Thermocline deepening and mixing alter zooplankton phenology, biomass and body size in a whole‐lake experiment

Abstract

1. Summer thermal stratification is thought to be one of the key structuring physical factors for north temperate lake zooplankton. Shifts associated with climate change may lead to altered thermocline depths in stratified lakes through changes to (i) wind stress and associated water column mixing or (ii) air temperature and precipitation. The effects of thermocline deepening through these two scenarios were simulated in a whole-lake experiment to assess the effects on the phenology, biomass and mean body size of the zooplankton. 2. The thermocline of a three-basin dimictic lake was deepened to 7-8 m in one basin using a lake mixer. Through heat transfer, the thermocline of the adjacent basin was deepened to 6-7 m, while the third basin served as an unmanipulated control (4-5 m deep thermocline). Zooplankton community dynamics were followed weekly in a control year and in two experimental years. A before-after-control-impact (BACI) statistical protocol was used to assess the effects of two treatments: Deepening and Deepening+Mixing. 3. Thermocline depth was the main factor influencing zooplankton changes. The biomass and late-season dominance by smaller-bodied and more fish-evasive species were favoured by deepening, leading to a decline in mean community body size, but an increase in total biomass. 4. While many effects could be attributed to thermocline deepening, additional or exacerbated responses were observed when mixing was also present. This resulted mostly from a loss of the hypolimnetic refuge for zooplankton, which accentuated predation effects by warm-water fish on larger cladocerans. Overall, our treatments promoted top-down (predation) effects that increased over the 2 years of the experiment.

Food Web Responses to Artificial Mixing in a Small Boreal Lake

“…We have earlier shown that when crustacean zooplankton was removed from a humic lake, ciliate numbers rapidly increased in the epilimnion [63]. The changes in the species relations of dominant cladocerans were comparable to those detected in the TIMEX experiment [19].…”

“…Due to mixing there was a loss of hypolimnetic refuge for zooplankton, which in turn accentuated stronger predation by fish on larger cladocerans, and subsequent changes (decline) in mean community body size of zooplankton, and an increase in zooplankton biomass [19].…”

The impacts of changing nutrient load and climate on a deep, eutrophic, monomictic lake

“…Climate also has a major effect on lakes via water temperature and stratification (Hutchinson & Löffler, 1956) and climate change has increasingly been recognised to have a substantial impact on lake structure and function globally (Paerl & Huisman, 2008;Tranvik et al, 2009;Williamson, Saros, Vincent, & Smol, 2009). Long-term incremental increase of air temperature influences the thermal structure (Gauthier, Prairie, & Beisner, 2014;Liu, Bocaniov, Lamb, & Smith, 2014) and timing of stratification in lakes (Izmest'eva et al, 2016;Meis, Thackeray, & Jones, 2009). The shorter-term impact of heatwaves, droughts, and flooding can alter the hydrological balance and ecological structure in many lakes (Bakker & Hilt, 2016;Bertani, Primicerio, & Rossetti, 2016;Wigdahl-Perry et al, 2016).…”

Mean weight and total biomass of zooplankton as a core indicator of biodiversity of the Marine Strategy Framework Directive: an example of the Gulf of Riga

“…Mean zooplankter weight correlated positively with biomass (r = 0.582; p = 0.001) and negatively with abundance (r = -0.546; p = 0.003). Large variation of mean zooplankter weight has been documented also in freshwater systems (Haberman, 1996;Jeppesen et al, 2011;Gauthier et al, 2014).…”