Rates of primary production and respiration were determined at a moderately eutrophic site (1--4 mg Chl a m-3) in the northeast Atlantic Ocean from in vitro changes in dissolved oxygen and 14C uptake. The oxygen rates were measured with new instruments called productivity autosamplers (PA's), which automate the process of water sampling, incubation, and O2 measurement. Measurements of net and gross production and respiration were obtained at two depths over an 8-day period in May and a 7-day period in August 1991. Carbon 14 uptake was measured in dawn-to-dusk and 24-hour incubations at six depths during the same periods. The results indicate that gross O2 production and 14C uptake were tightly coupled; the ratio of gross O2 production to dawn-to-dusk 14C uptake averaged 1.4¿0.1 (SE). Net autotrophic O2 production, estimated as gross O2 production minus a floristically determined chlorophyll a specific respiration rate times the chlorophyll a concentration, was compared for the first time with 14C uptake. These two processes were not tightly coupled; the ratio of net autotrophic O2 production to 14C uptake varied from 1.0 at 2 m to 0.2 at 15 m. Particulate organic carbon (POC) turnover rates (kPOC) in the mixed layer, estimated from day-to-day changes in the water column POC inventory, were compared with turnover rates estimated from the in vitro O2 and 14C incubations. The 24-hour 14C-based turnover rates (k14C) exhibited little relationship to in situ POC turnover rates. Turnover rate based on 24-hour net community O2 production (kO2) converted to carbon using a molar ratio of O2 produced to CO2 assimilated (PQ) of 1.4 was a more useful indicator of kPOC. ¿ American Geophysical Union 1995