Abstract
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.
Highlights
The bottom-up and top-down controls, with other environmental parameters are the main factors regulating the abundance and structure of microbial communities in aquatic ecosystems
Concentrations of total phosphorus (TP) in the euphotic zone varied between 37 ± 4 μg P l-1 and 91 ± 3 μg P l-1, while they were between 28 ± 4 and 130 ± 2 μg P l-1 in the bottom waters (Figure 2B)
The analysis revealed that in both water layers, ciliates were primarily associated with chlorophyll and nanoflagellates, while bacteria - with dissolved organic carbon (DOC) and TP
Summary
The 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 existence of specific periods of phytoplankton development, such as spring phytoplankton peak and its collapse, clear-water phase, and summer phytoplankton bloom, are characteristic features of lake ecosystems These three regularly occurring ecological events differ distinctly in many aspects such as water temperature, nutrient availability, quality and quantity of organic matter, abundance and taxonomic composition of phytoplankton and zooplankton, and the importance of two, bottom-up and top-down, control mechanisms in the ecological regulation of planktonic assemblages. Protists, mainly ciliates, can build up to 60% of the total zooplankton biomass [8] and are considered to be major consumers of spring phytoplankton blooms, leading to the clear-water phase [9,10,11]
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