Cyclonic cold-core eddy in the eastern North Atlantic. II. Nutrients, phytoplankton and bacterioplankton

Abstract

A cyclonic cold-core eddy in the Northeast Atlantic of about 100 km in diameter at the sea surface was investigated in May 1985, approximately 3 wk after it had separated from the Polar Front. A strong thermocline, which was shallower but more pronounced than in the ambient water, separated a warm surface layer within the eddy from deeper cold water, while horizontal salinity gradients marked the boundary to the ambient water. The cold-core eddy could be distinguished from ambient Northeast Atlantic water in terms of its nutrient chemistry, phytoplankton species distribution and abundance, bacterial numbers and cell size. The surface layer of the eddy was distinct from deeper eddy water, and was characterized by high concentrations of chlorophyll a, total phytoplankton biomass, dinoflagellates and bacteria. At the eddy's margin diatoms were predominant. It is argued that the physical isolation of the eddy surface due to the formation of a shallow thermocline led to rapid utilisation of nutrients.