Nutrient distributions in an anticyclonic eddy in the northeast Atlantic Ocean, with reference to nanomolar ammonium concentrations

Abstract

The micromolar concentrations of nitrate, nitrite, silicate, phosphate and ammonium; and the nanomolar concentrations of nitrate, nitrite and ammonia were studied during a Lagrangian study of an anti-cyclonic eddy situated in the Iceland Basin of the North Atlantic Ocean. Underway surface transects, carried out across the eddy at the beginning and end of the survey period of 2 weeks, showed a decrease in the surface-water concentrations of nitrate and phosphate, with a concomitant increase in the phytoplankton biomass and surface-water temperature. The trajectory of the eddy was tracked by a release of SF 6 tracer, and regular water column sampling was carried out at the centre of the tracer patch over a period of 11 days. A storm after the first 5 days of the cruise brought about a transport into the surface mixed layer of colder sub-thermocline waters, which resulted in significant nutrient enrichment of nitrate, phosphate and silicate in the euphotic zone. This transport of nutrients, in particular nitrate, into the surface-mixed layer demonstrates the importance of eddies in the open ocean for sustaining ‘new’ production within the euphotic zone. We also report observations of ammonium at concentrations from 45 to 935 nmol l −1 for the northeast Atlantic using an automated analytical system. The presence of an ammonium maximum in the water column related to the thermocline, chlorophyll maximum, and the primary nitrite maximum.