Ice cover can considerably influence the food web structure and dynamics of lake ecosystems during winter. The aim of this study was to test the hypothesis that, due to unfavourable conditions, abundances of planktonic organisms are low under ice. The components of the microbial loop (nanoflagellates, ciliates) and the classical food web (algae, rotifers, crustaceans) were investigated in a eutrophic lake from January to April, at 7-day intervals. The phytoplankton was dominated by the mixotrophic chrysophyte Synura uvella, which formed an intense phytoplankton bloom. Nanoflagellates (mostly heterotrophic forms) and ciliates (oligotrichs, prostomatids) were present in relatively high numbers. Rotifers were represented by the algivorous taxa (Notholca squamula, Polyarthra dolichoptera) and reached surprisingly high abundances. There were no significant correlations between physical parameters and protists, but correlations were apparent between the thickness of the ice and the numbers of cyanobacteria, diatoms (negative) and chrysophytes (positive). The absence of crustaceans and relationships between rotifers and both algae and nanoflagellates indicate that these groups were probably a crucial factor determining the abundance and composition of the rotifer community. These results suggest that the microbial loop had a central role in the functioning of the ecosystem under ice in this eutrophic lake.

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We conducted a mesocosm experiment to assess the impacts of large-bodied crustaceans on microbial communities. Three alien crustacean species (Daphnia pulex, Simocephalus vetulus and Macrocyclops albidus) were collected from the regional species pool and added to mesocosms that were filled with water from a eutrophic lake (Masurian Lake District, Poland). We then analysed chemical (total phosphorus and nitrogen concentrations) and biological (algae, bacteria, nanoflagellates, ciliates, rotifers, crustaceans) parameters over the course of the 40 day experiment. Alien crustacean species constituted 59-88% of the total crustacean biomass throughout the experiment. The final biomass of bacteria and copepods were not affected by the addition of alien species. However, rotifer and native cladoceran biomass tended to be lower while nanoflagellate and ciliate biomass were higher in mesocosms with alien species. Our research suggests that the large-bodied crustaceans altered the structure of the microbial loop. In the control, nanoflagellates were likely the main consumers of bacteria and thus constituted the main link between bacteria and higher trophic levels. In the mesocosms with large-bodied crustaceans, protists were likely not important as bacterial grazers because of strong top-down control of nanoflagellates by crustaceans. Combined, our results provide evidence that alien large-bodied crustaceans can significantly impact the microbial loop.

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BackgroundThe bottom-up (food resources) and top-down (grazing pressure) controls, with other environmental parameters (water temperature, pH) are the main factors regulating the abundance and structure of microbial communities in aquatic ecosystems. It is still not definitively decided which of the two control mechanisms is more important. The significance of bottom-up versus top-down controls may alter with lake productivity and season. In oligo- and/or mesotrophic environments, the bottom-up control is mostly important in regulating bacterial abundances, while in eutrophic systems, the top-down control may be more significant.ResultsThe abundance of bacteria, heterotrophic (HNF) and autotrophic (ANF) nanoflagellates and ciliates, as well as bacterial production (BP) and metabolically active cells of bacteria (CTC, NuCC, EST) were studied in eutrophic lakes (Mazurian Lake District, Poland) during spring. The studied lakes were characterized by high nanoflagellate (mean 17.36 ± 8.57 × 103 cells ml-1) and ciliate abundances (mean 59.9 ± 22.4 ind. ml-1) that were higher in the euphotic zone than in the bottom waters, with relatively low bacterial densities (4.76 ± 2.08 × 106 cells ml-1) that were lower in the euphotic zone compared to the profundal zone. Oligotrichida (Rimostrombidium spp.), Prostomatida (Urotricha spp.) and Scuticociliatida (Histiobalantium bodamicum) dominated in the euphotic zone, whereas oligotrichs Tintinnidium sp. and prostomatids Urotricha spp. were most numerous in the bottom waters. Among the staining methods used to examine bacterial cellular metabolic activity, the lowest percentage of active cells was recorded with the CTC (1.5–15.4%) and EST (2.7–14.2%) assay in contrast to the NuCC (28.8–97.3%) method.ConclusionsIn the euphotic zone, the bottom-up factors (TP and DOC concentrations) played a more important role than top-down control (grazing by protists) in regulating bacterial numbers and activity. None of the single analyzed factors controlled bacterial abundance in the bottom waters. The results of this study suggest that both control mechanisms, bottom-up and top-down, simultaneously regulated bacterial community and their activity in the profundal zone of the studied lakes during spring. In both lake water layers, food availability (algae, nanoflagellates) was probably the major factor determining ciliate abundance and their composition. In the bottom waters, both groups of protists appeared to be also influenced by oxygen, temperature, and total phosphorus.

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