Functional responses of zooplankton communities to depth, trophic status, and ion content in mine pit lakes

Functional traits can be used to identify various ecosystem processes that are influenced by natural and anthropogenic factors. We tested the hypothesis that the species composition and functional feeding traits of zooplankton are influenced by environmental gradients, such as depth, trophic status, and ion content. This hypothesis was tested in three lignite strip mine lakes in Central Poland. Two shallow, polymictic lakes were reclaimed (1994 and 2008), and the third deep, stratified lake has been reclaimed during the flooding process, since 2011. Samples were collected monthly in 2016 and 2017. The functional responses of zooplankton communities to environmental variables were tested through redundancy analyses. Both eutrophy and oligotrophy contributed to the taxonomic heterogeneity and functional homogeneity of zooplankton. In eutrophic condition small microphagous rotifers and small-sized crustaceans dominated, while oligotrophy and high ion concentrations favored dominance of large microphagous and stationary/suspended feeders. In turn, the depth gradient of a newly emerging, stratified lake resulted in taxonomic and functional heterogeneity of zooplankton. Our results complement existing knowledge about zooplankton in post-industrial areas, and also provide new information on the mechanisms of formation and functioning of mine pit lake ecosystems.

Contribution of the deep chlorophyll maximum to primary production, phytoplankton assemblages and diversity in a small stratified lake

This 6-month study characterized the contribution of deep chlorophyll maximum (DCM) to lake phytoplankton diversity and primary production, in relation to stratification during the ice-free season. Phytoplankton and zooplankton dynamics were examined with environmental drivers in a small stratified lake that presents vertical gradients of light and nutrients. The phytoplankton, first composed of diatoms and chrysophyceae, shifted to cyanobacteria in mid-July. With stratification increase, surface nutrient limitation appeared to favor motile species characteristic of oligotrophic environments above a deep layer of filamentous cyanobacteria, fueled by the vertical nutrient fluxes from sediment. The DCM contributed on average to 33% (but up to 60%) of total production during the strongest summer stratification period. In late summer, as stratification was eroding, the vertical gradient of nutrients was reduced, but light attenuation with depth increased. Distinct assemblages were identified between surface and deep layer with shade-adapted species. The contribution of DCM was reduced to 10%. Zooplankton community varied in conjunction with phytoplankton and stratification. Our study demonstrates no benefit of DCM for taxonomic and functional diversity and a limited contribution to total production. The depths over which phytoplankton use separate spatial niches may be lesser in a 6-m-deep lake compared with deeper stratified lakes.