‘ Excess Nitrate ’ in the East China Sea

‘ Excess nitrate ’ was found in waters with salinities below 30•5 in the top 10–15 m of waters covering one-third to one-half of the East China Sea in the summer of 1992. In these waters, significant concentrations of [nitrate+nitrite], up to about 6 μM, could be found while the corresponding concentrations of phosphate remained at or below 0•07 μM. Thus, the ratio of [nitrate+nitrite] to phosphate in these waters was far in excess of that at which these two essential nutrients are utilized by marine phytoplankton. (‘ Excess nitrate ’ is the concentration of [nitrate+nitrite] in excess of that which may be utilized by marine phytoplankton at the observed concentration of phosphate.) As a result, in contrast to the open marine systems in general, primary production in a significant portion of the East China Sea may be phosphate-limiting rather than nitrogen-limiting. The area covered by this layer of combined nitrogen-rich water could be temporally variable. While no ‘ excess nitrate ’ was detected in the south-eastern East China Sea along the Chinese coasts in the summer of 1992, significant quantities of ‘ excess nitrate ’, up to 4 μM, were found at the same location in the early spring of 1993 in waters with salinities as high as 34•5.