Chironomid Community Composition Research Articles

Acidification of freshwater lakes in Scandinavia: impacts and recovery of chironomid communities under accelerating environmental changes

We re-examined the effects of acidification on Scandinavian freshwater lakes using paleolimnological reconstructions focused on subfossil chironomids. Our study showed a widespread shift in chironomid community composition occurring at around 1960 and corresponding with the maximum fallout of anthropogenic SO42− deposition. Results also showed that taxonomic turn-over was higher in chironomid records from nitrogen-limited lakes, likely due to the cumulative effects of acidification and eutrophication driven by atmospheric nitrogen deposition. Despite strong evidence of chemical recovery from acidification, all lake records failed to show a return to pre-acidified community compositions and most chironomid communities instead continue to follow the trajectories triggered by acidification. We hypothesized that when SO42− deposition started to decrease after 1980, a period also marked by rapid environmental and climatic changes, the influence of pH and/or other acidification-related variables in structuring chironomid community became less important, thus, giving more relative importance to other drivers, such as brownification and climatic processes. Results, therefore, suggest the key role of acidification in shaping the response of chironomid communities to future environmental changes. Future paleolimnological studies will contribute to better manage aquatic ecosystems recovering from acidification worldwide by allowing managers to quantify the efficacy of different management actions taken to mitigate acidification as well as to redefine appropriate restoration targets.

Island ecosystem responses to the Kuwae eruption and precipitation change over the last 1600 years, Efate, Vanuatu

IntroductionIslands of the Southwest Pacific are exposed to geologic and climate-related disturbances that occur on a range of timescales and which probably affect, to varying degrees, their terrestrial ecosystems. Over the past ∼1100 years we know of two major events in the region: the Kuwae eruption which is thought to have occurred ∼500 cal. years BP and a shift to drier conditions which began ∼1100 cal. years BP.MethodsWe investigated terrestrial and lacustrine ecosystem responses to these events and also to a changing fire regime, likely human-caused, using a multi-proxy (C/N, charcoal, chironomids, pollen, and tephra) record from Lake Emaotul, Efate, Vanuatu.ResultsTephra from the Kuwae eruption was found across a 6 cm layer which our age-depth model suggests was deposited 650–510 cal. years BP (95% confidence). Forest and chironomid community turnover increased during the wet-dry shift 1100–1000 cal. years BP; subsequently, chironomid turnover rates decreased again within <135 years and vegetation had partially (but not fully) recovered after ∼80 years. Following Kuwae volcanic tephra deposition, vegetation turnover increased again, reflecting a reduction in small trees and shrubs and an increase in grasses. Subsequently, the forest vegetation did not regain its previous composition, whereas chironomid community composition remained fairly stable before and after tephra deposition. Within the last ∼90 years, enhanced local burning drove another increase in vegetation turnover.DiscussionTerrestrial and freshwater ecosystems in Efate are sensitive to changes in hydroclimate, volcanism, and anthropogenic fires, although to different degrees; while recent human impacts are often obvious, volcanic eruptions and climatic shifts have also structured Pacific-island ecosystems and will continue to do so.

Ecological impacts of N-deposition in a remote, high-elevation lake in the Three River Headwaters Region, Qinghai-Tibetan Plateau

This study provides a new record of nitrogen (N) deposition and ecological response at a remote, high-elevation lake within the ecologically fragile Three River Headwaters Region, northwest China, from the early 20thCentury to the past decade. A multi-proxy investigation of a lake sediment core including analyses of chironomids (non-biting midge), lake sediment geochemistry (δ15N, δ13C, TN, TOC, OC:TN), pollen, non-pollen palynomorphs and 210Pb-dating. Results were compared with regional pollution forcings, including fertiliser application, atmospheric nitrate and dust deposition, and a localised multiscale air quality modelling system for N-deposition. Our data reflect a mesotrophic lake for the majority of the 20th Century with minimal anthropogenic impact. A shift in the lake conditions is evident after 1985 with increased nutrient enrichment. Low δ15N, OC:TN and increasing TN values, together with an increase in chironomid taxa associated with eutrophic conditions and macrophytes are exhibited in the record, particularly since 2002. These changes are consistent with nutrient loading causing a change in trophic status. Redundancy analysis (RDA) and partial RDAs verified TN as an important driver behind the shift in chironomid community composition in recent decades, explaining 22.2% of the variance on its own and 16.9% with other environmental variables partialled out. While temperature change since 1960 was not a primary control on the chironomid community of Lake Bander, a step-change increase in summer temperatures since the 1990s coincided with the disappearance of cold stenotherms from the record and seems to have exacerbated the shift to more productive conditions. This study highlights the reach of airborne N from modern agricultural and industrial activity to remote locations, and demonstrates the utility of palaeolimnological techniques as part of modern ecosystem assessment for conservation.

Corrigendum: Using DNA metabarcoding for assessing chironomid diversity and community change in mosquito controlled temporary wetlands. MBMG 2: e21060. https://doi.org/10.3897/mbmg.2.21060

Chironomids have been proposed as important indicators for monitoring freshwater ecosystems, however, morphological determination is very challenging. In this study, we investigated the effectiveness of metabarcoding for chironomid diversity assessment and tested the retrieved chironomid operational taxonomic units (OTUs) for possible changes in relative abundance and species diversity in relation to mosquito control actions in four temporary wetlands. The biocide Bacillus thuringiensis var. israelensis (Bti) is widely applied for mosquito control in temporary wetlands of the German Upper Rhine Valley. Even though Bti is considered environmentally friendly, several studies have shown non-target effects on chironomids, a key food resource in wetland ecosystems Three of the studied wetlands were, for the first year after 20 years of Bti treatment, partly left Bti-untreated in a split field design, and one wetland has never been treated with Bti. Our metabarcoding approach detected 54 chironomid OTUs across all study sites, of which 70% could be identified to species level comparisons against the BOLD database. We showed that metabarcoding increased chironomid species determination by 70%. However, we found only minor significant effects of Bti on the chironomid community composition. Subsequent studies will have to address if and how the chironomid community composition might change in the now Bti-untreated temporary wetlands to assess effects of Bti.

Reduced abundance but increased diversity of chironomid larvae under higher trophic pressure from fish in semi-permanent ponds

Fish may affect macrobenthic communities through trophic interactions and modification of habitat conditions. We compared the density, diversity and taxonomic composition of larval chironomids under low vs high fish impact, created by stocking semi-natural ponds with either young/small common carp Cyprinus carpio at low biomass densities or with high biomass densities of older/large common carp, supplemented by other cyprinids and predatory fish species.Over the study season, May–August, total chironomid abundance was considerably lower in ponds with high fish biomass than in low fish biomass ponds. In July–August larval densities declined significantly, irrespective of the fish status of the ponds. The composition of larval communities diverged between ponds with different fish status. Chironomus riparius and C. plumosus accounted for most of the observed dissimilarity. C. plumosus densities were not affected by the fish status of the pond, constituting on average >38 % of chironomid specimens in both types of ponds. C. riparius was abundant in ponds with low fish biomass (on average 30 %), but rare in ponds with high fish biomass (4 %). Other common taxa were Glyptotendipes pallens (15 % and 13 %, respectively) and Procladius sp. (14 % in ponds with low fish impact). Canonical correspondence analysis showed that fish biomass was more important in determining chironomid community composition than environmental variables indicative of pond eutrophication (total N, P, chlorophyll a and conductivity).Despite adverse effects on total abundance, chironomid diversity was higher under apparently stronger trophic pressure by fish. Fish may have relieved some chironomid species from invertebrate predation and competition with dominant Chironomus larvae. Other important drivers of chironomid assemblage and diversity patterns may include species-specific feeding modes and prey size selectivity of fish, the ability of chironomid larvae to attain size- and depth-refuge from small fish but not larger fish, and differential vulnerability of free-living and tube-dwelling larvae relative to ontogenetic niche shifts of fish.

Chironomid communities from subalpine peatlands in subtropical China as indicators of environmental change

The composition of Chironomidae communities and the ecological significance of such assemblages in peatlands have received little attention, especially in subtropical alpine regions. We report on the spatial distribution of subfossil chironomids and environmental controls on the insects in three subalpine peatlands from subtropical, central China. We found 44 genera and 62 morphotypes in 41 surficial peat samples. The taxa are different from those found in shallow freshwater lakes in the same region, as terrestrial/semi-terrestrial taxa are common in the peatland chironomid communities. Dissimilarities were observed among the three investigated peatlands, and spatially heterogeneous microhabitats also showed notable differences in particular species abundances, even within the same peatland. Important taxa that are associated with specific peatlands, and/or illustrate within-peatland community differences, include Tanytarsus pallidicornis-type 2, which is related to lower conductivity, and Neozavrelia, which is associated with greater redox potential and pH. Multivariate statistical analysis revealed significant relationships between chironomid community composition and three environmental factors (conductivity, redox potential and pH). Anthropogenic influences and autogenic processes in peatlands likely modify the biotic communities, either through direct or indirect changes in peatland hydrology and chemistry. Low chironomid concentrations and diversity in Erxianyan Peatland may reflect ecological degradation caused by intense human activities, including the harvest of Sphagnum. This study provides information about subtropical peatland biodiversity and illustrates the utility of subfossil chironomids as proxies for past hydro-chemical changes in peatlands.

Effects of neonicotinoids on the emergence and composition of chironomids in the Prairie Pothole Region.

The use of neonicotinoid pesticides is widespread throughout agricultural regions, including the Prairie Pothole Region of North America. The occurrence of these pesticides to the abundant adjacent wetlands can result in impacts on nontarget insects, and cascading effects through wetland ecosystems. In the current study, field-based mesocosms were used to investigate the effects of multiple pulses of the neonicotinoid imidacloprid on the emergence and chironomid community composition, in an effort to simulate episodic rain events to Prairie Pothole Wetlands. Sediments from two local wetlands were placed into the mesocosm tanks and three imidacloprid pulses added, each 1week apart at nominal concentrations of 0.2, 2.0, and 20μg/L. Overall, a significant decrease in the emergence of adult chironomids was observed within the 2.0μg/L and greater concentrations, with the subfamilies Chironominae and Tanypodinae showing a greater sensitivity than the members of the subfamily Orthocladiinae. The chironomid community also had a dose-related response, followed by a recovery of the community composition near the end of the experiment. Our results provide additional evidence that repeated pulses of imidacloprid may have effects on chironomids and other sensitive aquatic insects living within Prairie Pothole Wetlands, resulting in reduced food availability. We stress the need for continued monitoring of US surface waters for neonicotinoid compounds and the continuation of additional experiments looking into the impacts on aquatic communities.

Spatial variation of benthic invertebrates at the whole-ecosystem scale in a large eutrophic lake

The spatial distribution of benthic invertebrates is fundamental to the ecology of lakes and has implications for higher trophic levels and ecosystem processes. We conducted a lake-wide study of the spatial variation of benthic invertebrates in Lake Winnebago, Wisconsin (USA) with emphasis on the chironomid community. We asked: 1) What is the extent of spatial variation in benthic invertebrate community structure and abundance among and within major lake zones in the lake? 2) What factors are correlated with spatial variation of benthic invertebrates at multiple scales? 3) How has the chironomid community and the abundance of Chironomus varied temporally at the multidecadal scale in the lake? We collected benthic invertebrates from the profundal (33 sampling locations), offshore reef (17), and littoral (15) zones in August 2013. Profundal locations were sampled again in August 2014. Habitat variables, including water depth and temperature, Secchi depth, and benthic organic matter were measured with the goal of investigating potential causes of variation in the biota. The highest mean invertebrate densities occurred in the offshore reef and littoral zones (13,987 and 10,638 individuals [ind]/m2, respectively), relative to the profundal zone (2103 ind/m2). Zebra mussels were the numerically dominant taxon in both the littoral and offshore reef zones, whereas chironomids were the most abundant macroinvertebrates in the profundal. Large differences in chironomid community composition occurred among the 3 major zones. Within the profundal zone, the pattern of spatial variation in total chironomid abundance differed between the 2013 and 2014 sampling events. Local factors (e.g., sediment characteristics) and stochastic factors (e.g., wind and current speed) probably interact to influence the spatial distribution of chironomid taxa between and within zones in large shallow lakes, such as Lake Winnebago.

Using DNA metabarcoding for assessing chironomid diversity and community change in mosquito controlled temporary wetlands

The biocide Bacillus thuringiensis var. israelensis (Bti) is widely applied for mosquito control in temporary wetlands of the German Upper Rhine Valley. Even though Bti is considered environmentally friendly, several studies have shown non-target effects on chironomids, a key food resource in wetland ecosystems. Chironomids have been proposed as important indicators for monitoring freshwater ecosystems, however, morphological determination is very challenging. In this study, we investigated the effectiveness of metabarcoding for chironomid diversity assessment and tested the retrieved chironomid operational taxonomic units (OTUs) for possible changes in relative abundance and species diversity in relation to mosquito control actions in four temporary wetlands. Three of these wetlands were, for the first year after 20 years of Bti treatment, partly left Bti-untreated in a split field design, and one wetland has never been treated with Bti. Our metabarcoding approach detected 54 chironomid OTUs across all study sites, of which almost 70% could be identified to species level comparisons against the BOLD database. We showed that metabarcoding increased chironomid species determination by 70%. However, we found only minor significant effects of Bti on the chironomid community composition, even though Bti reduced chironomid emergence by 65%. This could be due to a time lag of chironomid recolonization, since the study year was the first year of Bti intermittence after about 20 years of Bti application in the study area. Subsequent studies will have to address if and how the chironomid community composition will recover further in the now Bti-untreated temporary wetlands to assess effects of Bti.

Tracking recent human impacts on a nutrient sensitive Irish lake: integrating landscape to water linkages

High-resolution palaeolimnological analysis was implemented on a sediment core from Lough Muckno, a historically polluted inter-drumlin lake located in northeast Ireland. Focusing on chironomid and fossil pollen analyses, catchment conditions and human impacts on the lake environment during the last c. 200 years are identified and characterized. Chironomid community response to historical land-use change, reconstructed through a combination of pollen analysis and documentary evidence is evaluated in detail. The results provide insights into lake ecological conditions from a meso-eutrophic state in a nineteenth century agricultural setting to a highly eutrophic system in a landscape under modern pastoral management. Increasing livestock numbers and concomitant land fertilization are considered to be the main drivers affecting chironomid assemblages. Redundancy analysis indicates that grassland-based agriculture, inferred from Poaceae (grasses), the dominant component of palynological pastoral indicators, was the main variable controlling chironomid community composition. Lake resilience to nutrient abatement is reflected in a limited chironomid response to declining agricultural activity during the Great Irish Famine (c. 1846) and attempts at point and non-point pollution controls. Integrated interpretation of chironomid, pollen and non-pollen palynomorphs is shown to be a powerful tool for assessing the impacts of modern land-use change on lake ecology through time.

Response of chironomids to Neolithic land-use change in north-west Ireland

This study provides the first decadally resolved chironomid and organic geochemistry record of the Irish Neolithic from a small lake adjacent to the Carrowkeel-Keshcorran complex in County Sligo, Ireland. Chironomid (non-biting midge fly) sub-fossils and lake sediment geochemistry (δ13C, δ15N and C:N) from the Templevanny Lough core were used to assess the timing and magnitude of within-lake responses to Neolithic farming activity. When compared with decadally resolved pollen and macroscopic charcoal records from the same core, the limnological data show a direct influence of prehistoric farming on a freshwater lake system through nutrient loading and lake eutrophication. Elevated nutrient levels, suggesting a more productive lake system, and a subsequent turnover in the chironomid community indicate a period of intensive farming activity from c. 3790–3620 BC in the early Neolithic. This was followed by a decline in farming with short periods of small-scale human activity, exemplified through nutrient loading and short-lived increases in eutrophic chironomid taxa during the middle to late Neolithic. A return of farming activity can be seen in all proxy data in the late Neolithic ( c. 2720–2480 BC). The chironomid community composition typically lagged land-use change by c. 10–20 years and exhibited predictable and proportional responses to agricultural activity. The timing and magnitude of limnological changes show that land-use, rather than climate, is the main control on chironomids at Templevanny Lough, thus showing the potential prominence of the anthropogenic signal during the Neolithic.

Contrasting Patterns in Chironomid (Chironomidae) Communities of Shallow and Deep Boreal Lakes Since the 1960s

Chironomids have been widely used as indicators of trophic conditions of lakes due to their species-specific environmental requirements. In order to understand resilience and deterioration of aquatic ecosystems due to increases and/or decreases in external loading, recent chironomid community and water chemistry data of seven southern Finnish lakes with varying anthropogenic pressures and bathymetric properties were compared with similar data from the 1960s. Altogether 64 taxa were found. At present, the most numerous taxa are Procladius spp., Chironomus f.l. plumosus, Chironomus f.l. salinarius, Stictochironomus f.l. psammophilus, Benthalia spp., Tanytarsus spp. and Cladotanytarsus. In four lakes, current abundances of chironomid larvae are clearly lower, and in two lakes abundances are higher than during the 1960s. In two shallow, previously highly eutrophic lakes, the chironomid community composition revealed an improvement in the trophic status, while in three deep, previously moderately oligotrophic lakes, a slight deterioration was found. The most dramatic increase in chironomid density and diversity was found in a lake which was most heavily polluted in 1960s. In deep lakes, a slow gradual eutrophication seemed to be responsible for the observed replacement of the previous oligotrophic communities, dominated by Monodiamesa bathyphila, Stictochironomus spp., Polypedilum f.l. breviantennatum and Heterotanytarsus apicalis by eutrophic taxa such as Chironomus f.l. salinarius, Chironomus f.l. plumosus, Einfeldia spp., Sergentia coracina and Microtendipes spp. Among the key factors responsible for changes were dissolved oxygen concentration, food availability, sediment quality, and in one lake, toxic effluents from industry.

Intra- and inter-annual variations in Chironomidae (Insecta: Diptera) communities in subtropical streams

The structure and composition of stream benthic communities are strongly influenced by spatial and temporal factors. This study evaluated the intra and inter-annual variations in Chironomidae communities in subtropical streams. The organisms were sampled from 10 small-order streams during the summer and winter of 2010-2012. The number of chironomid specimens sampled was 7,568, distributed in 49 genera. Chironomid abundance and richness varied intra and inter-annually and community composition varied intra-annually (2010 and 2011). Water temperature, total organic carbon, nitrogen, and rainfall were correlated with chironomid community composition. The intra-annual variation of the community was dependent on climatic variations (temperature and rainfall) and changes caused by intensive agricultural use. We conclude that the temporal variation observed in the Chironomidae community correlates with climatic variations (rainfall) and changes in the total organic carbon and total nitrogen, caused by intensive agricultural land use.

Regional Climate Change Evidenced by Recent Shifts in Chironomid Community Composition in Subalpine and Alpine Lakes in the Great Basin of the United States

Chironomids (nonbiting midges) are used to develop centennial length temperature reconstructions for six subalpine and alpine lakes in the central Great Basin of the United States. Faunal turnover, assessed by detrended correspondence analysis (DCA), indicate that substantial compositional change in the midge communities has occurred during the past 100 years. Although the changes in composition are site-specific, increases in Dicrotendipes and decreases in Procladius characterize the late 20th century at a majority of the sites. Notable faunal turnover in midge community composition is observed at five of the six sites beginning at approximately A.D. 1970. Application of a chironomid-based mean July air temperature inference model (r 2 jack = 0.55, RMSEP = 0.9 °C) to the subfossil chironomid assemblages provides site-specific quantitative reconstructions of past temperature variability for the 20th and 21st centuries. Midge-inferred temperature estimates indicate that four of the six lakes were characterized by above average air temperatures during the post—A.D. 1980 interval and below average temperatures during the early 20th century. The rate of temperature change between A.D. 1920 and A.D. 2010 for these four lakes are: Smith Lake = 0.6 °C 100 yr-1; Birdeye Lake = 0.7 °C 100 yr-1; Cold Lake = 1.2 °C 100 yr-1; Stella Lake = 0.4 °C 100 yr-1. Correspondence between fluctuations in the midge-inferred temperature and instrumental measures of mean July air temperature for Nevada Climate Division #2 is also documented. This study adds to the growing body of evidence that subalpine and alpine lakes in the Intermountain West of the United States have been and are increasingly being affected by anthropogenic climate change in the early 21st century.

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.

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.

Spatial distribution and diversity of chironomid assemblages in a small hilly stream (Diptera: Chironomidae)

In 2006–2007, spatial variations of chironomid assemblages of a small chalky watercourse were investigated in the hilly region of Balaton Upland, Hungary. Samplings were carried out every three weeks at seven characteristically different sites along the whole length of the watercourse. Altogether 2966 specimens were collected and identified. The chironomid fauna of the stream was relatively rich with 40 taxa belonging to four subfamilies (eight Tanypodinae, one Prodiamesinae, 17 Orthocladiinae, 14 Chironominae). The lowest number of species and number of individuals was found at the spring, and the highest at the middle reach. Based on multivariate analysis the different sections of the creek were characterised by distinctive chironomid communities. Although longitudinal changes in the chironomid community of the Örvényesi creek were detectable due to changes in natural environmental factors, land-use changes also significantly influenced the chironomid community composition and diversity. The natural and anthropogenic effects were hardly distinguishable. Furthermore, we found that the response of different diversity indices to environmental variability was different. This draws attention to the difficulties in evaluating the relationships between stream communities and environmental variables using different diversity indices. The taxonomic diversity, which incorporates the taxonomical relationships between the specimens, proved to be one of the most useful diversity indices to characterise the stream chironomid assemblages.

Exploration of the influence of global warming on the chironomid community in a manipulated shallow groundwater system

Uncertainty about predicted effects of global warming on freshwater ecosystems led us to manipulate the thermal regime of a shallow groundwater ecosystem. The study area was separated into a control and treatment block using a sheet-metal groundwater divide to a depth of 1 m. Temperatures were increased according to General Circulation Model (GCM) projections for Southern Ontario, Canada. We examined the response of the groundwater chironomid community during pre-manipulation, manipulation and recovery periods. We found that warming decreased the total abundance of chironomids whereas no significant change in taxonomic richness was apparent. Interestingly, taxon composition changed markedly during both the manipulation and the recovery period. Whereas Heterotrissocladius disappeared during the manipulation in the treatment block, other coldstenothermal taxa such as Micropsectra, Parametriocnemus and Heleniella remained unaffected. Conversely, Corynoneura, Polypedilum and Thienemannia gracilis disappeared but were not reported as coldstenothermal. The chironomid community composition in the system changed from a Heterotrissocladius, Brillia, and Tanytarsini-dominated community during the pre-manipulation towards one dominated by Parametriocnemus, Polypedilum, Orthocladius/Cricotopus and Corynoneura during the recovery. Although increased temperature had a strong effect, chironomid occurrence was also influenced by a number of other abiotic variables, such as dissolved oxygen, depth, ammonia concentration and TDS (Total dissolved solids).

Paleolimnological evidence of the response of the central Canadian treeline zone to radiative forcing and hemispheric patterns of temperature change over the past 2000 years

Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record, we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W. A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition (LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing, namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century. As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA. Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen and lower temperatures in the hypolimnion of the more stratification-prone lake.

Larval Chironomid Community Organization in Four Tropical Rivers: Human Impacts and Longitudinal Zonation

Chironomid (Diptera) larvae were collected between February 1997 and December 1998 from four Trinidadian rivers, two relatively impacted (the Arima and Guanapo Rivers), and two comparatively pristine (the Marianne and Paria Rivers), in order to assess community changes as a result of both land-use and the natural longitudinal gradient. Multivariate analyses indicated changes in chironomid community composition at the impacted sites, with the most dramatic changes occurring in the low-reaches of the Arima and Guanapo Rivers, which were subjected to nutrient enrichment and general urban runoff. For example, genus richness, measured using Marglef’s Index, was lower in the low-reaches of both the Arima and Guanapo Rivers (0.9 and 1.1, respectively), compared with the low-reaches of the Marianne and Paria Rivers (1.8 and 2.8, respectively). The average chironomid densities in the low-reaches of both the Arima and Guanapo Rivers (1346 and 1029 chironomids/1 m2, respectively) were also much higher compared with those of the Marianne and Paria Rivers (130 and 123 chironomids/1 m2, respectively), most likely being the result of nutrient enrichment. Furthermore, the low-reaches of both the Arima and Guanapo Rivers demonstrated a shift in community structure, favouring those taxa more tolerant of non-point source pollution, including Thienemanniella spp., Paratrichocladius sp., Corynoneura spp., Polypedilum spp., Chironomus spp., Saetheria sp., Rheotanytarsus sp., and Thienemannimyia sp. In terms of natural longitudinal changes, the chironomid community shifted from one numerically dominated by Orthocladiinae in the headwaters to one numerically dominated by Chironominae in the low-reaches. Overall, this study shows that land-use can have a greater influence on the structure and diversity of chironomid communities than the natural longitudinal gradient.