To minimize loss of copepodamides through degradation and handling, the water samples were pulled onto SPE columns in situ, as described in (14). Syringes (100 ml) were interspersed on a submerged line and connected by silicone tubing to the Luer end of the SPE columns (Evolute ABN, 100 mg of sorbent). Pull speed, sorbent, and sample volume were optimized in pilot experiments with natural seawater spiked with known amounts of copepodamides. Copepodamides were sampled at depths of 0, 1, 5, 10, 15, 20, and 30 m. SPE columns were conditioned with 1.5 ml of methanol before deployment. The SPE openings were covered with 65-μm nylon plankton mesh to prevent copepods and copepod eggs from entering. Once deployed, the syringe plunger pulled water through the SPE by gravity from a weight connected to the syringe through a line. The weight was adjusted to achieve a flow rate of ~5 ml min−1. The rig was retrieved after 20 to 30 min, and the volume pulled was noted for each syringe. The procedure was repeated once to sample ~200 ml from each depth. Columns were transported to the laboratory (within 1 hour), desalted with 4 ml of Milli-Q water, and eluted in 3 ml of liquid chromatography–mass spectrometry (LC-MS)–grade methanol. One or two replicate profiles were obtained on each sampling occasion. On one occasion, we failed to get a sample from one of the depths; this value was interpolated linearly from the above and below sample at the same occasion. A single outlier sample, 40 times higher than the replicate from the same depth, was considered an artifact and removed. Sample head space was flushed with N2 gas before storage in glass vials at −80°C until analysis.

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