The minimum temperature of the sea just outside the fjord mouth is about 1°C in February but in the beginning of April a warming of the surface layer in the fjord starts. At the surface a thin layer is formed that gets progressively warmer and reaches a temperature of 8°C at the surface. In spring and summer a fresh surface layer forms caused mainly by the river runoff. In autumn and winter this fresh and warm surface layer almost disappears as the water in the fjord mixes down to great depths.

If we move to the inner part of Eyjafjörður to a station just inside of Hjalteyri where the bottom depth is about 90 m a somewhat different picture emerges. The minimum temperature is somewhat lower but the warming of the surface layer is faster than outside the fjord even though the maximum temperature only reaches 9°C. The difference in salinity between the inner part of the fjord and the station outside the fjord is larger, since there is a large freshwater discharge to the fjord. The surface layer becomes much more stratified by the freshwater in the inner part than outside the fjord.

The density of the seawater is determined by three factors, namely temperature, salinity and pressure. It increases with increasing salinity and pressure, but decreases with higher temperatures. In shallow waters, the effect of pressure is negligible. In fjords with considerable freshwater input the density is governed by the salinity. Around Iceland changes in the density are usually more affected by salinity than by temperature. The density of the seawater is critical for various processes that occur in the ocean, such as stratification and mixing, which in turn have a major impact on life in the sea, especially the phytoplankton that need to stay in the sunlit surface layer in order to thrive and multiply. Also, the distribution of density induces currents as lighter water tends to flow on top of heavier water.

Steingrímur Jónsson

Hiti og selta sjávar rétt utan við fjarðarkjaftinn á árunum 1992-1993 (gögn frá Hafrannsóknastofnuninni). Hiti og selta sjávar rétt utan við fjarðarkjaftinn á árunum 1992-1993 (gögn frá Hafrannsóknastofnuninni).

Hiti og selta sjávar í miðjum firðinum rétt sunnan Hjalteyrar á árunum 1992-1993 (gögn frá Hafrannsóknastofnuninni). Hiti og selta sjávar í miðjum firðinum rétt sunnan Hjalteyrar á árunum 1992-1993 (gögn frá Hafrannsóknastofnuninni).