The effects of light and nitrate levels on the relationship between nitrate reductase activity and (NO3-)-N-15 uptake: Field observations in the EastChina Sea

Nitrate reductase activity (NRA) and (NO3-)-N-15 uptake (NU) were determine d in the East China Sea and the adjoining Kuroshio in May 1996, at six stat ions covering a range of hydrographic conditions: the nutrient-rich and fre sher plume of Changjiang Diluted Water along the Chinese coast, the nutrien t-rich upwelling Kuroshio Subsurface Water at the shelf edge northeast of T aiwan, the oligotrophic Kuroshio Surface Water and the mixing zones among t hese water masses on the shelf. The values of NRA in the surface mixed laye r 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 betwe en the coastal and shelf waters and in the upwelling zone, whereas the lowe st values were found in the surface Kuroshio. The NRA/Chl a ratio increased linearly with increasing NU/primary production ratio in the sequence: Kuro shio < coastal plume < upwelling zone and mixing zones in the shelf. This i s 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. N RA 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 defic ient and light-replete waters, the average NU/NRA was 4 +/- 4. These high a nd variable values of NU/NRA might have been caused by an over-estimation o f NU as a result of the stimulatory effect of the added (NO3-)-N-15- on phy toplankton growth. Thus, NRA may be a more reliable indicator of the rate o f NO3- uptake in oligotrophic waters. In nitrate-replete and light-deficien t 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 decouplin g 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 an d 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.