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Autotrophic versus heterotrophic production and components of trophic chain in humic lakes: the role of microbial communities.

The ratio and rates of autotro- phic and heterotrophic pathways of organic mat- ter cycles constitute the basic functions of aquatic ecosystem and humic lakes are unique in this respect. The autotrophic and heterotrophic pro- duction, 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 bac- teria, autotrophic picoplankton (APP), nanofla- gellates (NF), ciliates, phytoplankton, rotifer and crustacean zooplankton as well as chlorophyll a and primary ( 14 C method) and bacterial pro- duction ( 3 H-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 car- bon (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 produc- tion. The lake was characterised by the highest crustaceans biomass and very variable chloro- phyll 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 expe- rienced 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 pre- vailing 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 det- rimental 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 acid- ic lakes like: Gonyostomum semen, Dictyosphaeri- um sphagnale from phytoplankton or Holopedium gibberum from crustacean zooplankton. Instead eurytopic species, common in eutrophic waters, were present.