MEASUREMENTS OF NET AND GROSS O-2 PRODUCTION, DARK O-2 RESPIRATION, AND C-14 ASSIMILATION AT THE MARINE LIGHT-MIXED LAYERS SITE (59-DEGREES-N, 21-DEGREES-W) IN THE NORTHEAST ATLANTIC-OCEAN

Rates of primary production and respiration were determined at a moder ately eutrophic site (1-4 mg Chl a m(-3)) in the northeast Atlantic Oc ean from in vitro changes in dissolved oxygen and C-14 uptake. The oxy gen rates were measured with new instruments called productivity autos amplers (PA's), which automate the process of water sampling, incubati on, and O-2 measurement. Measurements of net and gross production and respiration were obtained at two depths over an 8-day period in May an d 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 O-2 production and C uptake were tig htly coupled; the ratio of gross O-2 production to dawn-to-dusk C-14 u ptake averaged 1.4 +/- 0.1 (SE). Net autotrophic O-2 production, estim ated as gross O-2 production minus a floristically determined chloroph yll a specific respiration rate times the chlorophyll a concentration, was compared for the first time with C-14 uptake. These two processes were not tightly coupled; the ratio of net autotrophic O-2 production to C-14 uptake varied from 1.0 at 2 m to 0.2 at 15 m. Particulate org anic carbon (POC) turnover rates (k(POC)) in the mixed layer, estimate d from day-to-day changes in the water column POC inventory, were comp ared with turnover rates estimated from the in vitro O-2 and C-14 incu bations. The 24-hour C-14-based turnover rates (k(14C)) exhibited litt le relationship to in situ POC turnover rates. Turnover rate based on 24-hour net community O-2 production (k(O2)) converted to carbon using a molar ratio of O-2 produced to CO2 assimilated (PQ) of 1.4 was a mo re useful indicator of k(POC).