Response of phytoplankton to nitrogen addition in the Taiwan strait upwelling region: Nitrate reductase and glutamine synthetase activities

This study investigated the activities of two nitrogen assimilation enzymes, nitrate reductase (NR) and glutamine synthetase (GS) in phytoplankton in relation to sample, as well as the nutrient levels and phytoplankton biomass (Chl- a concentrations) during an upwelling event in the southern Taiwan Strait during an upwelling period from 6 to 12 July, 2005. The results showed that high NR activity (NRA) was always found with low nitrate and high Chl-a concentrations, while GS activity (GSA) exhibited positive correlations with ammonium and Chl-a concentration. Both NRA and GSA varied with the time and stage of upwelling: high NRA and GSA were observed initially at the subsurface layers in the early stage of upwelling, accompanied by the consumption of nutrients and the increase of Chl-a concentration; and then at the surface with high Chl-a concentrations in the middle and late stages of upwelling. Results from in situ enzyme bioassays on water samples along the tracing of upwelling track and on board mesocosm experiments on board the ship showed that there was a time-lag between nitrate addition and NRA and GSA, but. However, no time-lag was found between ammonium addition and GSA. The present results indicated that both NRA and GSA reflect the status of ambient nitrogen levels and the assimilation process of the phytoplankton, and could be used as effective parameters for the analysis of the physiological response of the phytoplankton to nitrogen variations during upwelling periods. Measurements of NRA and GSA in the phytoplankton in newly upwelled water appeared to provide ecophysiological indicators of phytoplankton, which will make it possible to trace during the sequence of upwelling events (such as nutrient supplementation) leading to enhanced productivity.