Emergence of Midges (Chironomidae, Diptera) from a Saline Lake as a Source of Organic Matter and Essential Biochemical Compounds for Arid Ecosystems in South Siberia

Midges (Chironomidae, Diptera) are dominant amphibiotic insects in zoobenthos of the large saline Lake Shira located in the arid landscape of South Siberia. During two vegetation seasons we measured the dynamics of chironomid emergence from littoral parts of four stations in the lake. During one of the vegetation seasons we also measured chironomid emergence from profundal parts (depth of 6 m) of the same stations using original floating traps. Both seasonal dynamics and daily means of emergence values significantly varied among the studied stations and depths and were related to the bottom sediment type. The annual export of wet biomass (calculated as the sum of daily values for a vegetation season) was equal to 0.61 g per square meter of water surface and included proportional contributions of the areas with various bottom sediment types. Based on the values of benthic secondary production, we estimated additional annual export of chironomid biomass from the profundal part of the lake of approximately 1 g of wet weight · m−2. Besides, the total export of polyunsaturated fatty acids (PUFA) calculated using the previous data on PUFA content in the studied chironomid taxa (~ 2 mg · m−2 year−1) appeared to be one of the lowest when compared to estimations for chironomids and some other amphibiotic taxa from similar steppe landscapes

Density-dependent effects of omnivorous stream crayfish on benthic trophic dynamics

Crayfish are abundant and important consumers in aquatic food webs and crayfish invasions have demonstrated strong effects of crayfish on multiple trophic levels. Density may be an important factor determining the role of omnivorous crayfish in benthic communities, especially if density alters the strength of trophic interactions. The effect of crayfish density on a simple benthic food web using ceramic tiles was examined in three treatments (crayfish exclusion cage, cage control (open to crayfish), and exposed ceramic tiles) in mesocosms stocked with 6, 12, or 18 crayfish·m-2. We hypothesized that at low densities crayfish consumption of herbivorous chironomids would increase algal abundance, but at high densities crayfish would reduce both periphyton and invertebrates. In the experiment, periphyton and chironomid abundance increased with declining crayfish biomass on day 30 but not day 15. The magnitude of crayfish effects on day 15 periphyton chlorophyll a abundance increased with crayfish biomass, but crayfish effects on day 30 periphyton chlorophyll a or chironomid biomass did not increase with crayfish biomass. In this experiment there was little evidence for a trophic cascade at low crayfish densities and strong omnivory by crayfish dominated trophic dynamics.

Effect of Prey Density and Prey Size on Growth and Survival of Juvenile Walleye (Stizostedion vitreum vitreum)

I investigated the influence of food availability on growth and survival of walleye (Stizostedion vitreum vitreum) fry stocked in six fertilized experimental ponds (density 40 fish∙m−3) and reared for 8 wk. Walleye fed largely on chironomid larvae and cyclopoid copepods in weeks 1 and 2, and chironomids thereafter. Prey choice and consumption were strongly influenced by chironomid biomass in the benthos, and to a lesser extent by zoo-plankton density. Weekly length increase of the populations and mean length in week 7 were significantly correlated with chironomid benthic biomass and mean prey length. Chironomid biomass and density of large zoo-plankton together explained 56% of the variation in the weekly population growth rate. Despite obvious food limitation in the second half of the experiment, pond survival rate was not significantly correlated with mean prey density, mean stomach fullness, or percentage of fish with empty stomachs. The results indicate that juvenile walleye growth can be regulated by the density and size of available prey. Prey availability apparently does not regulate short term juvenile walleye survival rates after the period around first feeding.