Phytoplankton growth and cyclonic eddies

Abstract

AROUND the British Isles thermal fronts¹ develop during the summer months between cold, well mixed waters that are relatively rich in inorganic nutrients and warm, well stratified waters that are nutrient depleted². The surface standing crop of phytoplankton is generally high in the marginally stratified frontal zone, particularly where the water is deep (for example, off Ushant and around Orkney and Shetland)², although these surface waters are generally low in inorganic nutrients. To understand the growth of these frontal blooms, therefore, the scales³ of non-tidal motion that determine the exchange of chemical and biological properties in frontal regions need to be identified. Although the frontal boundaries are almost in geostrophic balance, they are unstable and are characterised by cyclonic eddies with a spatial extent of 20–40 km which can persist for a few days⁴. These eddies probably have an important influence on phytoplankton growth through the transfer of nutrients as well as phytoplankton across the frontal zone. Furthermore, as the division time of the plant cells is relatively short (∼1 d), growth of the population may actually occur in the eddy transfer process. In this letter a series of observations on the Ushant frontal system, planned to investigate this problem, are described.