Contrasting nitrogen uptake by diatom and Phaeocystis-dominated phytoplankton assemblages in the North Sea

paper documents ambient concentrations of nutrients in the Belgian coastal waters of the North Sea during the spring of 1996 and 1997. The paper elaborates the differences of uptake rates of oxidised nitrogen (NO(3)(-)) and reduced nitrogen (NH(4) and urea) by Phaeocystis and diatoms. The nitrogen concentrations were dominated by NO(3)(-) with a maximum concentration of 30 gM (January 1997) and 40 gM (March 1996). In 1996, Phaeocystis dominated the spring biomass with a maximum of 521 mug C 1(-1), while maximum diatom biomass was 174 mug C 1(-1). In 1997, the maximum Phaeocystis spring biomass was 1600 mug C 1(-1) and diatom maximum biomass was below 100 mug C 1(-1). A maximum bacteria biomass of about 55 mug C 1(-1) was observed in mid-May 1996. The maximum nitrogen uptake rates were recorded during spring and were dominated by NO(3)(-) (0.005 h(-1) in 1996 and 0.032 h(-1) in 1997). Maximum specific NH(4) uptake rates were between 0.005 h(-1) in May 1996 and 0.006 h(-1) in April 1997. The NO(3)(-) uptake rates displayed exponential decrease versus increasing ambient reduced nitrogen concentrations (ammonium and urea), whereas the reduced nitrogen uptake increased but never compensated the decreased nitrate uptake. The NH(4) uptake kinetics of diatoms displayed lower v(max) compared to Phaeocystis. Consequently, Phaeocystis showed ability to increase their NH(4) uptake capacity when more NH(4) became available while diatoms failed to do so, after ammonium had exceeded their saturation concentration (>1 muM). Although reduced nitrogen has a negative effect on the uptake of NO(3)(-), Phaeocystis have more advantage than diatoms on the uptake of ammonium. This might be contributing to the biomass domination shown by Phaeocystis over extended periods in spring. (C) 2003 Elsevier Science B.V. All rights reserved.