The impact of a large lowland temperate zone reservoir on chironomid and fish biodiversity was investigated in the same upstream (U) and downstream (D) sites on the same sampling occasions in spring and autumn of several post-impoundment years. The true/Hill N1 diversity measure was used to construct diversity models of the impact, and partitioning of N1 to reveal the importance of ecological gradients. N1 is unique in being decomposable among multiple levels and easily comparable across ecosystems and animal groups.Chironomids were more diverse than fish in all community sets owing to their probably lower mobility, lower interspecific competition, opportunistic character and much shorter life spans than those of fish. Fish exerted consumption pressure on chironomids upstream by foraging mostly on benthic insects, but not downstream where they fed on microcrustaceans of reservoir origin or on epiphytic fauna (mainly Chironomidae).The reservoir impact on chironomids decreased their diversity in U and increased in D, while much the opposite was true in fish. In D in macroinvertebrates, the impact consisted in advancing over time intensification of seasonal development of submersed macrophytes, while in D in fish in a few species escaping from the reservoir after their populational explosions each year.Partitioning of diversity revealed that the spatial (upstream-downstream) gradient was the strongest factor of diversity change in chironomids as compared with season, and age of the impoundment. No gradient was dominant in fish. Comparisons of observed data with a null model testified to very strong intraspecific aggregation in both chironomids and fish.

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Chironomids (Diptera: Chironomidae) are a family of dipterans with a global distribution. Owing to their great functional diversity and ability to adapt to a wide range of environmental conditions, they often dominate in freshwater macroinvertebrate communities, playing a key role in the cycling of organic matter and the flow of energy in aquatic ecosystems. Our aim was to analyze the structure of chironomid assemblages and identify the environmental factors, including current velocity, river width, water depth, water temperature, dissolved oxygen, percentage of substrate covered by vascular plants, inorganic bottom substrate, and quantity of benthic (BPOM) and transported (TPOM) particulate organic matter, that underpin variation in species richness across a set of lowland rivers in central Poland, differing by stream order and abiotic parameters. Using an Information Theoretic Approach, we formulated a set of alternative models based on previously published work, with models fitted in a Bayesian framework using Integrated Nested Laplace Approximation. The species richness of chironomids increased with river order, achieving a maximum in third and fourth order rivers, but decreased at higher orders. The best-fitting models included a positive effect of inorganic substrate index and dissolved oxygen on chironomid species richness. The quality structure of chironomid assemblages reflected the assumptions of the River Continuum Concept showing that species richness was under the influence of factors operating at both a micro- (inorganic bottom substrate) and macro-scale (dissolved oxygen).

Spatial and temporal distribution, abundance and production of macroinvertebrate communities were estimated over two years in a fifth-order section of the Widawka River . Discharge of this river has been increased artificially by coal mine water inputs. Additionally, during the second year, one of the highest discharges of the current 20-year period was recorded . Chironomidae were co-dominant in macrobenthos, both in a straight reach (WIA) and in a meandering site (WIB) . More mosaic habitats resulted in higher densities of midges, reaching 6215 ind .m -2 in year 1 and 1141 ind .m -2 in year 2 at WIA, while at WIB 896 densities were ind .m-2 and 257 ind .m -2 , respectively . Flooding affected the distribution and abundance of the chironomid assemblages . Recolonization by psammophilous Polypedilum began after the various microhabitats were buried with sand . Chironomid production was estimated on a species-specific basis for the dominant taxa . In year 1 (mean annual water temperature 10 .0° C) chironomid production was 12 .4 g dry wt m -2 yr1 at WIA and 1 .9 g dry wt m -2 yr -1 at WIB . These values sharply decreased in year 2 (mean annual water temperature 9 .8° C) reaching 1 .9 g dry wt m -2 yr 1 at WIA and 0.4 g dry wt m-2 yr -1 at WIB, as effects of the high spate .