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CCE LTER analyzes and determins total organic carbon (TOC) and total nitrogen (TN) in sea water, expressed as micromoles of carbon (nitrogen) per liter of sea water. TOC (<200 µmol·L-1) includes both dissolved and particulate organic carbon (DOC and POC, respectively). TN (<50 µmol·L-1) includes particulate and dissolved organic nitrogen as well as dissolved inorganic nitrogen species. The instruments employed are from the Aluwihare Laboratory at the Scripps Institution of Oceanography, and whenever possible, protocols closely follow those used by Jonathan Sharp (U. of Delware), Craig Carlson (U. of California, Santa Barbara) and Dennis Hansell (RMSAS). SIO has the Shimadzu TOC-VSN, normal sensitivity, with TN unit.

Complete methods of TOC and TN determination can be found here.

Principle

An acidified seawater sample (< pH 3) is sparged with CO2 free-air to remove inorganic carbon. The water is then injected onto a combustion column packed with platinum-coated alumina beads held at 680°C. Non-purgeable organic carbon (NPOC) compounds are combusted and converted to CO2, which is detected by a nondispersive infrared detector (NDIR). Non-purgeable nitrogen compounds (which includes organic and inorganic nitrogen species) are combusted and converted to NO, which when mixed with ozone chemiluminesces for detection by a photomultiplier.

Sampling

All samples collected on CalCOFI and LTER cruises are TOC and TN samples. That is, no filtering of the sample is done to remove particulates. Filtering is time-intensive and filtration, especially by untrained personnel, can introduce significant contamination. So, to maximize efficiency and minimize sample handling, the filtration step common to DOC and TDN is omitted. In open ocean waters particulate organic carbon concentrations are a small fraction of the total organic carbon and does not pose too many problems. Here subtracting POC from TOC is likely to provide an accurate estimate of DOC because particles are typically small and homogeneously distributed in the sample. In coastal waters, and at stations where relatively high chlorophyll concentrations are present, the TOC measurement cannot be easily converted to DOC by subtracting POC values. Experience has shown that particles in these regions are large and inhomogeneously distributed. Therefore, samples collected in this study are reported as TOC and TN.

Sampling Depths

Water samples are take directly from the rosette. Full profiles are taken at all SCCOOS stations (4-5 vials per station). Line 93.3 most stations (except .55 and .90) are "normal" stations, which means samples are drawn from the surface niskin, the deep niskin, and three LTER depths (5 vials in all). At 93.3.55 and 93.3.90 samples a drawn from niskins triggered at 13 depths between 0 and 170 m, and 230 m, 320 m and the deepest niskin (16 vials). Line 90.0 contains normal, LTER cardinal, and full profile stations. Normal stations (.28, .30, .35, .45, .60, .80, and .100) sample surface, deep, 3 LTER depths, 140/145m, and 320 m. Cardinal stations (.37, .70, .90 and .120) sample surface, deep, 6 LTER depths, 140/145m, 230 m and 320 m. Full profiles are collected at station .100 and .53 (20-24 vials). Line 86.7 is primarily sampled at the surface, deepest, 3 LTER depths, 140/145m and 320m. A full profile is taken at the Santa Monica Basin station (86.7.40). 16 samples are collected at .60 and .100, which include deepest, 230 m, 320 m and 13 samples between 0-170 m. Line 83.3 is similar to 93.3. Most stations are sampled at the surface, deepest bottle, and 3 LTER depths. Station .90 and .70 are sampled at the deepest depth, 230 m, 320 m and 13 depths between 0-170 m. A full profile is collected at 81.7.46.9 (Santa Barbara Basin). Line 80.0 is similar to Line 90.0. Stations .51, .60, .90 are sampled at the surface, deepest bottle, 3 LTER depths, 140/145 m and 320 m. Full profiles are taken at station .55 and .100. Remaining stations are cardinal stations which include surface, deepest bottle, 6 LTER depths, 140/145 m and 320 m. Line 76.7 is samples primarily at the surface, deepest bottle, and 3 LTER depths. Stations .55, .70 and .90 are sampled at the deepest bottle, 230 m, 320 m and 13 depths between 0-170 m.

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