PRIMARY PRODUCTION AT 47-DEGREES-N AND 20-DEGREES-W IN THE NORTH-ATLANTIC OCEAN - A COMPARISON BETWEEN THE C-14 INCUBATION METHOD AND THE MIXED LAYER CARBON BUDGET

Primary production in the oceans has been estimated mainly on the basis of in vitro incubation measurements. An implicit assumption is that the growth rate of phytoplankton observed in vitro represents that occurring in the freely circulating water of the euphotic zone. We have tested this assumption at 47-degrees-N-20-degrees-W in the eastern North Atlantic Ocean during the initial stages of a spring phytoplankton bloom. The daily primary production was measured by means of the C-14 assimilation method, in which the incubation bottles were suspended in the ocean from dawn to dusk daily (about 14 h). The mean daily carbon assimilation rate in the photic zone and in the mixed layer was 107 +/- 23 and 84 +/- 18 mmol C m-2 day*-1 (where day* = 14 daylight hours), respectively, during the 12-day period between 26 April and 7 May 1989. The mixed layer carbon assimilation data are found to be consistent with the in situ CO2 utilization rate of 82 +/- 17 mmol C m-2 day*-1 estimated on the basis of the thickness of surface mixed layer, the CO2 concentration in it, and the air-sea CO2 flux. We conclude that primary production in the open ocean appears to be well represented by the in vitro measurements, if the samples are incubated under the in situ light and temperature conditions. The mean daily reduction rate of the total CO2 concentration observed in the mixed layer over the 12-day period is 2.3 mumol kg-1 day-1 , about 75% of the rate, 3.1 mumol kg-1 day*-1, expected from the rate of primary production. About 8.5% of this difference is explained by the atmospheric CO2 flux, and the remaining 16.5% may be attributed to the respiration and the influx of CO2-rich waters from the mixed layer.