Under Landfast Ice

Abstract

The layer of water under landfast ice has unique oceanographic characteristics, as described in this review of recent assessment information for the central Alaskan Beaufort Sea coast. Water circulation is very slow usually near the lower threshold of current meters. Barometric storms cause infrequent surges of water. The weak thermohaline-driven circulation is the reverse of that in ice-free estuaries. Water temperatures are always close to the slowly declining freezing point, and salinity gradually increases to high levels in bays because of flushing times of a month or more. Biological processes during the dark third of the year when there is no photosynthesis are dependent primarily on detritus and stored energy. Detritus is decomposed slowly by bacteria, and consumed by epibenthic invertebrates. Invertebrates and their main predators, fish, both reproduce under the ice cover. Food may limit biological activity in late winter, even in nearshore areas. Spring under-ice primary production totals possibly one-third of annual production with production of epontic algae attached to the bottom surface of the ice equalling only 5% of annual production. During breakup, river floods quickly flush under-ice areas; nearshore salinity drops to zero, and the wintertime thermohaline circulation is reversed. Year-to-year physical variations in the habitat cause the populations of three resident animals to vary up to sevenfold, but there are no regular cycles in abundance. Knowledge of these under-ice characteristics is important for understanding the Beaufort Sea coastal ecosystems, even for the relatively short open-water period.

Key words: Arctic: Beaufort Sea, Alaska winter, coastal, landfast ice, under-ice, oceanography, food web