15NO3- uptake rates were determined daily during experimental cycles from samples incubated on the in situ array at 8 depths spanning the euphotic zone. Each day samples were collected from Niskin rosette into polycarbonate bottles (a single 1.2-L bottle per depth). Samples were spiked with K15NO3 (final concentration 0.100 umol L-1). All samples were incubated in situ for 24 hours beginning and ending at ~04:00 am local time. Upon recovery, 5NO3 uptake samples were immediately filtered through pre-combusted GF/F filters at low pressure and frozen in liquid nitrogen. On land, they were thawed, acidified, dried out, and analyzed by mass spec at the SIO Analytical Facility.
Nitrate uptake was calculated following Dugdale and Wilkerson (1986). 15Ninit, 15Nfinal, 15Nnat, and 15Nspk were the 15:14N mass ratios for ambient PON in the water column (averaged for the cycle), final PON at the end of the incubation, water column NO3, and the K15NO3 spike, respectively. [NO3]nat was the water column nitrate values at the beginning of the incubation (determined from 0.1-um filtered, frozen samples analyzed at the UCSB Analytical Facility) and [15NO3]spk was the final concentration of added K15NO3 (0.1 umol L-1). Since [NO3]nat was not determined until the end of the cruise, the investigators used a fixed [15NO3]spk. The previous equation can significantly overestimate ambient nitrate uptake rates when [15NO3]spk is greater than 10% of [NO3]nat, so when this was determined to be the case, the investigators assumed that [NO3]nat was significantly than the half-saturation constant for NO3 and hence applied a correction such that \u03c1corrected=\u03c1\u00d7[NO3]nat/([NO3]nat+[NO3]spk).
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BCO-DMO added lat and lon values from the MV1008 event log.