Marine environment of the eastern and central Aleutian Islands

Carol Ladd,Phyllis J Stabeno,Sigrid A Salo,Calvin W Mordy,George L Hunt

doi:10.1111/j.1365-2419.2005.00373.x

Abstract

AbstractTo examine the marine habitat of the endangered western stock of the Steller's sea lion (Eumetopias jubatus), two interdisciplinary research cruises (June 2001 and May to June 2002) measured water properties in the eastern and central Aleutian Passes. Unimak, Akutan, Amukta, and Seguam Passes were sampled in both years, and three additional passes (Umnak, Samalga, and Tanaga) were sampled in 2002. In the North Pacific (and to a lesser extent in the Bering Sea), a strong front in water properties was observed near Samalga Pass in June of both years, with significantly warmer, fresher, and more nitrate‐poor water east of Samalga Pass than west of the pass. These water properties reflect differences in source waters (Alaska Coastal Current versus Alaskan Stream), mixing depth, and Bering Sea influence. Strong cross‐Aleutian gradients were also observed with warmer, fresher water on the North Pacific side of the archipelago. The nutrient content of the waters flowing through the passes, combined with the effects of mixing within the passes, influences the transport of nutrients into the Bering Sea. As water moves away from the strong mixing of the passes and becomes more stratified, phytoplankton can take advantage of the enhanced nutrient concentrations. Thus, the northern side of the Aleutian Islands (especially in the lee of the islands) appears to be more productive. Combined with evidence of coincident changes in many ecosystem parameters near Samalga Pass, it is hypothesized that Samalga Pass forms a physical and biogeographic boundary between the eastern and central Aleutian marine ecosystems.

Highlights

The Aleutian Islands are formed by the highest peaks of the submerged Aleutian ridge, between which waters flow from the North Pacific Ocean and the Bering Sea (Favorite, 1974)
Marked changes in physical properties of the water observed around Samalga Pass in 2001 and 2002 (Ladd et al, 2005a) showed that the Aleutian Archipelago could be divided into two distinct marine environments that extend over a much larger geographical scale than that determined solely by the local availability of shelf habitat (Springer et al, 1996)
The distributions and abundances of the two dominant seabird species could be partitioned into two regions that corresponded to the marine environments determined by the extent of the Alaska Coastal Current

Summary

Introduction

The Aleutian Islands are formed by the highest peaks of the submerged Aleutian ridge, between which waters flow from the North Pacific Ocean and the Bering Sea (Favorite, 1974). The upper ocean circulation on the North Pacific side of the Aleutian Islands is characterized by the westward flow of the Alaskan Stream and the Alaska Coastal Current, and on the Bering Sea side by the eastward flow of the Aleutian North Slope Current (Favorite, 1974; Reed and Stabeno, 1999; Ladd et al, 2005a; Stabeno et al, 2005) Within the passes, both northward and southward tidal flow occurs (Reed, 1971; Favorite, 1974; Ladd et al, 2005a), but the overall net flow of water through the passes is northward (Reed, 1990; Stabeno et al, 2005).

Methods

Results

Conclusion