Physical, chemical and biological features of a cyclonic eddy in the region of 61°10'N 19°50'W in the North Atlantic

Abstract The second leg (CD61) of a two cruise investigation of coccolithophore biogeochemistry in the NE subarctic Atlantic provided the opportunity to make a detailed study of a cyclonic eddy in the vicinity of 61°N 20°W. The eddy field in the NE Atlantic is thought to be particularly important with regard to the physics of this region, and may influence the resulting chemical and biological properties of subarctic Atlantic waters. This eddy was ca. 50 km in diameter, moved at ca. 1.5 km d −1 to the north of east, with a geostrophic circulation around the feature of ca. 25 cm s −1 and probably extended as far as the ocean floor, where it may have interacted with the bottom topography. The horizontal salinity, nitrate and biological gradients between adjacent waters and the eddy were less marked in the present study than in a previous investigation of a cyclonic eddy in the vicinity of 48N 22°W (Mittelstaedt, 1987), possibly due to the surface waters of the eddy mixing with surrounding waters. Satellite image sequences clearly link this feature with those studied in a mesoscale coccolithophore bloom studied in the same region on a previous cruise (CD60). Rates of primary production within the eddy were almost twice the mean values reported for Ocean Weather Station India (OWSI) at this time of year, but were similar to those noted during studies at the MLML site to the SE of the eddy location. Other biological rate measurements also indicated that the NE sub-polar Atlantic in mid-summer is more active than previously thought. Despite the extensive coccolithophore bloom studied immediately previously by CD60, there was no measurable coccolithophore calcification in the waters within the eddy in the present study. This is consistent with phytoplankton taxonomic data, which demonstrates that coccolithophore abundance was almost one hundred fold lower at this location on CD61 relative to CD60 and that lith and coccolithophore abundances were grestest in the water column beneath the mixed layer, suggesting sinking. These observations suggest that the decline of the bloom had occurred in the period between the two cruises.