Carbon cycling in primary production bottle incubations: inferences from grazing experiments and photosynthetic studies using C-14 and O-18 in the Arabian Sea

Estimates of photosynthesis based on the incorporation of C-14-labeled inor ganic carbon into particulate carbon were compared to estimates of gross ph otosynthesis 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 combi ned effects of the Mehler reaction, photorespiration, dark respiration, exc retion, and grazing effects on the two estimates of photosynthesis. The C-1 4 uptake:gross photosynthesis ratio was distinctly higher (0.62) for sample s 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 bott les, respectively. The C-14 uptake:gross photosynthesis ratio was lower (0. 31) for bottles incubated at optical depths > 3. This result probably refle cts an increase in the ratio of dark respiration to net photosynthesis in t he vicinity of the compensation light level. (C) 2000 Elsevier Science Ltd. All rights reserved.