In polar regions, sea ice is critical habitat for many marine birds and mammals. The quality of pack ice habitat depends on the duration and spacing of leads (openings in the ice), which determine access to water and air for diving endotherms, and how often and how far they must move as leads open and close. Recent warming trends have caused major changes in the extent and nature of sea ice at large scales used in climate models. However, no studies have analyzed lead structure in terms of habitat for ice-dependent endotherms, or effects of climate on ice habitat at scales relevant to their daily movements. Based on observations from an icebreaker and synthetic aperture radar (SAR) images, we developed methods to describe the dynamics and thermodynamics of lead structure relative to use by spectacled eiders (Somateria fischeri) wintering in pack ice of the Bering Sea. By correlating lead structure with weather variables, we then used these methods to estimate changes in lead dynamics from 1945 to 2002, and effects of such changes on flight costs of the eiders. For 1991-1992, when images were available about every 3 days throughout winter, SAR images were divided among five weather regimes defined by wind speed, wind direction, and air temperature. Based on 12.5-m pixels, lead shape, compass orientation, and fetch across leads did not differ among the weather regimes. However, the five regimes differed in total area of open water, leads per unit area, and distance between leads. Lead duration was modeled based on air temperature, wind, and fetch. Estimates of mean daily flight time for eiders, based on lead duration and distance between neighboring leads, differed among regimes by 0 to 15 min. Resulting flight costs varied from 0 to 158 kJ day-1, or from 0% to 11% of estimated field metabolic rate. Over 57 winters (1945-2002), variation among years in mean daily flight time was most influenced by the north-south wind component, which determined pack divergence (lead opening) during northerly winds or pack convergence (lead closing) during southerly winds. Mean daily flight time and flight cost during winter did not differ among proposed periods of decadal-scale climate shifts in the North Pacific Ocean. Although leads in mobile pack ice constantly open and close with variations in wind, under most conditions in the Bering Sea there appeared to be a shifting-mosaic steady-state of lead availability. Long-term trends in the extent and timing of Bering Sea pack ice may have affected spectacled eiders more by altering foodweb processes than by changing flight costs relative to lead structure.
Bibliographical noteFunding Information:
Jacqueline M. Grebmeier and Lee W. Cooper provided invaluable assistance, encouragement, knowledge, and friendship throughout this project. We thank Captain Terrence C. Julich and crew of the U.S. Coast Guard Cutter Polar Star , and Captain Gerald M. Davis and crew of the Polar Sea , for excellent logistic support and interest in our work. We also appreciate the outstanding performance of the aviation detachment under Lieutenant Commander Kyle G. Anderson. K.D. Powell, J.C. Hales, and C.A. Katje of the Alaska SAR Facility, Geophysical Institute, University of Alaska, facilitated image acquisition. J.J. Baranyi and N.P. Nibbelink helped design processing macros. R.W. Marrs helped with image processing and provided computing space, and K.G. Gerow aided with image sampling design. We especially thank J.B. Fadely for initially involving us in this effort, and G.R. Balogh for key financial and administrative assistance. This research was supported by National Science Foundation, Office of Polar Programs grant OPP-9813979 to JRL; NASA research credits at the Alaska SAR Facility, Geophysical Institute, University of Alaska to JRL; a contract from the Office of Ecological Services of the U.S. Fish and Wildlife Service in Anchorage, Alaska to JRL; and a Wyoming NASA Space Grant Graduate Research Fellowship to JKB.
- Bering Sea
- Flight energetics
- Ice lead dynamics
- Sea ice
- Spectacled eider
- Synthetic aperture radar