During March‐April 1983, measurements of14C uptake rate in both light and dark bottles were made on 276 sea‐surface samples distributed across a plume of upwelled water, from the northwest of South Island to the Taranaki Bight, New Zealand. The non‐photosynthetic14C uptake in the dark was extremely variable andoftenahigh(2–88%) proportion of the photosynihetic uptake in die light. Dark and light uptake was broadly related to the extent of spread of upwelled water. Within the plume there was a trend for both increasing phytoplanktonic biomass (as chlorophylla)and ammonium concentration to enhance dark uptake, but therapidity with which environmental conditions changed within the plume probably obscured aclose correlation between variables. High dark14C uptake probably mainly resulted from high bacterial nitrification rates, when average nitrification may have been c. 7% of total nitrogen uptake, plus some uptake by nitrogen‐deficient algae. Dark14C uptake varied too much for it to be used to adjust light14C uptake to estimate primary production.