Mud, Scales and Sea Lions

Changes in the abundance of microscopic plants (phytoplankton that form the base of marine food webs) probably play a key role in determining the structure and productivity of the Bering Sea ecosystem. However, current understanding of the timescales over which phytoplankton abundance has varied, and the factors that control it are limited.

Bruce Finney, Alan Springer and Amy Hirons from the University of Alaska Fairbanks have been digging in the mud for the past year to unravel the phytoplankton puzzle and gain insights into what has happened to sea lions at the top of the food chain.

One of the 10-ft sediment cores being examined on land.

The researchers have drawn 10-foot sediment cores from the bottom of Skan Bay on Unalaska Island. The layers of mud they removed contain fish scales and other pieces of organisms that accumulated like falling snow over hundreds to thousands of years. By counting the scales and knowing the age of each layer of sediment, the researchers can re-construct the relative abundance of fish that surrounded this Aleutian Island over the past 800 years.

The high-resolution sediment record is revealing century-to-decadal time scale oscillations in productivity and forage fish abundance. The researchers are now trying to link these changes to Steller sea lions using bones recovered from archaeological sites. When completed, the combination of mud sleuthing and bone crunching may help to put the recent changes observed in sea lion numbers over the past few decades into context with ecosystem level changes that have gone on for hundreds to thousands of years.

3 March 2003