Taxonomic composition, spatial distribution and seasonal changes in abundance of ciliates were investigated in hygroarenal (the zone of the beach adjacent to lake littoral and affected by wave action) of the eutrophic Lake Mikołajskie (area 498 ha, max. depth 26 m, Masurian Lake District, Poland). The choosen beach amounted 10.8 × 1.5 m in size and 30 sampling points were distributed according to regular network 60 × 30 cm. Three sampling points were located in each of 10 sampling stations parallel to water line and at an increasing 0, 30, 60 cm distance from it). The small-scale distribution of ciliates was recognized from the set of 30 samples taken once in June 2003 from all stations and sampling points. The seasonal changes of ciliate density were analysed on the set of 41 samples taken from March to November 2004 from the middle sampling points (about 30 cm from water line) in 3 sampling stations. Ciliate community was characterised by relatively high taxonomic richness (56 ciliate taxa) and numbers (up to 2750 ind. cm⁻³ of sand). The numbers of ciliates varied seasonally from 4.6 to 1630 ind. per 1 cm³ of sand, showed three distinct peaks (maximum in summer) and were markedly differentiated among the sampling stations distributed along the 10 m of homogenous beach. Significant correlations between ciliates and the share of largest grain size fraction suggest that grain size of sand may be responsible for the observed spatial heterogeneity of ciliates. Hymenostomes, particularly the omnivorous Pleuronema coronatum Kent, dominated the ciliate assemblages. The statistical analysis showed a significant correlation between ciliate numbers and total phosphorus and total nitrogen concentrations in interstitial water, but there was no correlation with chlorophyll a.
The experiment, with the use of sand deposit from hygroarenal (shore sand beach wetted by lake waves) of the beach of eutrophic Lake Mikołajskie (Masurian Lakeland, Poland), was performed in order to test the hypothesis that shore sand deposit is the bank of resting forms of ciliate, rotifer and crustacean species. The experiment was conducted over a 38 days period in March-April 2004. Frozen sand taken in winter was exposed in aquariums filled with pre-filtered (GF/C) lake water in stable temperature (20ºC), oxygen saturation and 12:12 light/dark conditions. Samples (three replicates) were taken each day. A total number of 44 ciliate, 59 rotifer and 9 crustacean taxa were identified during the studied period. Resting forms of different species and/ or taxonomic groups developed in different time and it may be the result of different strategies in colonization of new habitats. Organisms forming resting cysts such as ciliates (11 species) and bdelloid rotifers appeared after the first day of the incubation whereas organisms hatching from resting eggs (monogonont rotifers and crustacea) were observed from the 2nd-3rd day of the experiment. The numbers of all the studied groups of organisms increased gradually and then strongly decreased indicating probably nutrient and/or organic matter limitation. The highest numbers of ciliates (731 ind. cm⁻² of sand) was found on the 5th day, rotifers (987 ind. 100 cm⁻²) on the 23rd day, cladocerans (60 ind. 100 cm⁻²) on the 21st day and copepods (30 ind. 100 cm⁻²) on the 33rd day of the experiment. The results of this study suggests that shore sand deposits being the temporal refugium for the small-bodied invertebrates transported here with the wave action may also play an important role as the transfer for their further dispersal in addition to postulated ways of expansions such as wind, rain, animals and surface runoff.
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