Chironomid Community Structure Research Articles

Chironomid (Insecta: Chironomidae) community structure response to hydrological changes in the mid‐1950s in lake Nam Co, Tibetan Plateau

ABSTRACTThe recent rise in air temperatures detected at high altitudes of the Tibetan Plateau has accelerated glacier melt and retreat. Moreover, enhanced monsoonal precipitation has increased runoff and transport of allochthonous material to the lakes. Consequently, water levels are rising, modifying the spatial distribution and composition of local aquatic biota. To infer these environmental and biological changes in recent decades, a 30‐cm‐long sediment core, representing the past ~160 years, from Nam Co, an endorheic lake, was analyzed for subfossil chironomid assemblages and sediment geochemistry. In total, 25 chironomid morphotypes were identified. Nineteen were considered as non‐rare taxa (abundances ≥2%) and six as rare taxa (abundances <2%). Since 1956 ce, higher chironomid richness (S = 19) is evident compared to the previous 100 years. The simultaneous decrease in the abundance of profundal Micropsectra radialis‐type and increase of both Chironomus and Procladius, taxa adapted to more eurytopic and slightly warmer water bodies, indicate increasing water temperatures and intensified primary productivity. The dominance of littoral chironomid assemblages reflects increasing lake water levels, flooded shorelines and expansion of littoral areas driven by increased precipitation and glacial meltwater input both resulting from the increase in air temperatures. This scenario is confirmed by increases in total nitrogen and Zr/Rb ratios, indicating higher productivity and coarser grain size as a consequence of increased runoff via the Niya Qu. These hydrological changes have resulted in a positive water balance that can be linked to an increase in moisture supply from the Indian summer monsoon and glacier melt, reflecting increasing temperatures and precipitation since 1956 ce, ultimately driven by anthropogenic warming.

Stability of chironomid community structure during historic climatic and environmental change in subarctic Alaska

AbstractBy understanding lake ecosystem resilience in the face of increasing environmental and anthropogenic stress, we can hope to anticipate future ecosystem instability. We assess recent historic ecosystem resilience using composition and network analyses of empirical zoobenthos chironomid (Diptera: Chironomidae; nonbiting midges) reconstructions from three Subarctic Alaskan lakes, spanning the last c. 200 yr. We measured community richness, turnover and structure using taxon richness, beta diversity, and network skewness, respectively. Simulated taxonomic networks were created to establish the sensitivity of these metrics to changes in taxon connectivity, and to inform the interpretation of empirical chironomid records. The models indicated that beta diversity was more sensitive to taxon loss, while skewness was more sensitive to taxon gain. Both beta diversity and skewness were required to understand structural change under taxon replacement. The simulated arrival of strongly connected taxa caused a greater decrease in skewness than the arrival of weakly connected taxa. The empirical data sets indicated a rise in taxon richness (measured as rarefaction) and beta diversity in the recent samples. Changes in chironomid composition were associated with climate warming (replacement of cold taxa with temperate taxa) and increased lake biological productivity (the arrival of macrophyte‐associated taxa). Skewness was predominantly negative across the lakes, indicating high taxon connectivity and structural stress. However, little directional change in the skewness trends suggests some resilience within the chironomid community structures in relation to the current levels of climate and environmental stress. Continued climatic warming, and associated rises in nutrient levels, may cause further structural stress and ecological degradation.

Late Quaternary chironomid community structure shaped by rate and magnitude of climate change

ABSTRACTMuch is known about how climate change impacts ecosystem richness and turnover, but we have less understanding of its influence on ecosystem structures. Here, we use ecological metrics (beta diversity, compositional disorder and network skewness) to quantify the community structural responses of temperature‐sensitive chironomids (Diptera: Chironomidae) during the Late Glacial (14 700–11 700 cal a bp) and Holocene (11 700 cal a bp to present). Analyses demonstrate high turnover (beta diversity) of chironomid composition across both epochs; however, structural metrics stayed relatively intact. Compositional disorder and skewness show greatest structural change in the Younger Dryas, following the rapid, high‐magnitude climate change at the Bølling–Allerød to Younger Dryas transition. There were fewer climate‐related structural changes across the early to mid–late Holocene, where climate change was more gradual and lower in magnitude. The reduced impact on structural metrics could be due to greater functional resilience provided by the wider chironomid community, or to the replacement of same functional‐type taxa in the network structure. These results provide insight into how future rapid climate change may alter chironomid communities and could suggest that while turnover may remain high under a rapidly warming climate, community structural dynamics retain some resilience.

Ecosystem Responses to Climate-Related Changes in a Mediterranean Alpine Environment Over the Last ~ 180 Years

The effect of recent climatic warming is significant in the Mediterranean region, especially in high-mountain areas. This study uses multiple sedimentary proxies from Rio Seco Lake, a remote alpine lake in the Sierra Nevada, southeastern Spain, to reconstruct recent environmental and ecological changes in the lake and catchment. Two main climatic periods can be distinguished during the past 180 years: Period One (1820 to ~ 1920s) characterized by colder and wetter conditions than the more recent Period Two (~ 1920s to the present), characterized by warmer and drier conditions. Independent proxies such as subfossil chironomid assemblages, n-alkane indices, pollen data and/or spectrally inferred chlorophyll-a concentrations indicate a longer ice-cover period, colder water temperature and more pronounced accumulation of snow in the catchment during Period One than in Period Two, likely producing water stress for catchment plant growth because of the low rate of ice melting in Period One. As temperature increases and precipitation decreases from the 1920s onwards, a wider development of wetland plants is observed, which is associated with the longer warm season that contributed to snow and ice melting in the catchment. This continuing temperature rise and precipitation decrease over the past 60-years by ~ 0.24°C per decade and –0.92 mm/y, respectively, lead to an important increase in chlorophyll-a and changes in lake biotic assemblages. Major chironomid community structure changes to warmer water taxa were recorded, resulting in a 2°C increase in mean July air temperature inferred by chironomids from ~ 1950 onwards. An inferred increase in primary production for the past few decades is consistent with higher temperatures, while wider development of wetland plants is associated with longer warm season. The coherence between independent environmental proxies, each associated with distinct mechanistic linkages to climatic shifts, strengthens our interpretations of a recent warming trend and an intensification of summer drought in this high-mountain area leading to distinct changes in the lake and its catchment. The impact of this climate change on the summits of Sierra Nevada and its influence transcends its geographical limits because these systems provide ecosystem services to a vast area.

The response of chironomid taxonomy- and functional trait-based metrics to fish farm effluent pollution in lotic systems

The lotic habitats affected by trout farm waste are colonized with a particular invertebrate community of which chironomids are the most abundant group. However, there is little information available regarding how chironomid community structures respond to this type of pollution at the highest taxonomic resolution. Eight fish farms, together with their lotic systems as recipients, were used to test the variability of the chironomid community and its surrogates (taxonomic and functional metrics) across spatially arranged sampling sites to form a gradual decrease in the trout farm influence. The self organizing map (SOM) classified six different types of chironomid communities which were characteristic for both the control and affected habitats. The species indicator analyses listed 32 taxa as positive indicators of water pollution. The SOM and Kruskal-Wallis test revealed that the pattern of chironomid community structure obtained was mainly driven by six environmental parameters (Altitude, conductivity, distance from the outlet, hardness, HN4-N, NO3-N). Categorical principal components analysis (CATPCA) derived three models for each type of biotic metric, in which for diversity-, taxonomy- and functional feeding group-based metrics, the first two dimensions explained 55.2%, 58.3% and 55.4%, of the total variance respectively for 315 sampling sites. According to this analysis, the total number of taxa (S), abundance and the Shannon-Wiener index (H′) (as a diversity metric), as well as the proportion of Tanypodinae (as taxonomic group) and grazers/scraper (GRA) and gatherer collector (GAT)(as FFG metrics), were related to the outlet distance gradient, thus showing great potential to be used in the multimetric approach in bioassessment.

The potential of chironomid larvae-based metrics in the bioassessment of non-wadeable rivers

The chironomid community in non-wadeable lotic systems was tested as a source of information in the construction of biological metrics which could be used into the bioassessment protocols of large rivers. In order to achieve this, we simultaneously patterned the chironomid community structure and environmental factors along the catchment of the Danube and Sava River. The Self organizing map (SOM) recognized and visualized three different structural types of chironomid community for different environmental properties, described by means of 7 significant abiotic factors (a multi-stressor gradient). Indicator species analysis revealed that the chironomid taxa most responsible for structural changes significantly varied in their abundance and frequency along the established environmental gradients. Out of 40 biological metrics based on the chironomid community, the multilayer perceptron (MLP), an supervised type of artificial neural network, derived 5 models in which the abundance of Paratrichocladius rufiventis, Orthocladiinae, Cricotopus spp., Cricotopus triannulatus agg. and Cricotopus/Orthocladius ratio achieved a significant relationship (the r Pearson's linear correlation coefficient>0.7) with the multi stressor environment. The sensitivity analysis “partial derivatives” (PaD) method showed that all 5 biological metrics within the multi-stressor gradient were mostly influenced by dissolved organic carbon (DOC). Despite short and monotonous environmental gradients and the absence of reference conditions, the chironomid community structure and biological metrics predictably changed along the multistress range, showing a great potential for the bioassessment of large rivers.

Spring type influences invertebrate communities at cold spring sources

The invertebrate fauna from 22 lava spring sources throughout Iceland’s volcanically active zone was studied to investigate the role of spring type and environmental variables in shaping invertebrate communities. Springs were selected based on the ability to identify a discrete source, which was discharging water that appeared to be geothermally unaffected. Spring type, temperature, and pH significantly influenced invertebrate community structure. When looking only at chironomid community structure, the same variables were significant. Limnocrene spring sources were characterized by greater abundances of crustaceans, especially cladocerans. Rheocrene sources were characterized primarily by lotic taxa, such as Diamesa spp. Temperature was most strongly related to four species of Diamesa in the study, with the Diamesa being associated with the coldest sites. We conclude that associated habitat (i.e., spring type) structures invertebrate communities at spring sources, which are discrete habitats sensitive to change.

Artificial neural networks as an indicator search engine: The visualization of natural and man-caused taxa variability

One of the main challenges in selecting suitable biological indicators of environmental degradation is to recognize the stressor-specific response signal and to separate it from the natural background variability, which can be accomplished by setting an appropriate statistical design, with an output that enables understanding of the recorded indicator signal. In this study we used artificial neural networks (self organizing map (SOM) and geo-self-organizing map (Geo-SOM)) to model and visualize the variability in the chironomid community of the Danube basin, as a model for large non-wadeable rivers. Geo-SOM analysis visualized the longitudinal distribution of significant parameters defining different spatial-distributional types of anthropogenic disturbance. Chironomidae larvae, sampled in both shallow (river bank) and deep (middle) parts of the river, emphasized hydromorphological degradation and zinc as the most important stressing factors, with chlorophyll-a and suspended solids as accompanying variables influencing the community structure. Substrate specificity was shown to be a relevant factor influencing the variability within chironomid community structure bound to natural causes. Geo-SOM analysis also visualized the longitudinal distribution of chironomid taxa, following the distribution patterns of significant disturbance factors. The Kruskal–Wallis test validated 25 potential indicators for the shore area and 11 for the deep water area, which significantly changed their frequencies and abundances between classes with different extents of degradation. Due to its high taxonomical and ecological diversity, the Chironomidae family is a significant source of potential stress-specific indicators, which should be recognized and included in the future in relevant bioassessment methods. The artificial neural network could be a powerful tool for selecting reliable indicators to explain the variability found in the ecosystem and enable it to be specified and patterned together with environmental degradation.

Microhabitat influence on chironomid community structure and stable isotope signatures in West Greenland lakes

Most functional feeding types are represented within the species rich group of aquatic chironomids. Thus, we hypothesized that different lake types and microhabitats within lakes would (1) host specific chironomid communities and (2) that the individual communities would show specific δ 13C stable isotope signatures reflecting the prevailing origin of food source. To test our hypotheses, five lakes in southwest Greenland were investigated at a high taxonomic resolution and with detailed information on δ 13C signature of the chironomids and of individual microhabitats (macrophytes, sediment, stones, and profundal). We found that there was a significant difference in δ 13C between the chironomid assemblages of freshwater lakes and oligosaline lakes, while assemblages of the littoral microhabitats did not differ significantly. The δ 13C of chironomids reflected the wide variety of habitat signals, particularly in the freshwater lakes. Our results indicate that many chironomid taxa are ubiquitous and are found in several microhabitats, suggesting that they can adjust their feeding strategy according to the habitat. The implication is that chironomid assemblage composition has only limited use as indicator of littoral microhabitats in the Arctic. On the other hand, the δ 13C signature of fossil chironomids might have a potential as indicator of microhabitats in freshwater lakes.

Different aggregation approaches in the chironomid community and the threshold of acceptable information loss

Due to the problem of identification, Chironomidae larvae, although very abundant, are often avoided or not properly used in bioassessment programs. The aim of this work was to test how different aggregation processes—taxonomic resolution and the random aggregation approach (best practicable aggregation of species—BestAgg) affect the analysis of chironomid communities regarding any information loss. The self-organizing map method, together with classification strength analysis and Spearman’s rank correlation, revealed that the genus-level and BestAgg-abundance matrix most accurately approximated the species-level community pattern. The subfamily-level dataset was ineffective at presenting the chironomid community structure, with a substantially lower concordance with the species-level dataset. The biologic environmental gradients analyses presented the same set of important environmental variables for the species-level, genus-level, and BestAgg-abundance matrix. The indicator values analysis showed that indicator genera provide information very close to that gained from species indicators. According to our results, the numeric relationship between species and higher taxa influences taxonomic scaling, limiting Chironomidae family aggregation, with acceptable information loss only up to genus level. In addition, the BestAgg approach, with the maximum level of aggregation, properly assesses the community structure and consequently describes environmental conditions.

Chironomidae (Insecta: Diptera) of different habitats and microhabitats of the Vacacai-Mirim River microbasin, Southern Brazil

The Chironomidae family is one of the main groups of aquatic insects present in streams. This work aimed to investigate the influences of anthropogenic pressures and substrate types on chironomid community composition and structure. Chironomid larvae were collected during the summer of 2007 at four sites along the Vacacaí-Mirim River microbasin, chosen based on the different available benthic substrates. The organisms were identified in the laboratory at the genus level, and the density, the rarefied taxonomic richness and the Shannon diversity index were calculated. The faunal structure was subjected to an ANOVA to compare the metrics among sites and substrates. The community composition of the sites was subjected to a multivariate statistical analysis. Differences in the composition, richness, density, and diversity were observed among the sites due to differences in the levels of nutrients and solids generated by the presence of crops close to water bodies. Samples collected from sandy substrates exhibited a lower density and taxonomic richness because sand is a poorer substrate than the others that were sampled. In organic or mixed substrates with higher energy availability and better shelter conditions, the observed densities were higher. The physical and chemical conditions and the morphometric characteristics of the sites were more influential than the substrates.

How reliable are Malaise traps for biomonitoring? – A bivariate species abundance model evaluation using alpine Chironomidae (Diptera)

Abstract In this study, the potential of Malaise traps to collect representative portions of an insect community was investigated. To do so, the complete catch (nearly 22 000 specimens) of male Chironomidae (Diptera) from five parallel Malaise traps along an alpine stream was identified and assigned to 108 different species. The traps were run for 4 weeks in June and July, 2008. The similarity in community composition between parallel samples, that is, from different traps the same week, was evaluated by fitting a bivariate Poisson‐lognormal species abundance model. The estimated correlation in this bivariate distribution was used as a measure of similarity since this approach is utilising all the available species abundance information and accounts for the sampling process. Estimated similarities showed non‐significant differences in chironomid community structure between parallel samples. The five Malaise traps sampled equally representative portions of the Chironomidae community present at the site, so the traps were found to be very reliable in the monitoring of Chironomidae community structure. Application of the bivariate correlation as a similarity measure offers advantages over traditional measures because it takes account of the complete species abundance distributions. This approach provides an approximately unbiased estimate of similarity despite varying sample sizes and detection/non‐detection of species that are present, but rare.

Contrasting the effects of climatic, nutrient, and oxygen dynamics on subfossil chironomid assemblages: a paleolimnological experiment from eutrophic High Arctic ponds

Nutrients from the waste products of large seabird colonies can enter freshwater ecosystems, markedly altering water quality and biotic assemblages, especially in nutrient-poor regions like the Canadian High Arctic. Here, we investigate the influence of nutrient-rich seabird wastes on freshwater larval chironomid assemblages from two distinct seabird colonies. The study sites include four ponds dominated by northern fulmars (Fulmarus glacialis) at Cape Vera, Devon Island, as well as one pond near a large group of common eider ducks (Somateria mollissima borealis) on Tern Island (unofficial name) near Cornwallis Island, Nunavut. The rare combination of nutrient-enriched and well-oxygenated waters allowed us to contrast the effects of nutrients, oxygen, and temperature on chironomid community structure in shallow Arctic ponds using sediment cores. Despite highly elevated nutrient levels, the subfossil assemblages were dominated by cold stenotherms typical of oligotrophic waters. Although nutrient inputs appear to have increased chironomid head capsule numbers due to enhanced food availability, the fertilization had little direct effect on assemblage composition, at least at the taxonomic level achievable based on fossil material. The presence of low abundances of eutrophic/anoxic taxa, such as Chironomusplumosus-type, suggests that biogeographic barriers to dispersal are not influencing the assemblages. These data demonstrate that, in the presence of high concentrations of dissolved oxygen, nutrient enrichment had little direct effect on chironomid community composition in shallow Arctic ponds.

Diversity and structure of Chironomidae communities in relation to water quality differences in the Swartkops River

The Swartkops River is an important freshwater ecosystem in South Africa. But owing to its location, it suffers varying degrees of human induced impacts which include industrial and domestic effluent discharges, deforestation as well as agricultural land use which have negatively impacted on the water quality. Diversity and community composition of aquatic insects are frequently used to assess environmental water quality status. Chironomids occupy extremely varied biotopes. Their extraordinary ecological range and environmental sensitivity make them particularly useful for assessing and interpreting changes in water quality of aquatic ecosystems. The community structure of chironomid larvae was investigated at four sites in the Swartkops River and effects of different chemical and physical variables on their distribution were explored. Chironomid larvae were collected using the South African Scoring System version 5 (SASS5) protocol. A total of 26 taxa from four sampling sites in the Swartkops River were identified. Margalef’s species richness index, equitability, Shannon and Simpson diversity indices were highest at site 1 (reference site). The downstream sites contained 6–20 taxa compared to the 25 taxa at site 1. Site 1 was characterised by the subfamilies Orthocladiinae, Tanypodinae and the tribe Tanytarsini while the impacted sites were characterised by Orthocladiinae and Chironomini. Chironomus spp., Dirotendipes sp., Kiefferulus sp. and Tanypus sp. seemed to be tolerant to pollution, occurring in high abundance at sites 2, 3 and 4. In contrast, Polypedilum sp., Tanytarsus sp., Orthocladius sp., Cricotopus spp. and Ablabesmyia sp. appeared to be more sensitive taxa, being less common at the impacted sites (sites 2, 3 and 4). Five days biochemical oxygen demand, dissolved oxygen, electrical conductivity, orthophosphate–phosphorus and total inorganic nitrogen were among the important variables that determine the observed chironomid community structure. Results revealed that the family Chironomidae, identified to species or genus, can be use to assess environmental water quality status in South African freshwater ecosystems.

Colonisation of temporary macrophyte substratum by midges (Chironomidae: Diptera)

This study investigates a phytophylous community of Chironomidae larvae on the submerged plant species Myriophyllum spicatum L. in a eutrophic lake (Lake Sakadas, Danube floodplain area in Croatia) during summer 2004. This macrophyte species appeared for the first time in the lake in 2004, lasted approximately three months and was considered as a temporary habitat. The chironomid community was very abundant in the stands of this macrophyte species, which developed at three sites. The recorded species belong to three subfamilies: Chironominae (Chironomini and Tanytarsini), Orthocladiinae and Tanypodinae. Species composition varied in time, though Orthocladiinae with their representative Cricotopus sylvestris gr. dominated throughout the entire sampling period, accounting for approx. 60% of the total community. However, the presence of this species group experienced a marked drop to only 10% in September when Chironomini larvulae and Paratanytarsus sp. prevailed. Furthermore, the share of Endochironomus albipennis (Meigen 1830) in relation to other species was also consistently higher. NMDS ordination and cluster analysis separated three main colonization periods based on larval abundance. RDA analysis indicated the influence of environmental variables, especially Secchi depth, macrophyte dry weight, depth and water temperature, on chironomid community structure.

Effects of agricultural land use on chironomid communities: Comparisons among natural wetlands and farm ponds

Constructed farm ponds represent a major wetland habitat type in southeastern Minnesota. Farm ponds are subject to a variety of disturbances associated with agricultural land use, especially sedimentation and eutrophication. Chironomid community structure often reflects environmental changes in aquatic ecosystems. However, chironomid communities in farm ponds are poorly understood and their response to short-term increases in sedimentation and eutrophication remains undetermined. We studied relationships between pond-land cover condition and chironomid community structure. Of the 40 ponds selected for the study, 10 were natural, permanent palustrine wetlands. The remaining ponds were constructed habitats (i.e., farm ponds) with 10 in each of the following categories: non-grazed grassland, grazed grassland, and row crop agriculture. Larval chironomids and water quality parameters were collected during early, mid-, and late summer 2001. Total nitrogen concentrations were significantly greater and turbidity was generally greater in grazed grassland ponds as compared to all other pond types. Chironomid communities of grazed grassland ponds were characterized by lower taxonomic richness and abundant chironomids tolerant of increased sedimentation and nutrient enrichment (i.e., Chironomus and Glyptotendipes). This study determined that agricultural land use, particularly cattle grazing, can markedly affect pond water quality, thereby influencing the chironomid community structure in farm ponds.

The Dynamics of Macroinvertebrate Assemblages in Response to Environmental Change in Four Basins of the Etueffont Landfill Leachate (Belfort, France)

We investigated the relationships between the composition and structure of macroinvertebrate communities and some environmental variables over a year in four basins of the Etueffont landfill leachate (Belfort, France) using co-inertia analysis. Culicidae larvae were the dominant macroinvertebrate group in the studied basins, contributing to 87% of the total zoobenthos density, followed by Corixidae (8.8%), Chironomids (2.5%) and other larvae (each <1%). The lowest density of chironomid larvae was recorded in the first basin which is used as a discharge system for the leachate produced by the landfill. In basin 4, however, the Baetidae, Orthocladiinae (Orthocladius spp., Chaetocladius spp. and Isocladius spp.) and Tanypodinae (Psectrotanypus spp.) developed favoured by low levels in ammonia, COD, BOD, EC, metals and high oxygen concentrations. The co-inertia analysis illustrated both temporal and spatial variabilities in the basins and revealed a strong relationship between environmental conditions and benthic macroinvertebrates assemblages. This ordination technique showed that the chironomid community structure might be used successfully to differentiate between sites with different levels and types of pollution.

Effects of physical factors on chironomid larvae distribution and community structure at a mesohabitat scale

Effects of physical factors on chironomid larvae distribution and community structure at a mesohabitat scale

Incidence of Mentum Deformities in Midge Larvae (Diptera:Chironomidae) from Northern Nova Scotia, Canada

Deformities in the mouthparts of larval Chironomidae, particularly of the teeth on the mentum, have been proposed as a bioindicator of sediment quality and environmental stress. Most work to date has concentrated on relatively few abundant, responsive genera common in soft-bottom lakes. We examined mentum deformities in 25 genera of Chironominae, Orthocladiinae and Diamesinae (one genus) from streams and a lake in rural Nova Scotia where farming and forestry are the principal land uses. Incidence of deformity at similar stream sites varied across genera from zero to >10%. Average frequencies of deformity across all three subfamilies at sites with no known sources of contamination ranged from <4% to 8%, and increased to nearly 15% at a site receiving treated municipal sewage effluent. Differences in chironomid community structure and rates of leaf litter decomposition above and below the sewage effluent outfall were congruent with the difference in mentum deformities. Frequencies of deformity observed here are an order of magnitude greater than in similar studies of rural areas. Low-level stress from agriculture or forest harvesting may be widespread in rural regions even in aquatic ecosystems that are seemingly free of industrial discharges or sediment contamination.

Ten years trends in the oligochaete and chironomid fauna of Lake Neuchâtel (Switzerland)

Quantitative surveys of benthic macroinvertebrates (oligochaetes and chironomids) were conducted during 1992, 1997, 2000 and 2002 at a depth of 40 m to monitor the biological quality of sediments in Lake Neuchâtel. Recent declines in frequency of occurrence and abundance of oligochaete species characteristic of oligotrophic conditions (Stylodrilus heringianus, Embolocephalus velutinus) contrasted with the improvement of water quality metrics. Total phosphorus concentrations in lake waters decreased from 63 mg m - 3 in 1980 to 10 mg m - 3 in 2002. Since 1992, significant reductions of total zoobenthic biomass have been recorded and the chironomid community structure reflects typical oligomesotrophic conditions in the upper sediment layer. The population of the oligochaete species Potamothrix vejdovskyi is in clear expansion and indicates that this new species for the lake (1986) has found good conditions for a successful colonization. These divergent responses within the zoobenthic community are discussed according to three main hypotheses: implications of toxic pollutants like heavy metals and organic micropollutants in the sediment, impact of algae on oxygen conditions on the bottom layer and different biological responses of oligochaetes to competition for food and space.