Interannual and regional variability in winter nutrient concentrations in Icelandic Waters

We present spatial and temporal patterns of nutrients concentrations from February observations from two hydrographically different regions in the high latitude North-Atlantic: The northern Irminger Sea and the Iceland Sea. Our observations are from two time-series stations located off the continental shelf and from stations on the shelf. The Irminger Sea, southwest of Iceland, is primarily in the realm of Atlantic Water derived from the North Atlantic Current. The Iceland Sea is north of the Greenland-Iceland-Faroe Ridge separating the Nordic Seas from the sub-arctic North Atlantic. Both regions undergo strong seasonal variations and winter mixing is induced by strong winds and loss of heat to the atmosphere. The winter mixed layer depth is found to be variable in the Irminger and Iceland Seas, but it reaches much deeper in the Atlantic Water of the Irminger Sea as a halocline limits the vertical convection in the Arctic Waters of the Iceland Sea. Consequently, the winter mixed layer depths in the two regions range from or 300-700 m and 150-250 m respectively. The nutrient concentrations in the surface layer that result from the winter vertical mixing vary interannually and there is a significant spatial difference between the two regions where the long term (1991-2020) average for the nitrate concentration is 14.4 µmol kg^-1 in the Atlantic Water and 10.3 µmol kg-1 in the Arctic Waters. Higher spatial differences are in the silicate concentration, in the Atlantic Water the long-term average concentration is 6.8 µmol kg^-1 and 4.0 µmol kg^-1 in the Arctic Water. The interannual variations in the relative abundance of nitrate and silicate, the Si/N ratio, are likely to influence the productivity of silicious diatoms in the spring bloom. Nutrient concentrations at the end of winter on the shelf are also high, they reflect the state of the offshore waters. Our observations show hydrographic changes that have induced significant biogeochemical changes in these regions.