Introduction

The Cascade Project provided a perfect opportunity to study one of the great puzzles of limnology, a puzzle which has occupied numerous aquatic biologists over the past century-and-three-quarters. Hardy (1956) called it ‘the planktonic problem No. 1’ and more recent research has still not succeeded in providing an ultimate explanation that satisfies all cases. The adaptive significance of nocturnal diel vertical migration (DVM), a phenomenon whereby organisms throughout 15 aquatic phyla (Kerfoot, 1985) ascend through the water column around dusk and descend before dawn on a daily basis, is one of limnology's longest-standing enigmas.

Initial research on DVM was not published until almost sixty years after Baron Cuvier (1817) first documented its existence among fresh-water crustaceans. The phenomenon received attention from many of the crowned heads of nineteenth century European biology. August Weismann (1874, 1877), one of Darwin's strongest supporters on the Continent, and Forel (1876) both speculated on the causes of the behavior, as did Thienemann in the next century (1919). Most explanations from the nineteenth century, as well as from the first half of the twentieth, focused on the proximal causes of the behavior. Many abiotic factors were proposed as cues for the initiation of migratory behavior, including diel changes in temperature, pH, light intensity and density (Kikuchi, 1930). It eventually became apparent that no single factor could explain the many behavioral variations exhibited by migrators, including differences in the same species' migratory behavior in lakes near each other, and even substantial differences in migration by the same species in the same body of water (Juday, 1904; Kikuchi, 1930).