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 (58°56′N 02°52′E–58°20′N 03°28′E) 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 17 m 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 (NO 3 −, and the ratio of urea to NO 3 − 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 NO 3 − and NH 4 + from deep water was quantitatively important on three out of six days. Nitrification rates varied between 0.026 and 0.429 μmol N l −1 d −1. If nitrification rate was not included in the calculation, NO 3 − uptake was underestimated by a mean value of 2.4 times.