Table of Contents

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

Water samples for a variety of chemical and biological measurements are routinely collected from the surface to within 10 meters of the seafloor. To the extent possible, we collect samples for complementary biogeochemical measurements from the same or from contiguous casts to minimize aliasing caused by time-dependent changes in the density field. This approach is especially important for samples collected in the upper 350 m of the water column. 

Furthermore, we attempt to sample from common depths and specific density horizons each month to facilitate comparisons between cruises. Water samples for salinity determinations are collected from every water bottle to identify sampling errors. Approximately 20% of the water samples are collected and analyzed in duplicate or triplicate to assess and track our precision in sample analyses.

Sampling strategy

Timing: HOT cruises are conducted at approximately monthly intervals, and approximately 72 hours is spent on station. During this period, nearly continuous measurements of water column physical characteristics are made using a CTD. One CTD cast is made to 4700 m (maximum water depth is approximately 4750 m), and then 1000 m casts are made at approximately 3-hour intervals for at least 36 hours (see Chapter 4). Chemical constituents and relevant biogeochemical cycling rates are measured from water samples at discrete depths. 

Location: Our primary sampling site is centered at 22° 45'N, 158° 00'W. This location is approximately 100 km due north of Kahuku Point, Oahu and Station ALOHA is defined as the ocean within a 6 mi radius of the center. This location is believed to be free of significant biogeochemical island mass effects; it is located 50 km (one Rossby radius) away from the topography associated with the Hawaiian Ridge, it is northeast (upwind) of the Hawaiian Archipelago, and it is above flat topography in 4750 meters of water. 

High-resolution profiles: High-resolution profiles of relevant physical, chemical, and biological oceanic properties are collected using a Seabird CTD. The instrument is equipped with external temperature, conductivity, dissolved oxygen and fluorescence sensors and with an internal pressure sensor. A General Oceanics 24-place pylon and deck box are used to obtain water samples from discrete depths using 12-liter Niskin bottles on a Scripps- type aluminum rosette frame 

Discrete Depth measurements: Sampling depths range from the surface to approximately 50 meters above the seafloor (4750 m), with most of the samples being collected in the upper 1000 meters. Approximately 20% of the samples are collected and analyzed in triplicate. To the extent possible, samples are collected from the same depths and/or density levels each month in order to facilitate comparisons between monthly profiles. During each HOT cruise, at least 20% of the samples are routinely collected in duplicate or triplicate to evaluate field precision. Salinity samples are collected from all Niskin bottles in order to identify bottle trip errors and on-deck sample temperatures (for those casts where oxygen samples are drawn) are recorded for each water bottle sampled. Both procedures are useful for the identification of sample mistrips (i.e., the collection of water from a depth other than intended). Our sampling strategy is based on resolving depth profiles of specific chemical constituents from contiguous casts. This is done in order to produce profiles which minimize the effects of time-dependent density structure variation within the water column.

BioGeoChemical properties

Seawater is collected from known depths using CTD-rosette sampling procedures. 

Dissolved Oxygen: Subsamples are drawn into calibrated iodine flasks and dissolved oxygen (DO) is chemically bound by the formation of a flocculated form of manganese(III) hydroxide (Mn(OH)3 floc). The floc is subsequently dissolved under acidic conditions which stochiometrically converts the original DO oxidizing equivalents to iodine (I3- ion). The latter is quantitatively titrated with thiosulfate (S2O32-) ions to a potentiometric end-point using a high-precision computer-controlled titration system.

Dissolved Inorganic Carbon: Subsamples for dissolved inorganic carbon (DIC) are collected in glass bottles, immediately preserved with HgCl2 and stored for subsequent analysis in the laboratory using a manual commercial CO2 coulometer or a specially constructed automated system.

Alkalinity: Subsamples for alkalinity are collected and immediately preserved with mercuric chloride (HgCl2) for subsequent analysis in the laboratory. The alkalinity is determined by a potentiometric titration, and the end-point is computed using a non-linear least squares approach.

Measurement of pH: The pH is determined by spectrophotometric detection on the total hydrogen scale at 25 C (pHtotal@25), using m-cresol purple dye as the indicator.

Nitrate plus Nitrite: Subsamples are drawn and stored frozen (-20°C) in acid-washed, HDPE bottles. Nitrate+nitrite is measured from subsamples passed through a cadmium reduction column and reacted with aromatic amines to form an azo dye which is analyzed colorimetrically.

High-sensitivity Nitrate+Nitrite determinations by Chemiluminescence:

Subsamples are drawn and stored frozen (-20°C) in acid-washed, HDPE bottles. Nitrate/nitrite is converted to nitric oxide by wet chemical reduction in an acidic solution. The nitric oxide produced is measured using a commercial chemiluminescent detector.

Soluble Reactive Phosphorus: Subsamples are drawn and stored frozen (-20°C) in acid-washed, HDPE bottles. Soluble reactive phosphorus (SRP) is measured colorimetrically following the formation of phosphomolybdic acid.

High-sensitivity Soluble Reactive Phosphorus by MAGIC: Subsamples are collected in HDPE bottles and stored frozen (-20°C). The sample is preconcentrated and SRP is analyzed by standard colorimetry.

Soluble Reactive Silica: Subsamples are drawn into acid-washed, HDPE bottles. Soluble reactive silica (SRSi) is measured colorimetrically following the formation of silico-molybdic acid from the reaction of ammonium molybdate and silica at acidic pH.

Particulate Carbon and Nitrogen: Subsamples are prescreened and transferred to specially designed, calibrated filtration bottles, pressure-filtered through combusted GF/F filters and stored frozen for subsequent analysis. In the laboratory, the filters are dried, combusted and analyzed for C and N using a commercially available instrument.

Particulate Phosphorus: Subsamples are prefiltered (202 µm) and transferred to specially designed, calibrated filtration bottles, pressure filtered through combusted, acid-rinsed glass fiber filters and stored frozen for subsequent analysis. In the laboratory, the filters are combusted at 450°C, leached in dilute HCl and the concentration of the resulting inorganic phosphorus is determined by colorimetric analysis.

Particulate Inorganic Carbon: Particulate inorganic carbon (PIC), primarily calcium carbonate as both calcite and aragonite, is measured on filtered particulate matter from either water column seawater samples collected at discrete depths or sediment trap samples. In the laboratory, the filters are dried and acidified to convert PIC to CO2 which is analyzed by non-dispersive infrared analysis.

Particulate Biogenic Silica: Seawater samples are collected at discrete depths using CTD/rosette sampling procedures. A known volume is concentrated onto a 0.8 µm Nuclepore filter and stored at -20°C. In the laboratory, particulate Si is dissolved in a basic solution, with heat, over time.

Fluorometric Analysis of Chlorophyll a and pheopigments: Subsamples are filtered onto glass fiber filters and placed into cold, 100% acetone to extract photosynthetic pigments. Concentrations of chlorophyll a and phaeopigments are measured by fluorometry.

HPLC Analysis of Algal Pigments: Chlorophylls, carotenes and other accessory pigments are analyzed by high-performance liquid chromatography (HPLC). The diversity as well as their distributions and concentrations in the water column are used to describe the structure of the phytoplankton community.

Enumeration of Bacteria and Cyanobacteria by Flow Cytometry: Seawater is collected and preserved in the field, for later analysis in the lab using an InFlux Flow Cytometer run by the Spigot software package. The populations targeted for enumeration are Prochlorococcus, Synechococcus, pico-eukaryotes (cells sized around 1μm), and heterotrophic, non-fluorescing bacteria. Each sample is split with one aliquot analyzed for auto-fluorescence and the other stained with SYBR Green I, for enumerating heterotrophic bacteria. In both cases, cell excitation is produced using stacked 488nm and 457nm lasers. The data is analyzed using FlowJo software to determine cell numbers per target group and used to derive cell concentrations per milliliter sample.

Microbial ATP: Subsamples are filtered through glass fiber filters and ATP, an obligate constituent of all living organisms, is extracted using boiling TRIS buffer. The extracted ATP is analyzed in a photometer by the firefly bioluminescence reaction, and the ATP content is related to total living (biomass) microbial carbon by the application of a laboratory-derived extrapolation factor.

Please refer to the HOT Analytical Methods Web site: https://hahana.soest.hawaii.edu/hot/protocols/protocols.html

Quality Indicator Flags:
1 = not quality controlled
2 = good data
3 = suspect (i.e. questionable) data
4 = bad data
5 = missing data
9 = variable not measured during this cast

BCO-DMO Processing Notes:
- Imported data from source files hot340.gof, hot341.gof, etc. into the BCO-DMO data system. Data file imported using missing data identifiers -9 and -9.00 (v3)
- added Latitude and Longitude values in decimal degrees from cruise summary information (v1, v2, v3)
- combined separate dates and times to create a Sampling_Datetime field (v2, v3)
- added cruise summary information (v1, v2, v3)
- added field for Vessel based on the EXPOCODE (v2, v3)
- added field for HOT_ID based on EXPOCODE and filename (v2, v3)
- merged new data with previous data (v2, v3)
- sorted by Cruise number (largest to smallest) where most recent cruise is listed first (v3)

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.