Sudden disappearance of the amphipod Diporeia from Eastern Lake Ontario, 1993-1995

Diporeia, Oligochaeta, and Sphaeriidae were sampled at three sites (28 to 35 m depth) in the east basin of Lake Ontario, and at one mid-lake site (125 m) from 1982 and 1996. In addition, Diporeia was sampled at 14 more sites located in the south and eastern part of the lake in 1990 and 1995. Before 1991, densities of all three macroinvertebrate taxa fluctuated with peaks occurring between 1988 and 1991. Between 1992 and 1995, Diporeia declined rapidly from > 6,000 per m(2) to 0 at all three sites in the east basin, while increasing from 1,050 to 5,230 per m(2) at the mid lake site. Samples from the south and eastern part of the lake confirmed the decline was extensive at depths < 100 in. Abundance and biomass of the Oligochaeta and Sphaeriidae also fluctuated, but remained more similar before 1991 and after 1993. Direct cause for the disappearance of Diporeia is unknown but declines occurred after establishment of nearshore Dreissena colonies. Lake circulation patterns suggest that the nearshore mussel populations may intercept diatoms and other material that had been consumed by Diporeia located further offshore. A substantial biomass of Diporeia (3 g/m(2) dry) has been lost from the food web of eastern Lake Ontario, with serious consequences to the coldwater fisheries of the lake.