PHYSICAL, CHEMICAL AND BIOLOGICAL FEATURES OF A CYCLONIC EDDY IN THE REGION OF 61-DEGREES-N-10 19-DEGREES-50W IN THE NORTH-ATLANTIC

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 deg rees N 20 degrees W. The eddy held 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 sub arctic 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 topograp hy. The horizontal salinity, nitrate and biological gradients between adjacent waters and the eddy were less marked in the present study tha n in a previous investigation of a cyclonic eddy in the vicinity of 48 degrees N 22 degrees W (Mittelstaedt, 1987), possibly due to the surf ace 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 th is 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 measu rements also indicated that the NE sub-polar Atlantic in mid-summer is more active than previously thought. Despite the extensive coccolitho phore bloom studied immediately previously by CD60, there was no measu rable coccolithophore calcification in the waters within the eddy in t he present study. This is consistent with phytoplankton taxonomic data , which demonstrates that coccolithophore abundance was almost one hun dred fold lower at this location on CD61 relative to CD60 and that lit h and coccolithophore abundances were grestest in the water column ben eath the mixed layer, suggesting sinking. These observations suggest t hat the decline of the bloom had occurred in the period between the tw o cruises. (C) 1998 Published by Elsevier Science Ltd. All rights rese rved.