It's like night and day: Diel net-effects on Cercopagidae densities in the Laurentian Great Lakes

Abstract

In the Laurentian Great Lakes, zooplankters are often sampled using standard ≤ 153 µm mesh nets without regard to the time of day they are collected. We sampled Cercopagidae during 2013-2014 in northern Lake Huron during day, dusk, and night using two different nets (a 0.5 m wide 153 µm mesh "standard" net and a 0.75 m wide 285 µm mesh "Bythotrephes" net) to determine if there were any differences in their sampled densities. Bythotrephes densities with the standard net were approximately 2.07-fold greater when captured at night than during the day. No time of day bias occurred with the Bythotrephes net. Nighttime Bythotrephes densities did not differ between the two net types. Cercopagis densities did not vary with net type or the time of day in this study, but future work should revisit this result given our low sample size and the low occurrence of Cercopagis in Lake Huron. To reduce bias and calculate accurate density estimates, Cercopagidae should be sampled at night if using a standard net or any time of day with the Bythotrephes net. Given the large impact of invasive predatory cladocerans Bythotrephes longimanus and Cercopagis pengoi on food webs since their invasion in the Laurentian Great Lakes in the 1980s, proper estimation of their densities is essential.

Should we be sampling zooplankton at night?

“…Such an effect was evident in the predatory cladoceran Bythotrephes (Fig. ; Armenio et al ). However, night–day differences were consistent in groups that are not as strong swimmers as predatory cladocera, and thus net avoidance cannot explain our results.…”

“…Zooplankton behavior and sampling design and methods influence demographic estimates. For example, water column density of Bythotrephes longimanus in Lake Huron was ×2 higher at night than day with a 153-μm mesh net, but no different using a 285-μm mesh (Armenio et al 2017). Daytime sampling limited to surface layers can also lead to biased population estimates due to diel vertical migration (DVM) (Zaret and Suffern 1976).…”

Effects of Environmental Gradients on the Biomass of Alien Cercopagis pengoi in the Neva Estuary

“…We obtained a reliable positive relationship between the biomass of this crustacean and the water transparency and negative relationship with Chl concentration (Table 4). This may be explained by the fact that this predator sees worse in turbid eutrophic waters than, for example, the other planktonic predators Bythotrephes spp., because its compound eye is smaller (Armenio et al, 2017). This structure of the eyes prevents him from hunting in turbid waters.…”