Size-fractionated nitrogen uptake and carbon fixation during a developing coccolithophore bloom in the North Sea during June 1999

Phytoplankton biomass and rates of nitrogen and carbon uptake were determined during a six-day Lagrangian experiment in the northern North Sea (58degrees56'N 02degrees52'E-58degrees20'N 03degrees28E) in June 1999. Satellite remotely sensed images and shipboard observations were used to select an initial position within a bloom of Emiliania huxleyi. The euphotic zone was effectively separated into two layers by a pycnocline (27.0 isopycnal) at 17m on 18 June, which deepened, to 26 m on 24 June. The depth of the euphotic zone decreased with time over the experiment from 46 to 31 m and was significantly correlated (r(2) = 0.81) to increasing numbers of E huxleyi. Inorganic nutrient concentrations in surface waters were low (<0.05 mumol l(-1) NO3-, <0.1 mumol l(-1) NH4+ and <0.06 mumol l(-1) PO43-) The concentration of urea ranged between 0.1 and 1.5 mumol l(-1) and was the most important source of nitrogen to the phytoplankton. The phytoplankton biomass was dominated (58-89%) by cells > 5 pm, including E huxleyi, which contributed between 28% and 56% of chlorophyll a concentration in the surface 20m. Carbon fixation into particulate organic material ranged between 0.23 and 4.02 mumol Cl-1 d(-1). Rates were lowest at the beginning of the experiment and increased to a maximum equivalent to 91 mmol C m(-2) d-1 on 22 June. Calcification, the production of inorganic carbon by coccolithophores, reached a maximum of 11.54 mmol C m(-2) d(-1) on 21 June, and E huxleyi cell numbers continued to increase to 1500 cells ml(-1) on 23 June. Nitrogen uptake was always in the order urea > NH4+ > NO3-, and the ratio of urea to NO3 uptake varied between 2 and 26. The supply of nitrogen was dominated by regenerative processes except at the base of the euphotic zone where diffusion of NO3 and NH4+ from deep water was quantitatively important on three out of six days. Nitrification rates varied between 0.026 and 0.429 pmol N l(-1) d(-1). If nitrification rate was not included in the calculation, NO3 uptake was underestimated by a mean value of 2.4 times. (C) 2002 Elsevier Science Ltd. All rights reserved.