Fishpond sediments -: the source of palaeoecological information and algal "seed banks"

Sediment samples were taken for the first reconstruction of the eutrophication process in a man-managed fishpond using diatom frustules. Samples were also used for culture experiments in order to make good the lack of information about surviving and/or overwintering of most freshwater cyanobacteria and algae and to assess the significance of this ability in population recurrence. The extraction of a sediment core took place in January 2003, i.e. 43 years after the last restoration by sediment removal. A slow increase of eutrophication is shown by an increasing nitrogen and phosphorus content in the sediment along with changes in the diatom assemblage, especially marked in the shift from attached (Amphora copulata) to planktonic species (Stephanodiscus) tolerating high trophic level. Diatoms (Cyclotella) were observed among the first algae growing in culture experiments. The most frequent green algae encountered were species of Scenedesmus. Flagellates (Chlamydomonas) were observed in the upper layers of sediment. Cyanobacteria were represented by Nostoc and Anabaena. All germinated diatom taxa were also found in the palaeoecological part of the study, but they were not necessarily the most abundant species in the core. Thus the ability to form viable resting stages and/or to grow quickly can have a similar significance for the return of the population. The viable resting stages in the bottom sediment represent a "seed bank" for the potential return of various algal species, depending on the character of sediment disturbances. Superficial disturbances (wind, water circulation, macrozoobenthos) offer a chance for growth from resting stages of more recent populations, but profound disturbances (fish, man - restoration by sediment removal) can cause the rejuvenation of species from deep sediment layers.