The effects of light and nitrate levels on the relationship between nitrate reductase activity and 15NO3- uptake:: Field observations in the East China Sea

Nitrate reductase activity (NRA) and (NO3-)-N-15 uptake (NU) were determined in the East China Sea and the adjoining Kuroshio in May 1996, at six stations covering a range of hydrographic conditions: the nutrient-rich and fresher plume of Changjiang Diluted Water along the Chinese coast, the nutrient-rich upwelling Kuroshio Subsurface Water at the shelf edge northeast of Taiwan, the oligotrophic Kuroshio Surface Water and the mixing zones among these water masses on the shelf. The values of NRA in the surface mixed layer ranged between 16 and 0.1 nM-N h(-1), whereas those of NU ranged between 37 and 1 nM-N h(-1). Higher NRA and NU were found in the frontal zone between the coastal and shelf waters and in the upwelling zone, whereas the lowest values were found in the surface Kuroshio. The NRA/Chl a ratio increased linearly with increasing NU/primary production ratio in the sequence: Kuroshio < coastal plume < upwelling zone and mixing zones in the shelf. This is probably a reflection of the varying nutrient condition and the relative importance of NU in sustaining the biomass in these regions. In nitrate- and light-replete waters, the average NU/NRA was 1.0 +/- 0.3. NRA was linearly related to NU so that NU = 1.08 (+/- 0.07)NRA (r(2) = 0.79). Thus, NRA may be used for estimating NU in these waters. In nitrate deficient and light-replete waters, the average NU/NRA was 4 +/- 4. These high and variable values of NU/NRA might have been caused by an over-estimation of NU as a result of the stimulatory effect of the added (NO3-)-N-15- on phytoplankton growth. Thus, NRA may be a more reliable indicator of the rate of NO3- uptake in oligotrophic waters. In nitrate-replete and light-deficient waters, NU did not correlate well with NRA. The average NU/NRA was 0.7 +/- 0.7. These low and variable values of NU/NRA suggest a possible decoupling between NRA and NU. By using the relationship between NU and NRA in nitrate- and light-replete waters and the depth-integrated inventory of NRA in the photic zone at each station, NU in oligotrophic waters, the coastal plume, upwelling waters and shelf waters can be estimated to be 0.45, 1.55, 3.12, and 3.59 mg-N m(-2) h(-1) respectively. These values fall well within the range of previously reported values in similar types of water.