NAFO Sci. Coun. Studies, 24: 69–80 Long-time Series in Icelandic Waters in Relation to Physical Variability in the Northern North Atlantic Svend-Aage Malmberg, Hédinn Valdimarsson and John Mortensen Marine Research Institute, Skúlagata 4, P. O. Box 1390 121 – Reykjavik, Iceland Abstract This review is mainly based on two papers the author has published in the ICES Journal of Marine Science Symposia. One dealing with the hydro-biological variability in the ICES Area (Malmberg and Kristmannsson, 1992) and the other on capelin and cod, and climatic change (Malmberg and Blindheim, 1994). These papers deal with hydrographic conditions in Icelandic waters and adjacent seas in relation to large scale pan Atlantic conditions. Additional updated information from Icelandic waters are included in this review and compared with hydrographic variations elsewhere along the Subpolar Gyre in the North Atlantic. Some ecological impacts of the observed hydrographic conditions in the northern North Atlantic are also discussed. Key words : Hydrography, Iceland, salinity Introduction This review is partly based on two papers, with some additional remarks, in the ICES journal of Marine Science Symposia. The first dealing with hydro-biological variability in the ICES Area (Malmberg and Kristmannsson, 1992) and the other on capelin and cod, and climatic change (Malmberg and Blindheim, 1994). These papers deal with hy- dro-biological conditions in Icelandic waters and adjacent seas in relation to large scale pan Atlan- tic conditions. Additional information was available to update the above studies, and it was found use- ful to review these and compare them with varia- tions elsewhere, along the Subpolar Gyre in the North Atlantic. Some ecological impacts of the ob- served hydrographic conditions in the northern North Atlantic are discussed. Data The hydrographic data used in this paper are partly from joint multidisciplinary Danish – Icelan- dic research cruises into the western part of the Iceland Sea in September during 1987–91 (Anon., MS 1991; Fig. 1). The investigations were under- taken as a part of the international Greenland Sea Project (Meincke et al. , 1990). Also routine long-time investigations in Icelandic waters are included (Fig. 2), as well as some more remote information from adjacent ocean regions. Some emphasis is laid on the hydrographic conditions in time and space in relation to ecological conditions. Results The joint Danish – Icelandic hydrographic pro- gram in the western Iceland Sea in September dur- ing 1987–91 (Fig. 1) and long-time investigations in Icelandic waters (Fig. 2) revealed the following: a) The bottom water from the north at 1 000–1 500 m depth flowing southwards into the Denmark Strait area was of Arctic origin (Fig. 3; t = -0,8 , S>34.92; Swift and Koltermann, 1988; Malmberg et al ., MS 1990; Rudels and Quadfasel, MS 1991; Buch et al ., MS 1992). b) The intermediate water also from the north (West Spitzbergen Current origin) at 200–600 m showed some interannual variability as re- gards the strength of its salinity maximum (Malmberg et al. , MS 1990). Thus, in 1987, 1988 and 1991 salinity in the intermediate layer was above 34.92 ppt in a wider extend, and in gen- eral with higher core values, than in 1989 and 1990 (Fig. 1; Malmberg and Blindheim, 1994). c) The polar water component in 1988 (t<0 , S<34.7) showed by far the strongest input into the Iceland Sea during the period 1987–92 (Fig. 4; Malmberg and Blindheim, 1994). The condi- tions found in the Iceland Sea in 1988 are in agreement with the hydrographic conditions found in the East-Icelandic Current, where tem- perature and salinity were extremely low in 1988 together with severe ice conditions in North Ice- landic waters (Fig. 5; Malmberg and Krist- mannsson, 1992). d) In North Icelandic waters, three different hydrographic regimes have been identified in the water column structure (Malmberg and Kristmannsson, 1992; Malmberg and Blindheim, 1994):