Emergence of steeply stratified permafrost thaw ponds changes zooplankton ecology in subarctic freshwaters

Climate change and associated permafrost thaw are creating new shallow waterbodies in vast regions of the circumpolar Arctic. These thaw ponds are characterized by high concentrations of colored dissolved organic matter originating from the degrading watershed, inducing a strong vertical thermal and oxygen (O₂) stratification. We investigated the zooplankton community and biomass in eight subarctic thaw ponds and evaluated how zooplankton respond to this stratification. In a subset of three ponds, we further examined how other environmental variables, including essential fatty acids (EFA) concentration and phytoplankton, bacteria, and larval phantom midge <i>Chaoborus</i> biomass stratify and contribute to the vertical distribution of zooplankton in this increasingly common type of arctic freshwater system. The zooplankton community was extremely abundant in all ponds (up to 3,548 ind L⁻¹) and dominated mainly by rotifers (35–93 percent of the biomass). Most zooplankton aggregated at the interface between the shallow well-oxygenated mixed surface layer and the deeper hypoxic but algal-rich stratified layer, and their distribution was affected by a combination of O₂, <i>Chaoborus</i>, phytoplankton, and EFA that were supplied from opposite directions. Our findings show how water column stratification deeply affects the ecology of planktonic organisms in circumpolar freshwaters and indicate Arctic zooplankton species composition is expected to deeply change with the ongoing warming and browning.