C-13 AND N-15 UPTAKE BY MARINE-PHYTOPLANKTON .3. INTERACTIONS IN EUOPHOTIC ZONE PROFILES OF STRATIFIED OCEANIC AREAS

Upon addition of inorganic N to phytoplankton in various nutritional states, transient changes in net C uptake were observed over a few hours in daylight, both in euphotic zone profiles and in time series of natural and cultured samples. The reductions in C fixation, probably due to increased respiration, led to inverse relations C and N uptake. They were sometimes followed by stimulations of C uptake, corresponding or not to exhaustion of the added nutrient. Such phenomena could not be readily interpreted in terms of nutrient deficiency, as they also occurred in waters in which dissolved N compounds were around 3 .mu.M. Under controlled laboratory conditions, interspecific variability in responses of the C uptake system to a N addition was very high, but the absolute irradiance level appeared to be determinant in the manifestation of such patterns.