General Effects of Ice on Birds
Negative Effects
Sea Ice Decreases the Surface Area of Water
The decrease in the surface area of water is the simplest and most immediate effect that sea ice has on birds. Ice acts as a barrier that restricts the availability of food in the water. Surface feeders are the most severely affected since, in general, ice cover of 50% reduces the possible feeding area by half. The effect on diving species is not as severe since, if open water is scattered throughout the ice, diving species still have access to much of the prey in the water column and benthos. When open water is scarce, however, diving species can become concentrated in the available water, resulting in intense competition for available prey. In certain situations the open water is used only as a migratory pathway, but open water is necessary for birds that must roost or feed.
Sea Ice Reduces Primary Productivity in the Water Column
Ice inhibits phytoplankton blooms in the water column, thus decreasing the biological productivity of ice-covered waters. This inhibition occurs in two ways:
• By decreasing light penetration of the water column.—Much of the sunlight reaching the ice is reflected by the ice and by snow on the ice. The amount of light reaching the water depends on the angle of the light, thickness of ice, and amount of snow cover. When the layer of under-ice algae forms, it absorbs light and further reduces the amount of light reaching the water (Bunt 1963). This reduction in light reduces the depth of the euphotic zone.
• By increasing the stability of the water column.—Increased stability of the water column reduces the upwelling of nutrient-rich waters into the euphotic zone. Ice stabilizes the water column primarily by preventing wind-driven movement of surface waters and by forming a layer of meltwater at the surface in the spring and summer (Dunbar 1968).