Unravelling the determinants of stream midge biodiversity in a boreal drainage basin

1. Few extensive lotic studies have examined patterns in the biodiversity of non‐biting midges (Diptera: Chironomidae) along major environmental gradients. Our aim was to fill this gap by describing patterns in species diversity, assemblage composition and distributions of midges across a boreal drainage basin. 2. We found that the diversity of midges, as measured by rarefied species richness, Fisher’s α and Pielou’s evenness, responded positively to stream size in regression analysis. By contrast, species density was most strongly correlated to a gradient in suspended solids and phosphorus in stream water, as well as macrophyte cover. Spatial variables were not significantly correlated with species diversity. 3. Midge assemblage composition was best explained by a model incorporating five composite environmental gradients in canonical correspondence analysis. The environmental gradients were stream size, macrophyte cover, alkalinity, nitrogen and suspended solids. Spatial variables did not overcome the effects of environmental gradients on assemblage composition. 4. Cluster analysis divided the 27 study sites into four groups with relatively similar midge assemblages. These groups were statistically significant in multi‐response permutation procedure, and 15 of the 49 midge taxa recorded varied significantly among the groups in indicator value analysis. Discriminant function analysis showed that stream size, macrophyte cover and habitat structure predicted 66.7% of sites into correct groups. 5. The information provided by the present analyses may be of considerable importance in conservation planning at the drainage basin level. The fact that species diversity and assemblage composition varied primarily along the stream size gradient suggests that sites belonging to the different size classes (first to fifth order) are needed to conserve the biodiversity of midges. The other environmental gradients should also be considered in conservation planning, because they explained significant amounts of variability in midge assemblage composition.     

Lake‐derived midges increase abundance of shoreline terrestrial arthropods via multiple trophic pathways

Aquatic insects link adjacent ecosystems by transporting nutrients, energy, and material as they move from bodies of water into terrestrial habitats. Insects emerging from streams and rivers are known to benefit arthropod predators such as spiders, but their influence may extend to other arthropod feeding groups as well. We conducted a terrestrial arthropod survey at a series of lakes spanning a strong gradient of midge (Chironomidae, Diptera) emergence. These small, short‐lived insects reach high densities in some areas such that their carcasses litter the ground, and serve as a potential resource for non‐predatory arthropods. Our study revealed that arthropod assemblages in areas of high midge density were significantly different from those with few midges, the result of an increase of all taxa rather than changes in taxonomic composition. Eight of nine terrestrial arthropod taxa sampled showed a strong positive response to the presence of midges including detritivores and herbivores in addition to predators. Taxa that could consume living or dead midges directly responded especially strongly to midge gradients. Our results strongly suggest that midges enter the terrestrial arthropod food web through multiple pathways, increasing numbers of a wide range of arthropods. Furthermore, they emphasize the importance of lakes as sources of aquatic insects that significantly alter processes in the neighboring terrestrial environment.     

Field experiments on interactions between vertebrate predators and larval midges (Diptera: Chironomidae) in the littoral zone of a reservoir

Summary A field experiment was designed to test a frequent assumption in the literature that vertebrate predators (in this case, fish and turtles) are capable of regulating the seasonal abundance and diversity of benthic communities in the littoral zone of lentic environments. Effects of thermal effluents from a nuclear reactor on predator-prey relationships were also examined. Benthic samples were removed after each of three, 3-month test periods from 36 predator exclusion cages (4m²) and 36 control plots located along a thermal gradient in Par Pond, an 1,100 ha freshwater reservoir in the southeastern United States.Results of our field experiments provide little evidence to suggest that either a single keystone species or vertebrate predators as a group were capable of regulating the abundance, diversity or productivity of chironomids in Par Pond. The relationship between predator treatment and community response (changes in density and species richness) was generally unaffected by either plot location or temperature fluctuations. When data from caged and control plots were pooled, however, both location and water temperature individually had significant impacts on the chironomid community. Alternative hypotheses are proposed to explain the lack of regulatory control of the benthic community by individual species or guilds of predators.     

The freshwater littoral meiofauna in a South Carolina reservoir receiving thermal effluents*

SUMMARY. The freshwater littoral meiofauna along a temperature gradient in Par Pond (a cooling reservoir receiving thermal effluents from a nuclear reactor) was sampled from September 1975 to October 1976. Monthly samples were taken at three stations; ‘hot-water’ (15–40°C), ‘warm-water’ (13–37°C) and ambient or ‘cold-water’ (8–37°C) sites. Total numbers of individuals at the ambient site ranged from 971 to 3674 per 10 cm² (mean = 2263), approximating the density reported from productive estuarine environments. Nematodes, rotifers, ostracods, cladocerans and mites comprised 80% of overall density. When compared to the ambient site, thermally affected sites demonstrated reduced faunal density. Contrary to other environmental perturbation studies, the Shannon-Weaver diversity index (H') did not reflect alteration of structural complexity within the rotifer taxocene when hot-water, warm-water and cold-water sites were compared. Though there was a significant reduction in number of species at the thermally altered sites, high ‘equitability’ among the reduced species resulted in H’ values comparable to those in natural communities. Distinct winter-spring and summer-autumn assemblages were evident in the ambient community while no seasonal clustering was apparent at the thermally affected sites. It is hypothesized that the maintenance of temperatures above ambient normals masks environmental cues that normally would elicit seasonal changes in the rotifer fauna at those two sites.     

BIOCHEMICAL AND MORPHOLOGICAL EFFECTS OF TEMPERATURE ON TYPHA LATIFOLIA L. (TYPHACEAE) ORIGINATING FROM DIFFERENT ENDS OF A THERMAL GRADIENT. I. CONTROLLED ENVIRONMENTAL STUDIES

T. latifolia grows along most of the shoreline of Par Pond, a cooling reservoir for the reactors at the Savannah River Plant in Aiken, South Carolina. A thermal gradient exists because of the movement of hot water from one end of the pond to the other. This study was undertaken to determine the biochemical and morphological effects of temperature on Typha latifolia clones originating from different ends of this thermal gradient. The data indicated that both “hot” and “cold” end plants react similarly to temperature. Forty-eight rhizome plants from both ends of Par Pond were transplanted into tanks at 20 C and 30 C. Total levels of malic dehydrogenase activity, proportions of mitochondrially bound MDH and growth rates were significantly different in plants grown in the two tanks, but no significant differences were observed within each tank between individuals from the “hot” and “cold” end. The relation of these results to hypotheses concerning quantitative strategies of enzyme adaptation are discussed in the paper.     

Observations on the ecology of Clinostomum marginatum in largemouth bass (Micropterus salmoides)

Over a fifteen month period, beginning October, 1974, approximately 13 500 centrarchids were examined for evidence of infection with metacercaria of Clinostomum marginatum. Species checked included Lepomis macrochirus, L. gulosus, L. auritus, Pomoxis nigromaculatus and Micropterus salmoides. The study site was Par Pond (South Carolina, U.S.A.), an 1120 ha reservoir receiving thermal effluent from a nuclear production reactor. Except for the largemouth bass, M. salmoides , infection percentages among the five species were less than 1%. Among bass, infection varied seasonally, being highest from January to June. From the spring highs of approximately 25 %, the percentages dropped to lows of < 10% in July and August; there was a jump in September-October to another peak of 30% and then a steady decline through December when infection percentages were again less than 10%. Neither body condition nor length of the bass were related to infection percentage or metacercaria density. Infection percentage could not be related to the influence of thermal effluent. Infection percentages did vary from location to location within the Par Pond system. A significant rank correlation could be established between infection percentage and the amount of littoral zone present in the locality from which the bass were taken. It is suggested that the local 'bay effects' are the result of limited home and foraging ranges of the bass in relation to the amounts of littoral zone present in various locations of the reservoir.     

Colonisation of pitcher plant leaves at several spatial scales

Abstract.  1. The effect of meso-scale (zone within bog and local plant density) and fine-scale (leaf length and resource availability) factors on the colonisation of pitcher plant leaves by arthropods was examined in an eastern Canadian bog.
2. In spring, the abundances of three arthropods, the mosquito Wyeomyia smithii , the midge Metriocnemus knabi , and the mite Sarraceniopus gibsoni , were determined for plots with low, moderate, and high densities of pitcher plants. All overwintering inhabitants were then removed from the plots. Newly opening leaves were colonised from outside the plots, and arthropod abundances were assessed again in autumn.
3. Pitcher plant fauna varied in their response to the meso-scale factors. In autumn (soon after colonisation), midges were more abundant in areas with high densities of pitcher plants. The relationship between mosquito abundance and plant density, and the variation in abundance among zones within the bog in the spring, were probably due to overwintering mortality.
4. All taxa responded to the fine-scale factors, leaf length, and capture rate, in the autumn, but the strength of the responses frequently depended on a meso-scale factor (plant density), in which responses were usually strongest where plants were sparse. Thus, the interaction between meso- and fine-scale processes needs to be considered when interpreting patterns of species abundance within arthropod assemblages in pitcher plant leaves.     

Phylogenetic community patterns suggest Central Indian tropical dry forests are structured by montane climate refuges

Aim

We used an eco-phylogenetic approach to investigate the diversity and assembly patterns of tropical dry forests (TDFs) in Central India. We aimed at informing conservation and restoration practices in these anthropogenically disturbed forests by identifying potential habitats of conservation significance and elements of regional biodiversity most vulnerable to human impact and climate change.

Location

Tropical dry forests of Madhya Pradesh, Central India.

Methods

We analysed the species richness, stem density, basal area and phylogenetic structure (standardized effect size of MNTD, MPD, PD and community evolutionary distinctiveness cED) of 117 tree species assemblages distributed across a ~230 to ~940 m elevational gradient. We examined how these community measures and taxonomic (Sørensen) and phylogenetic (UniFrac) beta diversity varied with elevation, precipitation, temperature and climatic stress.

Results

Species richness, phylogenetic diversity, stem density and basal area were positively correlated with elevation, with high-elevation plots exhibiting cooler temperatures, higher precipitation and lower stress. High-elevation assemblages also trended towards greater phylogenetic dispersion, which diminished at lower elevations and in drier, more stressful plots. Phylogenetic turnover was observed across the elevation gradient, and species evolutionary distinctiveness increased at lower elevations and under harsher abiotic conditions.

Main Conclusions

Harsher abiotic conditions at low elevations may act as a selective filter on plant lineages, leading to phylogenetically clustered low-diversity assemblages. These assemblages contained more evolutionarily distinct species that may contribute disproportionately to biodiversity. Conversely, milder abiotic conditions at high elevations may serve as refuges for drought-sensitive species, resulting in more diverse assemblages. Conservation practices that prioritize both high- and low-elevation habitats could promote the persistence of evolutionarily distinct species and areas of high biodiversity within the Central Indian landscape. Establishing connectivity between these habitats may provide a range of climatic conditions for species to retreat to or persist within as climates change.     

Seasonal variation in abundance and diversity of eavesdropping frog‐biting midges (Diptera,Corethrellidae) in a neotropical rainforest

1. In the tropics, precipitation patterns result in seasonal fluctuations in the abundance and distribution of plant and animal species. Tropical predators and parasites are therefore faced with seasonal changes in prey and host availability. 2. This study investigates the seasonal interaction among a specialised ectoparasite, eavesdropping frog‐biting midges (Corethrella spp.), and their anuran hosts, examining how the abundance and diversity of the frog‐biting midge community fluctuate between the rainy (host abundant) and dry (host sparse) seasons. 3. Midges were captured in both the rainy and dry seasons using acoustic playbacks of calls from a common frog species that breeds during the rainy season, the túngara frog (Engystomops, Physalaemus, pustulosus). During the dry season túngara frog choruses are absent. To explore seasonal shifts in host preference or changes in the midge community due to host specificity, midges were also captured using playbacks of calls from a frog that breeds during the dry season, the pug‐nosed tree frog (Smilisca sila). 4. While the overall abundance of midges decreased in the dry season, only slight differences in the relative abundance between midge species were found. These results suggest that midge populations can shift between hosts as they become available across seasons, allowing adult populations of frog‐biting midges to persist year‐round. To overcome the challenge of detecting and localising different host species, it is proposed that frog‐biting midges have evolved a generalised acoustic template, allowing them to respond to a broad range of available hosts, regardless of seasonal host composition.     

Ecosystem Linkages Between Lakes and the Surrounding Terrestrial Landscape in Northeast Iceland

Despite a recent emphasis on understanding cross-habitat interactions, few studies have examined the ecological linkages between lakes and surrounding terrestrial habitats. The current paradigm of land–lake interactions is typically unidirectional: the view is that nutrients and matter are transported downslope from the surrounding watershed to their ultimate lacustrine destination. Emergent aquatic insects, which spend their larval stages in lake sediments and emerge as adults to mate over land, can act as vectors of material, energy and nutrients from aquatic to terrestrial habitats. In this study, we document a gradient of midge (Diptera: Chironomidae) infall rates into terrestrial habitats (measured as g dw midges m^?2 d^?1) surrounding eight lakes in Northern Iceland (≈66°N latitude). Lakes ranged from having virtually no midge infall (for example, Helluvaðstjörn, 0.03 g m^?2 d^?1) to extreme levels (for example, Mývatn, 19 g m^?2 d^?1) with abundances of midges decreasing logarithmically with distance from shore. Annual midge input rates are estimated as high as 1200–2500 kg midges ha^?1 y^?1. As midges are approximately 9.2% total N, this can result in a significant fertilization effect of terrestrial habitats with consequences for plant quality and community structure. In addition, we used naturally-occurring δ¹³C and δ¹⁵N isotopes to examine food web structure and diet sources of terrestrial arthropod consumers surrounding lakes with differing amounts of midge input. Terrestrial arthropods showed increased utilization of aquatic-derived (that is, midge) C relative to terrestrial sources as midge infall increased. This pattern was particularly pronounced for predators, such as spiders and opiliones, and some detritivores (Collembola). These findings suggest that, despite being largely ignored, aquatic-to-terrestrial linkages can be large and midges can fuel terrestrial communities by directly serving as resources for predators and decomposers.     

BIOCHEMICAL GENETICS OF MOSQUITOFISH. IV. CHANGES OF ALLELE FREQUENCIES THROUGH TIME AND SPACE

Gene frequency data from samples of Gambusia affinis populations at 76 localities across the Savannah River drainage were used to investigate temporal and spatial patterns in population genetic structure. Localities in the Par Pond system on the Savannah River Plant were sampled in 1971, 1977, and 1979. Allelic frequencies in these populations were generally stable through time, although significant temporal changes were observed among samples from Pond C, an impoundment receiving thermal effluent. Significant spatial heterogeneity in allele frequencies was observed on both microgeographic and regional scales. Populations within the Par Pond system were spatially subdivided at four of the five loci surveyed (mean F_ST = 0.051). Subdivision was even more pronounced when samples from across the Savannah River drainage were compared (mean F_ST = 0.196). A hierarchial analysis of gene diversity (G_ST) demonstrated that most of the genic diversity across the drainage exists as within-subdivision diversity. Even when populations from such contrasting habitats as rivers, creeks, ponds, and reservoirs are compared, an average of only 13% of the total gene diversity was attributed to between-group diversity. Greatest between-group gene diversity was observed when reservoirs were compared with one another. This general pattern of low between-habitat diversity suggests that differential selection pressures are not playing a major role in producing the observed levels of subdivision. In the Par Pond system, neither single locus nor multilocus genetic distances were significantly associated with geographic distance or with its reciprocal. For samples from over the Savannah River drainage, significant correlations between genetic and geographic distance were observed only for the Gpi-2 and Pgm-2 loci. Thus, there was a general lack of concordance between genetic and geographic distances. Spatial autocorrelation demonstrated patterns consistent with Wright's isolation by distance model. Significant positive correlations in allelic frequencies among neighboring populations were observed for five of six alleles; allelic frequencies in more distantly separated populations were typically not correlated.     

Microbial community structure and function in response to larval chironomid feeding pressure in a microcosm experiment

Studies concerning the interactions between macroinvertebrate and microbial communities have been carried out for some time. However, most of these studies have been simple feeding relationships that as a group have produced ambiguous results. We perceive these relationships to be more complex, encompassing not only microbial population density but also structure and function. To further understand these relationships, we employed molecular and biochemical techniques to study microbial structural and functional diversity in relation to macrobenthic feeding pressure. We studied the effect of feeding by the three population densities of larval midge, Chironomus tentans, (Diptera: Chironomidae) on microbial community organization. No significant difference in microbial biomass carbon (10.0 mg/g DWS ±1.97) was seen between the three treatments. However, we did detect significant shifts in microbial structure and function with increases in midge population density. The activities of carbon (C) and nitrogen (N) acquiring enzymes were negatively correlated with midge population density. While the C:N ratio was positively correlated with midge population density, suggesting that while the concentration of nitrogen decreased, its availability to the chironomids increased. There was also a marked difference in microbial community structure with increasing midge population density. These shifts in microbial organization are indicative of a complex set of interactions between the microbial community and the chironomid larvae.     

Factors influencing macroinvertebrate colonization of seasonal wetlands: responses to emergent plant cover

1. We conducted field experiments to examine factors influencing macroinvertebrate colonization of seasonally flooded marshes. Few macroinvertebrate species were found aestivating in soils within non-flooded wetlands indicating that most taxa colonize these marshes from other flooded habitats.
2. We manipulated amounts of salt grass ( Distichlis spicata ) to examine how emergent plant cover affects aerial colonization by macroinvertebrates. Areas mowed 3 weeks before flooding had low plant cover, areas mowed 5 and 9 weeks before flooding had medium and high plant cover, respectively, and non-mowed control areas had the most plant cover. Macroinvertebrate numbers and biomass were generally higher in mowed treatment areas than in control areas, but overall diversity was generally higher in high plant cover and control areas than in low plant cover areas.
3. Mosquitoes (Culicidae), brine flies (Ephydridae) and hover flies (Syrphidae) were positively correlated with amount of plant cover, and waterboatmen (Corixidae), midges (Chironomidae) and water scavenger beetles (Hydrophilidae) were negatively correlated with plant cover. Species assemblages changed seasonally among treatment areas because these taxa colonize wetlands at different times in the year.
4. These results demonstrate that invertebrate communities may be different within plant stands with heterogeneous amounts of emergent cover, and management practices that alter the structure of wetland vegetation can influence macroinvertebrate communities colonizing seasonal marshes.     

Host community similarity and geography shape the diversity and distribution of haemosporidian parasites in Amazonian birds

Identifying the mechanisms driving the distribution and diversity of parasitic organisms and characterizing the structure of parasite assemblages are critical to understanding host–parasite evolution, community dynamics, and disease transmission risk. Haemosporidian parasites of the genera Plasmodium and Haemoproteus are a diverse and cosmopolitan group of bird pathogens. Despite their global distribution, the ecological and historical factors shaping the diversity and distribution of these protozoan parasites across avian communities and geographic regions remain unclear. Here we used a region of the mitochondrial cytochrome b gene to characterize the diversity, biogeographical patterns, and phylogenetic relationships of Plasmodium and Haemoproteus infecting Amazonian birds. Specifically, we asked whether, and how, host community similarity and geography (latitude and area of endemism) structure parasite assemblages across 15 avian communities in the Amazon Basin. We identified 265 lineages of haemosporidians recovered from 2661 sampled birds from 330 species. Infection prevalence varied widely among host species, avian communities, areas of endemism, and latitude. Composition analysis demonstrated that both malarial parasites and host communities differed across areas of endemism and as a function of latitude. Thus, areas with similar avian community composition were similar in their parasite communities. Our analyses, within a regional biogeographic context, imply that host switching is the main event promoting diversification in malarial parasites. Although dispersal of haemosporidian parasites was constrained across six areas of endemism, these pathogens are not dispersal‐limited among communities within the same area of endemism. Our findings indicate that the distribution of malarial parasites in Amazonian birds is largely dependent on local ecological conditions and host evolutionary relationships.     

Australian Tropical and Subtropical Rain Forest Community Assembly: Phylogeny,Functional Biogeography,and Environmental Gradients

To compare community assemblage patterns in tropical northeastern and subtropical central eastern Australia across selected gradients and scales, we tested the relationship of species traits with phylogenetic structure, and niche breadth. We considered phylogenetic relationships across current‐day species in assemblages in relation to rain forest species pool sizes, and trait values along gradients including elevation and latitude. Trait values were quantified across scales for seed size, leaf area, wood density and maximum height at maturity for 1137 species and 596 assemblages using trait gradient analysis (TGA). Local assemblages of subtropical species had narrower trait ranges, and higher niche breadth values than corresponding assemblages of tropical species. Leaf size and seed size increased at low latitudes, and community phylogenetic structure was most strongly correlated with seed traits in the subtropics, reflecting dispersal and re‐colonization processes. Elevation accounted for little of the variance in community phylogenetic structure or trait variation across local and regional scales. Stable moist forest areas retained many species from ancestral rain forest lineages across a range of temporally conserved habitats; species within assemblages were less related; and rain forest assemblages had higher functional diversity, but lower niche breadth. This suggests that on average, assemblages of species in stable areas had greater trait variation and narrower distributions. Historic and recent rain forest contraction and re‐expansion can result in recolonized areas that are dominated by species that are more related (phylogenetically) than by chance, have smaller, widely dispersed seeds, and greater niche breadth (broader distributions).     

Plant diversity in wooded moderate-rich fens across boreal western Canada: an ecoregional perspective

Ecoregions are increasingly being used as a framework for conservation planning. The Mid-Boreal Uplands Ecoregion stretches across Canada from Manitoba to British Columbia. From the perspective of conservation and to understand the dynamics of plant diversity and community composition in a common wetland type, we examined the plant communities and environmental variables in 80 wooded moderate-rich fens within this ecoregion. Regional diversity totalled 273 species, with 86 bryophytes and 187 vascular plants. Total diversity was greatest in Manitoba and decreased in a longitudinal trend west through Saskatchewan and Alberta. This may be related, in part, to orographic precipitation at Manitoba sites and a gradient of growing degree days. Richness of locally rare vascular plants exhibited a clear west to east gradient. Ten species of provincially rare vascular plants were observed across the ecoregion, but without pattern. Ordinations and other analyses revealed distinct plant communities for all three locations, with vascular plant assemblages more discrete than bryophyte assemblages. Bryophyte diversity increased with latitude and longitude, whereas vascular plant diversity decreased. Additionally, elevation, precipitation, surface water alkalinity, water temperature, percent overstory density, and peat organic C played a role in determining species richness and community composition. Overall, species composition and diversity in a single wetland type exhibited continuous change across multiple political jurisdictions at the ecoregion scale. Conservation plans for wetlands at the ecodistrict scale may be preferable.     

Community concordance between bryophyte and insect assemblages in boreal springs: a broad-scale study in isolated habitats

1. We examined species–environment relationships and community concordance between aquatic bryophytes and insects in boreal springs. We sampled bryophytes (Marchantiophyta and Bryophyta), benthic macroinvertebrates and environmental variables in 138 springs in Finland, spanning a latitudinal gradient of 1000 km. Macroinvertebrates were subdivided into two groups: Ephemeroptera, Plecoptera, Trichoptera and Coleoptera (EPTC taxa) and chironomid midges (Diptera; Chironomidae). Our aim was to test whether EPTC taxa could be used as surrogates in biodiversity surveys and bioassessment for the two less-well known organism groups, chironomids and bryophytes.
2. Bryophyte assemblages were clearly differentiated along gradients in thermal conditions and water chemistry (pH, conductivity). Chironomids and EPTC were also differentiated in relation to thermal conditions and, to a lesser extent, physical habitat variables, but were only weakly associated to spring water chemistry. Chironomid and EPTC assemblages were more concordant with each other than with bryophytes, but all concordances were relatively weak.
3. Our results suggest that even if the overall compositional patterns of the three taxonomic groups were significantly concordant, the relative importance of environmental drivers underlying their community compositions differed strongly. The results thus imply that spring bryophytes and insects are relatively poor surrogates for each other. The proportion of spring specialists was highest in bryophytes, promoting their primacy for spring bioassessment and biodiversity conservation. We suggest that adequate variation in water chemistry be assured to protect spring bryophyte biodiversity, whereas preserving the physical variation of springs is more important for macroinvertebrates.     

Midge (Chironomidae,Chaoboridae, Ceratopogonidae) assemblages and their relationship with biological and physicochemical variables in shallow,polymictic lakes

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Metacommunity patterns of highly diverse stream midges: gradients,chequerboards, and nestedness,or is there only randomness?

Abstract.  1. Several non-random patterns in the distribution of species have been observed, including Clementsian gradients, Gleasonian gradients, nestedness, chequerboards, and evenly spaced gradients. Few studies have examined these patterns simultaneously, although they have often been studied in isolation and contrasted with random distribution of species across sites.
2. This study examined whether assemblages of chironomid midges exhibit any of the idealised distribution patterns as opposed to random distribution of species across sites within the metacommunity context in a boreal drainage system. Analyses were based on stream surveys conducted during three consecutive years. Analytical approaches included ordinations, cluster analysis, null models, and associated randomisation methods.
3. Midge assemblages did not conform to Clementsian gradients, which was evidenced by the absence of clearly definable assemblage types with numerous species exclusive to each assemblage type. Rather, there were signs of continuous Gleasonian variability of assemblage composition, as well as significant nested subset patterns of species distribution.
4. Midge assemblages showed only weak relationships with any of the measured environmental variables, and even these weak environmental relationships varied among years.
5. Midge assemblages did not appear to be structured by competition. This finding was somewhat problematic, however, because the two indices measuring co-occurrence provided rather different signs of distribution patterns. This was probably a consequence of how they actually measure co-occurrence.
6. Although midge assemblages did not show a perfect match with any of the idealised distribution patterns, they nevertheless showed a resemblance to the empirical patterns found previously for several plant and animal groups.     

Temporal and spatial variability in midge assemblages from a backwater lake in Pool 2, Mississippi River

The temporal and spatial variability in the midge assemblage found in a backwater lake of the Mississippi River was examined. Bi-weekly samples were taken during the summers of 1987 and 1988. Four taxa of chironomids were found. The population density and sizes of chironomids, and the proportion that each taxon comprised of the midge assemblage, varied significantly in time and space. In an attempt to examine whether the variability noted was related to sediment accretion in the lake, sedimentation rates were measured. There were few correlations among sedimentation rate or the organic matter content of the collected sediment and the structure of the midge assemblage. Those few correlations that were statistically significant were low and often paradoxical. For example negative relationships were found between the density and size of Chironomus spp., a detritivore, and the amount of sediment deposited or its organic matter content. Also no significant relationships were found between the density of the predator Cryptochironomus spp. and the density of other chironomids (potential prey). These negative findings indicate that factors other than food, such as sediment texture, may be more important than food availability in structuring chironomid assemblages in backwater areas. There was spatial variability in the densities and sizes of chironomids that may be explained by differences in sediment texture and organic matter. This suggests that short-term inputs of sediment (as ascertained by sediment traps) may not greatly influence midge assemblages while the long-term changes in sediment composition may have large influences.