Diel Vertical Migration of Bythotrephes Longimanus and Daphnia in Eutrophic Lake Mendota

Noah Nicol with Dr. Jake Vander Zanden, Michael J. Spear, Tommy Shannon, Michaela Kromrey, and Marco Scarasso

UW Department of Limnology

Invasive species are organisms that are transported to ecosystems, become established, and disrupt the natural processes of that ecosystem. This can reduce the ecosystem services of the region and result in great economic costs. Aquatic invasive species are of particular interest to the Great Lakes region because of the potent vector of transatlantic shipping.  One of these alien species, the spiny water flea (Bythotrephes longimanus), is estimated to have taken an economic toll of US$140 million (Walsh, 2016). To help determine the impact of B. longimanus on their preferred native prey, Daphnia, the densities of the two species were found at various depths in eutrophic Lake Mendota over a 24 hour period. It was hypothesized that B. longimanus densities would be greater at the surface at night, and would be greater at the bottom during the day.  However, its densities did not seem to correlate with depth or time of day. Conversely, the daphnia population was expected to be denser near the surface at night, and denser near the bottom during the day, because of Daphnia’s historically known diel vertical migration pattern (W. LAMPERT, 1989).  However, we saw the opposite of this relationship in our data.  Daphnia were denser at the surface during the day, and denser at the bottom at night.  While the Daphnia distributions were the inverse of what we expected, the data does not support their density distribution being driven by B. longimanus predation.