Diel and horizontal patterns in the densities of Bythotrephes, Chaoborus, and Leptodora in a Minnesota Reservoir

Abstract

Understanding the simultaneous distributions of three invertebrate planktonic predators—Bythotrephes cederstroemii, Chaoborus punctipennis, and Leptodora kindtii—in one system can help describe food web dynamics and inform management decisions. The population densities of these three species were examined in a tannin-stained lake (Island Lake Reservoir, Duluth, MN, USA) on eight dates in 2020 and 2021. Four shallow to deep (nearshore to offshore) sites, ranging from 2-16 m, were sampled during the day and the same night to target diel movement and nearshore to offshore population density differences across seasons. Night population densities increased on average by 364% (Bythotrephes), 129% (Chaoborus), and 493% (Leptodora) compared to daytime densities. Increases were likely driven by reduced water column net avoidance and nighttime diel vertical migration from a daytime refuge. Differences in nearshore to offshore density were variable, with marginally significant differences in the Bythotrephes distribution during the night (larger nearshore) and significant differences in the Chaoborus distribution during the day (larger offshore). The nearshore to offshore densities at night may be controlled by food and predator avoidance. Contemporary zooplankton sampling practices that focus on daytime, pelagic collections may underestimate invertebrate predator densities, thus misrepresenting the roles of predatory invertebrates in food webs. Further analysis of day-to-night and nearshore-to-offshore distributions of Bythotrephes, Chaoborus, and Leptodora are necessary to explain the extent of these differences and how they may differ between tannin-stained and clear-water ecosystems.