Six to eight depths will be sampled, including some below the photic zone, with the Kevlar cable/rosette system. Samples to be used for determination of nutrient uptake rates will be transferred immediately to acid cleaned polycarbonate bottles.
Standard autoanalyzer techniques will be relied upon when N concentrations are above ~100 nM. The low level techniques will be used when N concentrations are <100 nM, i.e., chemiluminescent technique (Garside, 1982) for NO¯ and the solvent extraction technique (Brzezinski, 1987) for NH.
Additions of 10 % ambient substrate concentration will be used when substrate concentrations are above the analytical limit of detection. When substrate concentrations are at or below the limit of detection (whether by conventional methods or the new low level methods), tracer additions will be at the detection level.
Rates of nitrogen uptake will be determined during the course of 3--4 hour simulated in situ incubations. Subsurface light will be approximated with a combination of neutral density and colored filters. Temperature will be maintained ± a few C of the depth from which samples were collected. For each station uptake rates will be determined twice during the day and once at night.
Preparations of enriched and unenriched plankton nitrogen will be prepared by each lab group and exchanged for intercalibration.