Taxonomic and functional differences between winter and summer crustacean zooplankton communities in lakes across a trophic gradient

Abstract

Despite increasing interest in winter limnology, few studies have examined under-ice zooplankton communities and the factors shaping them in different types of temperate lakes. To better understand drivers of zooplankton community structure in winter and summer, we sampled 13 lakes across a large trophic status gradient for crustacean zooplankton abundance, taxonomic and functional community composition and C/N stable isotopes. Average winter zooplankton densities were one-third of summer densities across the study lakes. Proportionally, cladocerans were more abundant in summer than winter, with the opposite pattern for calanoids and cyclopoids. In green (eutrophic) lakes, zooplankton densities were higher under the ice than in brown (dystrophic) and blue (oligotrophic) lakes, suggesting better conditions for zooplankton in productive lakes during winter. Overall, zooplankton communities were more similar across lakes under the ice than during the open water season. Feeding group classification showed a decrease in herbivore abundance and an increase in predators from summer to winter. C/N stable isotope results suggested higher lipid content in overwintering zooplankton and potentially increased reliance on the microbial loop by winter zooplankton. Our results show substantial variation in the seasonality of zooplankton communities in different lake types and identify some of the factors responsible for this variation.

Opening the black box of winter: Full‐year dynamics of crustacean zooplankton along a nearshore depth gradient in a large lake

“…Along the depth gradient (Duluth Harbor, Chequamegon Bay, Duluth Entry, Madeline Island, McQuade Harbor), summer–winter densities varied 329‐, 53‐, 8‐, 10‐, and 33‐fold, respectively. The lower zooplankton densities we observed in winter agree with other studies (Hampton et al 2017; Kalinowska et al 2019; Kalinowska and Karpowicz 2020, Shchapov et al 2021) where winter zooplankton were reported to be 1.2‐ to 38‐fold lower compared to summer values. The reason for this large variability of summer‐to‐winter differences in zooplankton densities across systems is an open question, but our results suggest that shallow and productive systems that warm rapidly after ice‐off may display larger summer‐to‐winter variations than deeper and more oligotrophic ones.…”

“…Deeper sites (especially Duluth Entry and Madeline Island) also had less seasonal variation in seston concentrations than shallow stations. This suggests that more productive and shallow regions of large lakes can maintain relatively high phytoplankton concentration during winter, an observation supported from studies of smaller lakes across productivity gradients (Hampton et al 2017; Shchapov et al 2021).…”

“…Egg‐ and embryo‐carrying cladocerans were absent in all deeper stations during winter but were present in low densities in shallower Duluth Harbor and Chequamegon Bay stations. Generally, cladocerans (especially gravid cladocerans) are absent during winter due to cold temperatures but have been reported in systems with high phytoplankton biomass (Slusarczyk 2009; Shchapov et al 2021).…”

“…Light measurements were used to calculate the euphotic depth ( Z eu ) for each station by accounting for water column, ice (combined with snow) k d values (Table S1). Z eu was calculated as the depth at which mean daily PAR equals 1 μ mol m −2 s −1 according to Silsbe et al (2016) and Shchapov et al (2021) by using water column, ice, and snow k d values in combination with MODIS‐Aqua satellite data on daily PAR at the earth surface for our study locations (https://earthdata.nasa.gov).…”

“…Depth‐integrated values for environmental parameters (Chl a , DOC, POC, PON, and TP) were calculated by using trapezoidal integration (Shchapov et al 2021). Seasonal changes in zooplankton communities were visualized by nonmetric multidimensional scaling (NMDS) using the R vegan package (Oksanen et al 2018), on forth‐root transformed zooplankton abundance.…”