Metalimnion as an ecotone in stratified freshwater lakes: A case study on benthic testate amoebae in Lake Valdayskoe

Abstract Testate amoebae are widely used as bioindicators in the study of modern ecotopes and as proxies in palaeoecological reconstructions of lacustrine environments. The metalimnion with thermocline in stratified water bodies strongly affects the species structure of a number of aquatic animals and often plays a role as an ecotone for pelagic animals, but our knowledge on the influence of the metalinmion on benthic organisms, especially on testate amoebae, is still limited. This study aims to describe the influence of the metalimnion and its thermocline on benthic testate amoebae assemblages along a temperature gradient in the stratified freshwater Lake Valdayskoe. Twenty‐eight samples from nine sites were collected along the transect starting in the littoral zone, passing through a long metalimnion and its thermocline and finishing in the hypolimnion on the bottom slope. We identified four species assemblages, six non‐attributed samples and five morphotrait compositions in the transect, including those corresponding to the area inside the metalimnetic zone. We observed similarities in the transitions in benthic species assemblages, morphotype compositions, and species and morphotrait structures from shallow to deep water, and two shifts corresponded well with the upper and lower limnological boundaries of the metalimnion. The combination of the observed biological and hydrological characteristics suggests that the metalimnion plays a role of an ecotone between shallow and deep water in stratified freshwater lakes, and maintains the integrity and sustainability of benthic communities of testate amoebae. We explored a multivariate approach to demonstrate that the combination of highly abundant species and dominant morphotypes may be an efficient indicator of related environmental conditions. Thus, the high abundances of Cylindrifflugia acuminata , Centropyxis aculeata, Difflugia lacustris, D. linearis and C. discoides in combination with the dominance of elongate‐spiky and flattened‐spiky shells are closely related to the littoral zone with sandy sediments covered by many macrophytes. The combination of a high abundance of D. petricola and D. penardi, and the dominance of pyriform, necked and oval shells are closely related to deep depth. The studied species and morphotrait transition from littoral to deep water provides additional information about the morphological adaptations of testaceans, and about the processes of forming and maintaining benthic testate amoebae assemblages along the depth gradient. The described species and morphotrait assemblages related to specific water depths may be useful for palaeoreconstructions.