Status of the amphipod Diporeia spp. in Lake Superior, 1994-2000

The amphipod Diporeia spp. is the dominant component of the Great Lakes benthic macroinvertebrate fauna, and plays an important role in the ecosystem. The Great Lakes Water Quality Agreement of 1978 (GLWQA) calls for the use of Diporeia as an indicator of ecological condition, with a goal of 220 to 320/m(2) at depths less than 100 m, and 30-160/m(2) at greater depths. To ascertain the status of Diporeia in Lake Superior, a probability-based survey of 27 sites representing the U.S. nearshore (10 to 110 m) waters of Lake Superior was conducted in 1994, and again in 2000. During 1995 to 1998, ten nearshore non-depositional sites and five sites in major depositional basins in the western half of the lake were revisited yearly to examine variability of Diporeia abundance. In 1994, nearshore Diporeia abundance ranged from 550 to 5500/m(2), and the entire nearshore area met or exceeded the GLWQA ecosystem objective. In 2000, abundance ranged from less than 10 to 2,800/m(2), and 11% of the nearshore area did not meet the GLWQA objective. The area that did not meet the GLWQA objective was located in the eastern half of the lake. Examination of yearly abundance data in the western half of Lake Superior did not reveal a significant trend at nearshore or offshore sites. Although Diporeia abundance in the eastern half of the lake was lower in 2000 than 1994, the severe declines in Diporeia populations that have been observed in the lower Great Lakes are not evident in Lake Superior. Abundances of Diporeia observed in the present study are higher than those reported in the 1970s by a factor of seven.