Sampling strategy
\nTiming: 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.\u00a0
\nLocation: Our primary sampling site is centered at 22\u00b0 45'N, 158\u00b0 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.\u00a0
\nHigh-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\u00a0
\nDiscrete 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.
\nBioGeoChemical properties
\nSeawater is collected from known depths using CTD-rosette sampling procedures.\u00a0
\nDissolved 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.
\nDissolved 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.
\nAlkalinity: 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.
\nMeasurement 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.
\nNitrate plus Nitrite: Subsamples are drawn and stored frozen (-20\u00b0C) 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.
\nHigh-sensitivity Nitrate+Nitrite determinations by Chemiluminescence:
\nSubsamples are drawn and stored frozen (-20\u00b0C) 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.
\nSoluble Reactive Phosphorus: Subsamples are drawn and stored frozen (-20\u00b0C) in acid-washed, HDPE bottles. Soluble reactive phosphorus (SRP) is measured colorimetrically following the formation of phosphomolybdic acid.
\nHigh-sensitivity Soluble Reactive Phosphorus by MAGIC: Subsamples are collected in HDPE bottles and stored frozen (-20\u00b0C). The sample is preconcentrated and SRP is analyzed by standard colorimetry.
\nSoluble 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.
\nParticulate 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.
\nParticulate Phosphorus: Subsamples are prefiltered (202 \u00b5m) 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\u00b0C, leached in dilute HCl and the concentration of the resulting inorganic phosphorus is determined by colorimetric analysis.
\nParticulate 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.
\nParticulate Biogenic Silica: Seawater samples are collected at discrete depths using CTD/rosette sampling procedures. A known volume is concentrated onto a 0.8 \u00b5m Nuclepore filter and stored at -20\u00b0C. In the laboratory, particulate Si is dissolved in a basic solution, with heat, over time.
\nFluorometric 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.
\nHPLC 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.
\nEnumeration 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 1\u03bcm), 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.
\nMicrobial 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.
Water samples for a variety of chemical and biological measurements are routinely collected from the surface to within 10 meters\u00a0of 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.\u00a0
\nFurthermore, 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.
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Please refer to the HOT Analytical Methods Web site: https://hahana.soest.hawaii.edu/hot/protocols/protocols.html
\nQuality Indicator Flags:
\n1 = not quality controlled
\n2 = good data
\n3 = suspect (i.e. questionable) data
\n4 = bad data
\n5 = missing data
\n9 = variable not measured during this cast