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 (r2=0.81) to increasing numbers of E. huxleyi. Inorganic nutrient concentrations in surface waters were low (<0.05 μmol l−1 NO3−, <0.1 μmol l−1 NH4+ and <0.06 μmol l−1 PO43−). The concentration of urea ranged between 0.1 and 1.5 μmol l−1 and was the most important source of nitrogen to the phytoplankton. The phytoplankton biomass was dominated (58–89%) by cells >5 μm, including E. huxleyi, which contributed between 28% and 56% of chlorophyll a concentration in the surface 20 m. Carbon fixation into particulate organic material ranged between 0.23 and 4.02 μmol C l−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 μmol 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.