Measurements of net and gross O₂ production, dark O₂ respiration, and ¹⁴C assimilation at the Marine Light‐Mixed Layers site (59°N, 21°W) in the northeast Atlantic Ocean

Abstract

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 14 C uptake. The oxygen rates were measured with new instruments called productivity autosamplers (PA's), which automate the process of water sampling, incubation, and O 2 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 O 2 production and 14 C uptake were tightly coupled; the ratio of gross O 2 production to dawn‐to‐dusk 14 C uptake averaged 1.4±0.1 (SE). Net autotrophic O 2 production, estimated as gross O 2 production minus a floristically determined chlorophyll a specific respiration rate times the chlorophyll a concentration, was compared for the first time with 14 C uptake. These two processes were not tightly coupled; the ratio of net autotrophic O 2 production to 14 C uptake varied from 1.0 at 2 m to 0.2 at 15 m. Particulate organic carbon (POC) turnover rates ( k poc ) 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 O 2 and 14 C incubations. The 24‐hour 14 C‐based turnover rates ( k 14C ) exhibited little 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 CO 2 assimilated (PQ) of 1.4 was a more useful indicator of k POC .