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Infrapopulations of trematode metacercariae were monitored in the snail Lymnaea stagnalis over 17 yr (1982-1999) at Chany Lake, Novosibirskaya Oblast', Russia. Eighteen trematode species were recorded. Patterns of occurrence varied from 4 species (Echinoparyphium aconiatum, Echinoparyphium recurvatum, Moliniella anceps, and Cotylurus cornutus) that persisted at relatively high prevalence (> 60% of samples) across sites, seasons, and years, to species that were very rare and sporadic in occurrence. The stability of the 4 common species was probably because of their occurrence either in a wide range of definitive hosts or in a host adapted to the extreme abiotic changes that occurred from year to year in these wetlands. The prevalence and mean abundance of C. cornutus were negatively correlated with water level in the wetlands; its prevalence was also correlated with water temperature. The mean abundance of M. anceps was positively correlated with water level. The most probable explanation for the cyclic dynamics of infections of the common species is change in population sizes and densities of definitive and intermediate hosts, which mediated cyclic alterations in water levels.

The ionic concentrations,conductivity and pH of water in the Lake Chany complex in West Siberian Russia change from the mouth to the interior of the lake. This difference is indicative of marked evaporation of lake water from the closed water body system in the dry climate of Western Siberia. The carbon isotope composition of particulate organic matter (POM, composed mainly of phytoplankton) clearly changes, along with the pH of the water, reflecting the concentration of dissolved CO 2 . Carbon and nitrogen isotope signatures of Chironomus plumosus larvae, a benthic invertebrate that may feed on bulk lake sediment, systematically increase, along with those of POM and sediment organic matter (SOM), through the lake chain. Both sulfate-sulfur and nitrogen isotope compositions of the POM and SOM increase with distance from the estuary into the Lake Chany complex. Heavier sulfur and nitrogen isotope recycling from the sediment, caused by microbial sulfate reduction and denitrification, respectively, may have led to the increased sulfate-sulfur and nitrogen isotope compositions of the POM and SOM.

In lacustrine ecosystems, benthic grazers are generally thought to feed on sediment detritus and microalgae, although there is a paucity of information on food resource use within populations. In this study, we investigated individual level trophic signatures for grazing snails, Lymnaea stagnalis, along with primary consumers and producers in the same habitat, using carbon and nitrogen stableisotope analyses. In addition, we tested whether ontogeny and parasite presence influence food resource use. The large variation in d 13 C and d 15 N isotopic signatures indicated that individuals within a population feed on different food sources. Snails appear to have much greater individual variance in trophic behaviour than the other lacustrine species sampled (larval chironomid Chironomus plumosus, amphipods, Gammarus lacustris, zooplankton Ceriodaphnia sp. and Simocephalus vetulus). Moreover, variation among snails was not explained by shell length or the presence of parasitic infections. Habitat heterogeneity and resource availability at the microhabitat level may be the primary factors determining individual food sources.

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