Effect of incubation time and substrate concentration on N-uptake rates by phytoplankton in the Bay of Bengal

Abstract. We report here the results of three experiments, which are slight variations of the ¹⁵N method (JGOFS protocol) for determination of new production. The first two test the effect of (i) duration of incubation time and (ii) concentration of tracer added on the uptake rates of various N-species (nitrate, ammonium and urea) by marine phytoplankton; while the third compares in situ and deck incubations from dawn to dusk. Results indicate that nitrate uptake can be underestimated by experiments where incubation times shorter than 4h or when more than 10% of the ambient concentration of nitrate is added prior to incubation. The f-ratio increases from 0.28 to 0.42 when the incubation time increases from two to four hours. This may be due to the observed increase in the uptake rate of nitrate and decrease in the urea uptake rate. Unlike ammonium [y{=}2.07x{-}0.002\, (r²=0.55)] and urea uptakes [y{=}1.88x{+}0.004 (r²=0.88)], the nitrate uptake decreases as the concentration of the substrate (x) increases, showing a negative correlation [y{=}-0.76x+0.05 (r₂=0.86)], possibly due to production of glutamine, which might suppress nitrate uptake. This leads to decline in the f-ratio from 0.47 to 0.10, when concentration of tracer varies from 0.01 to 0.04μ M. The column integrated total productions are 519 mg C m^-2 d^-1 and 251 mg C m^-2 d^-1 for in situ and deck incubations, respectively. The ¹⁴C based production at the same location is ~200 mg C m^-2 d^-1, which is in closer agreement to the ¹⁵N based total production measured by deck incubation.