Dataset: EN616 Discrete Data

Techniques used are described in Balch et al. (2008).

Biogenic Silicas:
To determine reactive silicate, 200 mL of seawater sample is filtered onto a 25 mm, 0.4um pore size polycarbonate filter. Filters are folded and placed in a super clear polypropylene centrifuge tube and dried in a drying oven at 60°C for 24 hours then tightly capped and stored until analysis. On shore, 0.2N NaOH is added and the sample is placed in a 95C water bath. The digestions are then cooled and neutralized with 1N HCl. After centrifuging, the supernatant is transferred to a new tube and diluted with MilliQ water. Molybdate reagent is added and then a reducing agent is added to reduce silicomolybdate to silicomolybdous acid. The transmission at 810 nm is read on a Hitachi U-3010 spectrophotometer (SN 0947-010). Reactive silicate is calculated using a silicate standard solution standard curve prepared at least every 5 days or whenever new reagents are prepared. Readings are corrected using a reagent blank run at the same time as the standard curve and three tube blanks interspersed in each batch.

References: Brzezinski & Nelson (1989); JGOFS (1994); Strickland & Parsons (1977).

PIC (Particulate Organic Carbon):
Water samples are filtered through a 25mm, 0.4 µm pore size polycarbonate filter. The dry filter is rinsed with Potassium tetraborate (6.11 g/l K₂B₄O₇ · 4H₂O) buffer while still in the filter tower to remove as much seawater salt and also to maintain a high pH (~8.1) during sample storage and preserve the CaCO₃ on the filter. Filters are placed into trace metal clean polypropylene centrifuge tubes and dried at approximately 60°.

For analysis, the filters are currently sent out to the Sawyer Environmental Chemistry Laboratory at the University of Maine or Department of Earth Sciences at Boston University. Filters are digested in a 5% nitric acid solution for 12 hours to dissolve all CaCO₃ and the solution is analyzed by ICP-AES (Inductively Couple Plasma – Atomic Emission Spectrometry) for Ca concentration. We have filter and dissolution blanks as well as QC standards run with each batch of samples. We also use the concentration of dissolved Na in the digestate to correct for any Ca present in sea salts left on the filter. PIC concentrations are calculated using the volumes of water filtered and the volume of the digestions, and assuming all Particulate Inorganic Carbon is in the form of CaCO₃.

POC (Particulate Organic Carbon):
Water samples are filtered onto 25mm GF/F filters which have been pre-combusted (450°, 5 hours). Filters are rinsed with filtered seawater (FSW) and then stored in individual petri-plates and dried (60°) for storage. Prior to analysis, the plates are opened and placed overnight in a sealed container like a dessicator with saturated HCL fumes to remove any PIC. We send these samples to the University of Maine’s Darling Marine Center for analysis. The filters are packed into pre-combusted nickel sleeves and analyzed on a Perkin Elmer 2400 Series II CHNS/O for C, N, and H.

The analyzer is calibrated using tin capsules as blanks and acetanilide to calibrate instrument response to carbon and nitrogen. NIST certified check standards consisting of either low organic content soil or sediment are analyzed to determine accuracy of carbon detection. NIST certified organic check standards such as corn flour or rice flour are analyzed to determine the accuracy of nitrogen detection. If values vary by more than 4% from stated values, instrument is examined, any problems are addressed and instrument is recalibrated and check standards rerun until error is within acceptable limits. Duplicate samples are run during each sample run to ensure results are reproducible. If duplicates cannot be run on actual samples, as in the case of filter samples, duplicate check standards are analyzed. Duplicate samples typically vary less than 2%.

One instrument blank is analyzed for every 12 samples run. One acetanilide standard is analyzed for every 15 samples run. If blank or acetanilide values differ significantly from previous values, a new series of standards and blanks are analyzed to recalibrate the instrument.

The actual minimum detection limit (3 times the standard error) determined from the standard error of the instrument blanks is 2 micrograms for carbon and 4 micrograms for nitrogen.

References: JGOFS (1994).

Nutrients:
Water samples are collected in clean 60ml plastic bottles and immediately frozen (-20°). These samples are kept frozen with dry ice and sent to the University of California, Santa Barbara’s Marine Science Analytical Lab. They are analyzed on a Lachat QuickChem 8000 for Nitrite, Nitrate plus Nitrate, Phosphate, and Silcate.

Chlorophyll a:
Water samples are filtered onto a 25mm Millipore HA filter (mixed cellulose ester, 0.45 µm pore size). The filters are transferred to test tubes filled with chilled 90% acetone for extraction and vortexed until the filter dissolves. Tubes are stored in the dark in a freezer for 24 hours before analysis. Tubes are then re-vortexed and gently centrifuged (~1300g) for 5 minutes before being decanted into a glass cuvette for the fluorometer. We use a Turner Designs 10AU to read Fb of the sample and then add 50 µl of 10% HCL and read Fa. The fluorometer was calibrated previously with a pure chlorophyll extract (Turner Designs part# 10-850) to determine Tau τ=(Fb/Fa pure chl a) and chlorophyll a can then be calculated from: (Fb – Fa) * (τ/ τ-1) * (Vfiltered/Vextracted). Generally all surface measurements are made in triplicate.

The fluorometers (Turner 10-AUs) are calibrated at least annually using the calibration method defined by Turner Designs using standards purchased from Turner Designs. Additionally, for long cruises (e.g. Great Belt, COPAS, etc), a calibration is performed on the ship.

References: Trees, et al.