Sea Ice as a Factor in Seabird Distribution and 

 Ecology in the Beaufort, Chukchi, and Bering Seas 



by 



George J. Divoky 1 



U.S. Fish and Wildlife Service 

 Fairbanks, Alaska 



Abstract 



Arctic sea ice has a variety of effects on seabirds. Although the decrease in sur- 

 face area available for feeding and roosting is probably the major restrictive 

 effect, also important are productivity of water covered by ice and the reduced 

 prey abundance in nearshore areas due to ice scour. The most important benefit 

 that sea ice provides to seabirds is the plankton bloom that occurs in the ice in 

 the spring. In the Beaufort and Chukchi seas this bloom supports an under-ice 

 fauna that is an important food source for seabirds. 



Sea ice is a major factor in the distribution 

 and ecology of many of the birds treated in 

 this symposium. Sea ice is defined here as ice 

 formed by the freezing of seawater and in- 

 cludes both free floating pack ice and the 

 more stable shorefast ice. Since icebergs are 

 composed of ice of land origin, they are not 

 discussed. 



Before discussing the specific relationship 

 of birds and sea ice in the Beaufort, Chukchi, 

 and Bering seas, I list the general effects that 

 arctic ice can have on seabirds. For purposes 

 of discussion these effects can be divided into 

 negative effects, or disadvantages, and posi- 

 tive effects, or advantages. 



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 



'Present address: Point Reyes Bird Observatory, 

 4990 State Route 1, Stinson Beach, California 

 94970. 



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 inhi- 

 bition 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, thick- 

 ness 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 reduc- 

 tion in light reduces the depth of the euphotic 

 zone. 



