraduate students, graduate students and a postdoctoral scholar. The research will also serve as the basis for activities aimed at K-12 and junior high school students.

The research will broadly impact our understanding of the biology of the biological pump (the transport of photo-synthetically fixed organic carbon to the deep sea) by forming a mechanistic basis for predicting the export of diatom carbon. It is hypothesized that the type and degree of diatom physiological stress are vital aspects of ecosystem state that drive export. To test this hypothesis, the genetic composition, rates of nutrient use and growth response of diatom communities will be evaluated and supported with measurements of silicon and iron stress to evaluate stress as a predictor of the path of diatom carbon export. The subarctic N. Pacific ecosystem is characterized as high nutrient low chlorophyll (HNLC) due to low iron (Fe) levels that are primary controllers constraining phytoplankton utilization of other nutrients. It has been a paradigm in low Fe, HNLC systems that diatoms grow at elevated Si:C and Si:N ratios and should be efficiently exported as particles significantly enriched in Si relative to C. However, Fe limitation also alters diatoms species composition and the high Si demand imposed by low Fe can drive HNLC regions to Si limitation or Si/Fe co-limitation. Thus, the degree of Si and/or Fe stress in HNLC waters can all alter diatom taxonomic composition, the elemental composition of diatom cells, and the path cells follow through the food web ultimately altering diatom carbon export.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Website: http://oceanexports.org/

EXport Processes in the Ocean from Remote Sensing (EXPORTS) is a large-scale NASA-led field campaign that will provide critical information for quantifying the export and fate of upper ocean net primary production (NPP) using satellite observations and state of the art ocean technologies.

Ocean ecosystems play a critical role in the Earth’s carbon cycle and the quantification of their impacts for both present conditions and for predictions into the future remains one of the greatest challenges in oceanography. The goal of the EXport Processes in the Ocean from Remote Sensing (EXPORTS) Science Plan is to develop a predictive understanding of the export and fate of global ocean net primary production (NPP) and its implications for present and future climates. The achievement of this goal requires a quantification of the mechanisms that control the export of carbon from the euphotic zone as well as its fate in the underlying "twilight zone" where some fraction of exported carbon will be sequestered in the ocean’s interior on time scales of months to millennia. In particular, EXPORTS will advance satellite diagnostic and numerical prognostic models by comparing relationships among the ecological, biogeochemical and physical oceanographic processes that control carbon cycling across a range of ecosystem and carbon cycling states. EXPORTS will achieve this through a combination of ship and robotic field sampling, satellite remote sensing and numerical modeling. Through a coordinated, process-oriented approach, EXPORTS will foster new insights on ocean carbon cycling that maximizes its societal relevance through the achievement of U.S. and International research agency goals and will be a key step towards our understanding of the Earth as an integrated system.