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 amblent 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 layer due to the formation of a shallow thermocline led to rapid utilisation of nutrients. This probably enabled the development of a dinoflagellate-dominated phytoplankton population and of organisms capable of heterotrophic regenerative processes.

The metabolic balance of the planktonic community in the North Atlantic Subtropical Gyre: The role of mesoscale instabilities

“…‡ Minimum front length of 1,800 km, if a simple linear shape between 20ЊW and 40ЊW and minimum front width of 55 km, calculated by a marked depression of isotherms ( centration, and picoplankton biomass could be due to surface heating and mixing processes (Harris et al 1997) or the dynamic equilibrium with the surrounding waters (Ring Group 1981) when the eddy depicts a low-energy stage. High heterotrophic bacterial abundance has been observed both in mature (Harris et al 1997) and in young eddies (Lochte and Pfannkuche 1987). The mean bacterial abundance within the photic layer observed by Campbell et al (1997) in the oligotrophic subtropical central Pacific was 4.2 ϫ 10 5 cell ml Ϫ1 , similar to the values measured in this study in the oligotrophic waters outside the mesoscale features (4.4 ϫ 10 5 cell ml…”

“…Boyd et al (1997) observed little variation between rates of new production inside and outside of this eddy. By contrast, high differences between cyclonic eddies and surrounding waters have been observed in primary production rates and Chl a concentration (e.g., Falkowski et al 1991;McNeil et al 1999) and bacterial abundance (Lochte and Pfannkuche 1987). The TOPEX-POSEIDON altimetry data received indicated that Leticia was detected for ϳ400 d (24 May 1998-28 June 1999) but with a disminishing signal for ϳ3 months after the cruise (Mouriño et al pers.…”

Otolith elemental signatures reflect residency in coastal water masses

“…Lochte and Pfannkuche (1987) observed nutrient depletion in the upper 50 m within the center of a cyclonic eddy. They proposed that the physical isolation of eddy surface waters due to the formation of a shallow thermocline lead to rapid utilization of nutrients.…”

Cyclonic cold-core eddy in the eastern North Atlantic. I Physical description

“…Within the eddy centre, we noticed the characteristic odour of plankton. Using the CTD profiles to obtain a rough idea about the extension of the eddy, stations were selected for biological sampling along the SW-NE section (Stn 159 to 164) to study the eddy's ecosystem (Beckmann et al 1987, Lochte & Pfannkuche 1987.…”

“…Some of them, for instance Gulf Stream rings, have a lifetime from a month to more than a year (see reviews in Robinson 1983). An example of such an oceanic ring is described in this and the following 2 articles (Beckmann et al 1987, Lochte & Pfannkuche 1987. Although a turbulent element, the geostrophy and the long Lifetime allow such an eddy to be considered as a systematic and orderly phenomenon.…”