Estimates of photosynthesis based on the incorporation of C-14-labeled inorganic carbon into particulate carbon were compared to estimates of gross photosynthesis based on net O-2 production and the production of O-18(2) from (H2O)-O-18 during the US Joint Global Ocean Flux Study (US JGOFS) Arabian Sea process cruises. For samples incubated below the surface and at optical depths < 3, the C-14 uptake : gross photosynthesis ratio averaged 0.45 +/- 0.1. This result is in accord with theoretical considerations of the combined effects of the Mehler reaction, photorespiration, dark respiration, excretion, and grazing effects on the two estimates of photosynthesis. The C-14 uptake:gross photosynthesis ratio was distinctly higher (0.62) for samples incubated at the surface. This result is likely due to UV light effects, since the O-2 and C-14 incubations were done in quartz and polysulfone bottles, respectively. The C-14 uptake:gross photosynthesis ratio was lower (0.31) for bottles incubated at optical depths > 3. This result probably reflects an increase in the ratio of dark respiration to net photosynthesis in the vicinity of the compensation light level. (C) 2000 Elsevier Science Ltd. All rights reserved.
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