The ratio and rates of autotrophic and heterotrophic pathways of organic matter cycles constitute the basic functions of aquatic ecosystem and humic lakes are unique in this respect. The autotrophic and heterotrophic production, the food web structure and the role of microbial communities in three humic lakes (area 1.3–9.2 ha) were studied. The abundance of bacteria, autotrophic picoplankton (APP), nanoflagellates (NF), ciliates, phytoplankton, rotifer and crustacean zooplankton as well as chlorophyll a and primary (¹⁴C method) and bacterial production (³H–thymidine method) were measured. The lakes differed in humic matter content, water colour, pH and hydrology. Two lakes were acidic (pH 5.2–4.9) with different dissolved organic carbon (DOC) content: oligo/mesohumic – 7.1 mg C L⁻¹ , and polyhumic lake – 21 mg C L⁻¹. Due to draining of surrounding meadows, the third lake – formerly humic – experienced changes in the hydrological regime together with liming and fertilisation. Despite low DOC, the oligohumic lake resembled a low productive, typically humic, acidic lake with dominating bacterial production. The lake was characterised by the highest crustaceans biomass and very variable chlorophyll a concentration (between 1.5 and 71 mg Chl a m⁻³). The polyhumic lake had the highest mean and maximal chlorophyll a content but the lowest crustacean biomass, and functioned more like a eutrophic lake. The formerly humic lake had lost probably most of its humic features and experienced a eutrophication process that resulted in a food web structure typical of a shallow eutrophic pond-like environment. The mean chlorophyll a concentration there was at the same level as in an oligohumic lake, but the variability was much lower. This lake can be considered as an example of the posthumic lakes abundant in the managed wetland regions. Microbial communities were numerous in both humic lakes, with bacteria prevailing in microbial biomass in the oligo-humic and APP in the polyhumic lake. In the former humic lake the microbial communities, especially APP, seemed to play a lesser role, while the whole planktonic food web was more balanced. The results demonstrated that uncontrolled drainage and reclamation of wetland can be detrimental to biodiversity of small, mid-forest lakes. Although biodiversity in almost all plankton groups was the highest in the posthumic lake but this lake lacked rare species typical of humic acidic lakes like: Gonyostomum semen, Dictyosphaerium sphagnale from phytoplankton or Holopedium gibberum from crustacean zooplankton. Instead eurytopic species, common in eutrophic waters, were present.
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