Environmental control of nitrate uptake in the East China Sea

Abstract

Abstract Nitrate uptake rate was determined in the East China Sea by a simulated in situ 15 N incubation technique during four cruises representing different seasons in the period from 1993 to 1996. Stations were occupied on an observational line extending from the inner continental shelf to the adjacent oceanic water in the Pacific. The nitrate uptake rate integrated over the euphotic zone was higher on the average on the continental shelf (56 mg N m −2 d −1 ) than at the outer waters of the shelf-break and pelagic regions (21 mg N m −2 d −1 ). Both seasonal and spatial variations were larger on the shelf. Uptake rates were low in summer when the water columns were stratified. Nitrate in the summer surface waters was found to be almost completely depleted by determination with a chemiluminescence technique, and kinetic uptake experiments conducted concurrently suggested severe limitation of uptake due to the low nitrate concentrations. Uptake rates were also low in well-mixed water columns in winter and spring when the water temperature was low. High rates of uptake were observed in a spring bloom as well as at some of the inner-shelf stations in autumn. In the latter case, high phytoplankton biomass was accompanied by detectable amounts of nutrients and by lower salinity at the surface of the stratified water columns. Chlorophyll specific uptake capacity was higher and was significantly correlated with water temperature at a group of stations mostly on the shelf, while the capacity was lower and unaccountably varied at other stations. An empirical prediction model of nitrate uptake from chlorophyll concentration, temperature, light intensity, and nitrate concentration was developed and applied to these data.