Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Nitrous oxide concentrations (depths of 5-175 m) at Station ALOHA (2008-2016). This dataset includes pressure, temperature, salinity, density, dissolved oxygen, N2O (atmospheric), N2O (equilibrium), and N2O (measured).

The following describes the methodology for both this dataset (methane) as well as the nitrous oxide dataset because the analyses were completed concurrently. Please see Related Datasets section below for N2O results.

Water sampling
Samples were collected using specially-designed 12L Niskin® bottles connected to a rosette with a Sea-Bird SBE 911Plus CTD package integrated into the system. The bottles were designed by John Bullister (NOAA Pacific Marine Environmental Laboratory (PMEL)) to specifically minimize contamination for trace gases, in particular chlorofluorocarbons and sulfur hexafluoride (Bullister and Wisegarver, 2008). Seawater was dispensed using Tygon® tubing from the bottles into the bottom of 240 mL borosilicate bottles to at least two times overflowing ensuring the absence of bubbles.

Sample preservation
The samples were preserved using 200 microliters of saturated mercuric chloride solution, crimp-sealed and stored in the dark at room temperature until analysis. 

Analysis
Analysis of methane (CH4) and nitrous oxide (N2O) was conducted simultaneously using a gas chromatography method (del Valle & Karl, 2014; Wilson et al., 2014), typically 1 to 6 months after sample collection. Briefly, the water sample was transferred under positive pressure (supplied by helium gas) from the glass vials to a purge chamber fitted with a porous frit. The sample was then purged with ultra-high purity helium and the gas stream passed through a Nafion® dryer (Perma Pure LLC) and a Drierite® trap (VWR) to remove water vapor and then passed through an Ascarite trap to remove CO2, before being trapped on a packed sample loop (Porapak Q 80/100; Sigma-Aldrich) immersed in liquid nitrogen. After sparging for 10 minutes, the sample loop was heated and injected onto an analytical column (30 m x 0.32 mm GS-CarbonPLOT capillary column; J&W Scientific) within a gas chromatograph (Agilent 7890A) equipped with a flame ionization detector (FID) and an electron capture detector (ECD). The carrier flow was alternated from the FID to the ECD using a Dean’s switch® (Agilent Technologies) which allowed the quantification of both CH4 and N2O from a single sample. The oven temperature was typically maintained at 38°C, although this subsequently was decreased to 30°C in August 2016 to better resolve the CH4  peak when partial overlap with an O2 peak occurred.

Calibration
Calibration of the analytical system was conducted using gaseous standards purchased from Scott-Marin (CH4: 20.15 ± 1% ppmv; N2O: 4.81 ± 2% ppmv in a balance of N2) and NOAA (CH4: 1965.32 ppbv; N2O: 357.56 ppbv in a balance of air). From March 2016 onwards, the calibration for CH4 and N2O was compared against reference standards prepared by John Bullister at NOAA PMEL on behalf of SCOR Working Group #143. In all instances, standards were injected prior to the purge and trap set-up and therefore passed through the purge chamber and gas drying apparatus. A linear curve was applied to the CH4 calibration values and a polynomial curve was fitted to the N2O calibration values. 

Precision and Accuracy
The precision of CH4 measurements for surface seawater with concentrations of 2.57 ± 0.07 (SD) nmol kg-1 (n=14), as calculated by the coefficient of variation, was 3%. The accuracy of CH4 measurements was evaluated by analyzing filtered (0.2 μm) seawater samples that had been equilibrated with atmospheric air at a range of set temperatures between 19–27°C which were maintained using a water bath. The measured values agreed to within 2.4 ± 0.9% of predicted values. The precision of N2O measurements in surface seawater, with concentrations of 6.47 ± 0.14 (SD) nmol kg-1 (n=14), as calculated by the coefficient of variation was 2%. Using the same air-equilibrated seawater set-up, as described for CH4, the accuracy of N2O measurements was 2.6 ± 1.9% of predicted values.

Calculation
The sea-air flux (F, μmol m-2 d-1 ) of CH4 and N2O was calculated as : 
                   F = k (Cw -Ceq)
where k is the gas transfer velocity (m d-1), Cw is the ambient concentration of the gas dissolved in water (μmol m-3), and Ceq is the concentration of the gas at equilibrium with the atmosphere. Ceq was determined from the solubility equations for N2O (Weiss and Price, 1980) and for CH4 (Wiesenburg & Guinasso, 1979), and using the atmospheric concentrations measured at the NOAA Mauna Loa Observatory and available online from NOAA's Global Monitoring Laboratory (Lan & Keeling, 2025; Dlugokencky et al., 2016). The wind speed data, used to calculate k, for January 2009 to May 2016 were taken from the Woods Hole Oceanographic Institution-Hawaii Ocean Time-series Site (WHOTS) mooring (Upper Ocean Processes Group, 2024) and normalized to a height of 10 meters above sea surface (Smith, 1988). The wind data from July–December 2016 derive from the Blended Sea Winds data product for the coordinates 22°45' N and 158°00' W (Zhang et al., 2006). k was determined for each gas using the wind speed parameterization of Wanninkhof (2014) and the Schmidt numbers for each gas, calculated using the updated empirical temperature dependence formulations (Wanninkhof, 2014). To calculate the sea-air fluxes, the mean k value for the week prior to sample collection was used.

* processing originally completed 2017-08-28 by Amber York. Updated in 2025 by Dana Gerlach.
* added a conventional header with dataset name, PI name, version date
* modified parameter names to conform with BCO-DMO naming conventions (e.g. no spaces, slashes, special characters)
* converted latitude and longitude values to decimal degree values
* added column for ISO_DateTime_UTC converted from local date and time (HST time zone).

Hawai'i Ocean Time-Series Project Summary
This continuing award for the HOT research program sustains the open-ocean climatology of biological, chemical, and physical observations into a 4^th decade.

Intellectual Merit
The scientific mission of HOT continues to be monitoring of temporal dynamics in the cycling of carbon and associated bioelements, and observations of the variability of hydrological and ecological properties, heat fluxes, and circulation of the North Pacific Subtropical Gyre (NPSG).  The proposed research will rely on shipboard observations and experiments conducted on 10 separate 5-day expeditions per annum along with near-continuous moored platform measurements of air-sea interactions, ocean mixing, and physical characteristics of the deep sea.  The HOT program maintains the high-quality suite of biogeochemical and physical measurements required for continued assessment of dynamics in ocean carbon and nutrient pools and fluxes, plankton community structure, ecosystem productivity, and inherent optical properties of the water column. Continuity of these observations improves the value of the dataset for deciphering how low-frequency natural and anthropogenic climate signals influence ecosystem structure in the NPSG as well as providing up-to-date measurements to place current signals in the longer-term context.  Such efforts will continue to aid on-going modeling efforts required for predicting how future habitat perturbations may influence ecosystem dynamics in the NPSG.  All HOT program data are publicly available and are frequently used by researchers and policy makers around the world. HOT data provide reference baselines for essential ocean variables, allow for characterization of natural patterns of ocean system variability and associated links to regional climate indices, and support calibration/validation of autonomous in situ and remote (satellite, airborne) sensors.

Broader Impacts
The long-term, continuous HOT data are critical to assess variability on seasonal to decadal time-scales and thus are essential to determine the emergence of anthropogenic signals in the oligotrophic North Pacific.  Further sustaining HOT measurements will strengthen our capacity to test hypotheses about poorly understood interactions between ocean dynamics, climate, and biogeochemistry and increase the value of HOT data for understanding the response of ocean ecosystems to both natural and anthropogenic climate perturbations. Over the next 5 years, we will continue to promote the value of HOT research through high quality, high visibility peer-reviewed journal and book articles, newspaper and newsletter articles, and community outreach. With partners BCO-DMO and OceanSITES we will also continue to strive for a FAIR data model (see data management plan) as metadata standards and conventions evolve in the community. We will continue working with an Earthcube Research Coordination Network for Marine Ecological Time Series (METS) to support efforts that bring together different cross-sections of METS data producers, data users, data scientists, and data managers in large- and small-group formats to foster the necessary dialog to develop FAIR data solutions across multiple time-series.  In addition, HOT is a community resource that helps support the research of numerous ocean scientists who rely on the program’s infrastructure (ship time, staff, laboratories, equipment) to conduct their research, education, and outreach activities.  Moreover, HOT PIs maintain a strong commitment to mentoring and training of undergraduate and graduate students, and will continue these activities as well as facilitates access to the sea by a number of ancillary students and scientists. 

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NSF Award Abstract:
Long-term observations of ocean physics, biology, and chemistry across decades provide a powerful lens for understanding the response of the oceans to environmental change. This award will continue the Hawaii Ocean Time-series (HOT) research program, which began in 1988, for an additional five years. Continuity of these observations will improve the value of the dataset for deciphering how natural and human-influenced climate signals affect ecosystem structure in the Pacific Ocean. All HOT program data are publicly available and are frequently used by researchers and policy makers around the world. HOT also serves as (1) a testbed for the development of new sensors and methodologies, (2) a calibration/validation site, (3) an invaluable training ground that attracts students and researchers from around the globe, and (4) a forum for international collaboration and capacity building.

The proposed research will rely on shipboard observations and experiments conducted on ten separate five-day expeditions per year along with near-continuous moored platform measurements of air-sea interactions, ocean mixing, and physical characteristics of the deep sea. Observations include biogeochemical and physical measurements required for continued assessment of dynamics in ocean carbon and nutrient pools and fluxes, plankton community structure, ecosystem productivity, and inherent optical properties of the water column. The major program goals and objectives over the next 5 years remain as in prior years and include: (1) sustain high quality, time-resolved oceanographic measurements on the interactions between ocean-climate and ecosystem variability in the North Pacific Subtropical Gyre (NPSG), (2) quantify time-varying (seasonal to decadal) changes in reservoirs and fluxes of carbon and associated bioelements (nitrogen, phosphorus, and silicon), (3) constrain processes controlling air-sea carbon exchange, rates of carbon transformation through the planktonic food web, and fluxes of carbon into the ocean's interior, (4) extend to 40 years a climatology of hydrographic and biogeochemical dynamics from which to gauge anomalous or extreme changes to the NPSG habitat, forming a multi-decadal baseline from which to decipher natural and anthropogenic influences on the NPSG ecosystem, (5) continue to provide scientific and logistical support to ancillary programs that benefit from the temporal context, interdisciplinary science, and regular access to the open sea afforded by HOT program occupation of Station ALOHA, including projects implementing, testing, and validating new methodologies and transformative ocean sampling technologies, and (6) provide unique training and educational opportunities for the next generation of ocean scientists.

The Ocean Carbon and Biogeochemistry (OCB) program focuses on the ocean's role as a component of the global Earth system, bringing together research in geochemistry, ocean physics, and ecology that inform on and advance our understanding of ocean biogeochemistry. The overall program goals are to promote, plan, and coordinate collaborative, multidisciplinary research opportunities within the U.S. research community and with international partners. Important OCB-related activities currently include: the Ocean Carbon and Climate Change (OCCC) and the North American Carbon Program (NACP); U.S. contributions to IMBER, SOLAS, CARBOOCEAN; and numerous U.S. single-investigator and medium-size research projects funded by U.S. federal agencies including NASA, NOAA, and NSF.

The scientific mission of OCB is to study the evolving role of the ocean in the global carbon cycle, in the face of environmental variability and change through studies of marine biogeochemical cycles and associated ecosystems.

The overarching OCB science themes include improved understanding and prediction of: 1) oceanic uptake and release of atmospheric CO2 and other greenhouse gases and 2) environmental sensitivities of biogeochemical cycles, marine ecosystems, and interactions between the two.

The OCB Research Priorities (updated January 2012) include: ocean acidification; terrestrial/coastal carbon fluxes and exchanges; climate sensitivities of and change in ecosystem structure and associated impacts on biogeochemical cycles; mesopelagic ecological and biogeochemical interactions; benthic-pelagic feedbacks on biogeochemical cycles; ocean carbon uptake and storage; and expanding low-oxygen conditions in the coastal and open oceans.

The United States Joint Global Ocean Flux Study was a national component of international JGOFS and an integral part of global climate change research.

The U.S. launched the Joint Global Ocean Flux Study (JGOFS) in the late 1980s to study the ocean carbon cycle. An ambitious goal was set to understand the controls on the concentrations and fluxes of carbon and associated nutrients in the ocean. A new field of ocean biogeochemistry emerged with an emphasis on quality measurements of carbon system parameters and interdisciplinary field studies of the biological, chemical and physical process which control the ocean carbon cycle. As we studied ocean biogeochemistry, we learned that our simple views of carbon uptake and transport were severely limited, and a new "wave" of ocean science was born. U.S. JGOFS has been supported primarily by the U.S. National Science Foundation in collaboration with the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, the Department of Energy and the Office of Naval Research. U.S. JGOFS, ended in 2005 with the conclusion of the Synthesis and Modeling Project (SMP).

Program description text taken from Chapter 1: Introduction from the Global Intercomparability in a Changing Ocean: An International Time-Series Methods Workshop report published following the workshop held November 28-30, 2012 at the Bermuda Institute of Ocean Sciences. The full report is available from the workshop Web site hosted by US OCB: http://www.whoi.edu/website/TS-workshop/home

Decades of research have demonstrated that the ocean varies across a range of time scales, with anthropogenic forcing contributing an added layer of complexity. In a growing effort to distinguish between natural and human-induced earth system variability, sustained ocean time-series measurements have taken on a renewed importance. Shipboard biogeochemical time-series represent one of the most valuable tools scientists have to characterize and quantify ocean carbon fluxes and biogeochemical processes and their links to changing climate (Karl, 2010; Chavez et al., 2011; Church et al., 2013). They provide the oceanographic community with the long, temporally resolved datasets needed to characterize ocean climate, biogeochemistry, and ecosystem change.

The temporal scale of shifts in marine ecosystem variations in response to climate change are on the order of several decades.  The long-term, consistent and comprehensive monitoring programs conducted by time-series sites are essential to understand large-scale atmosphere-ocean interactions that occur on interannual to decadal time scales.  Ocean time-series represent one of the most valuable tools scientists have to characterize and quantify ocean carbon fluxes and biogeochemical processes and their links to changing climate.

Launched in the late 1980s, the US JGOFS (Joint Global Ocean Flux Study; http://usjgofs.whoi.edu) research program initiated two time-series measurement programs at Hawaii and Bermuda (HOT and BATS, respectively) to measure key oceanographic measurements in oligotrophic waters. Begun in 1995 as part of the US JGOFS Synthesis and Modeling Project, the CARIACO Ocean Time-Series (formerly known as the CArbon Retention In A Colored Ocean) Program has studied the relationship between surface primary production, physical forcing variables like the wind, and the settling flux of particulate carbon in the Cariaco Basin.

The objective of these time-series effort is to provide well-sampled seasonal resolution of biogeochemical variability at a limited number of ocean observatories, provide support and background measurements for process-oriented research, as well as test and validate observations for biogeochemical models. Since their creation, the BATS, CARIACO and HOT time-series site data have been available for use by a large community of researchers.
 
Data from those three US funded, ship-based, time-series sites can be accessed at each site directly or by selecting the site name from the Projects section below.