Preliminary study of the development of the subfossil chironomid fauna (Diptera) of Lake Taylor,South Island,New Zealand,during the younger Holocene

The composition and succession of the subfossil chironomid fauna has been studied in a sediment core from Lake Taylor. The sediments of the core are all within the Nothofagus phase of the last 3000 years within the Aranuian. In the older sediments of the core the chironomid fauna was dominated by Tanytarsini and Orthocladiinae whereas Chironomini dominated in the more recent sediments. The species diversity is very high compared with the results of other studies of the recent chironomid fauna of lakes in the South Island. The most abundant head capsules belong to the taxa Chironomus zealandicus, Tanytarsus vespertinus, Corynocera sp., and Eukiefferiella sp. Several additional taxa which have not yet been described also occurred in large numbers. It is not possible to correlate any definite changes in the composition of the subfossil chironomid fauna with a change in climatic conditions or the trophic status of the lake.     

A preliminary survey of the Chironomidae (Diptera) of Costa Rica,with emphasis on the lotic fauna

Extensive collections of Chironomidae were made in Costa Rica, Central America, during 1986 and 1987. Fifty-five genera and at least 148 species belonging to the subfamilies Podonominae, Tanypodinae, Orthocladiinae and Chironomidae were found. Chironominae and Orthocladiinae predominated. Only one species of Podonominae was collected. Tanypodinae was represented by many genera, but species richness was low.Cricotopus was the most widespread and diverse genus of Orthocladiinae. Among the Chironominae the generaPolypedilum, Pseudochironomus, Tanytarsus andRheotanytarsus showed high species richness. Several species were collected that could not be assigned to genus. A number of range extensions were recorded for taxa found in the Neotropical region for the first time and for Neotropical taxa recorded outside of South America for the first time. The Costa Rican chironomid fauna consists of cosmopolitan, holarctic and neotropical components. There is probably an endemic Central American chironomid fauna at the species level.     

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a ¹³C‐depleted stable carbon isotopic composition (δ¹³C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ¹³C of other abundant chironomid groups and the relationship between chironomid δ¹³C and methane production in lakes. 2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ¹³C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini). 3. Remains of Orthocladiinae in the seven study lakes had the highest δ¹³C values (?31.3 to ?27.0‰), most likely reflecting δ¹³C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ¹³C values (?33.2 to ?27.6‰ and ?33.6 to ?28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ¹³C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ¹³C of Chironomini (r = ?0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05). 4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ¹³C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ¹³C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ¹³C. More detailed studies with a higher number of analysed samples and detailed measurement of δ¹³C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ¹³C of chironomid remains.     

Littoral Chironomid Communities of Alpine Lakes in Relation to Environmental Factors

The results of a study of littoral chironomid communities from 89 lakes in the Alps are presented. The lakes are located on the northern and southern sides of the Alps from the western Bernese to the eastern Julian Alps. Due to the different origins of the data set here considered, different sampling years and sampling methodologies were adopted. All of the lakes were sampled during the ice-free season, but 79/89 lakes were sampled mainly in autumn 2000, while for a few of them the sample dated back to 1996 or the early 90s. Samples were taken by kicking the different substrata or a stretch of lakeshore (10–30 m) for at least 2–5 min with a standard net and were then preserved in alcohol. Larvae and pupal exuviae were sampled by skimming the lake surface with the same net, to extend identification to species level. Notwithstanding the large altitudinal gradient, almost all of the lakes are situated above the timberline, and show a wide range of morphological (depth, watershed and lake area) and chemical characteristics. Chironomids were the most abundant insects represented in these lakes, with highest abundances at pH between 6.0 and 7.5. In particular, Orthocladiinae and Chironominae were the two subfamilies with the widest distribution. Altitude and temperature played an important role in determining their distribution, the higher altitudes were dominated by the former, whereas the latter were more numerous in lower lakes. The prevalence of Orthocladiinae and Tanytarsini at high altitudes was regarded as a general pattern of taxonomic composition in harsh climatic areas. A reverse condition occurred only in the Julian Alps, where Tanytarsini dominate, followed by Tanypodinae and rare Orthocladiinae. Although our results demonstrated that the composition of chironomid assemblages was also determined by pH, the pattern was unbalanced. Due to the fact that no cases of strong water acidification were present, taxa restricted to very low pH were poorly represented (only three taxa—Limnophyes, Endochironomus and Orthocladius (O.) spp.) compared with circumneutral (8) and alkaline (14) taxa, which occurred in slightly acid and non-acidified soft water lakes. As regard conductivity and altitude, we found that almost all the taxa present at higher pH were contemporary present at conductivity higher than 100 μeq l⁻¹ and altitude lower than 2100 m a.s.l. At species level, Pseudodiamesa branickii, Corynoneura arctica, Heterotrissocladius marcidus and Paratanytarsus austriacus resulted to be the most ubiquitous and abundant species in the Alps. When the west to east distribution of taxa was taken into account, Chironomini (particularly the genera Chironomus and Dicrotendipes) and Tanypodinae (Larsia and Paramerina) were more typical of the eastern lakes (JA). They may be better indicators of more alkaline waters. Assembling all the existing data and presenting a comprehensive panorama of lake-littoral chironomid taxa of the Alps was one of the aims of this paper. Further monitoring, but also harmonization of the sampling methodologies and taxonomical identification including inter-calibration exercises among laboratories are recommended, to provide reliable basis and more knowledge for future studies of comparative biogeography, and to preserve these environments where global climatic changes may cause abrupt shifts in the faunal assemblages with a loss of taxa typical of these mountainous areas.     

Stratigraphy of the fossil Chironomidae (Diptera) from Lake Grasmere,South Island,New Zealand,during the last 6000 years

The fossil chironomid fauna of a 3.26 m long sediment core from Lake Grasmere has been analysed. The fossil chironomid taxa belong mainly to the subgroups Tanytarsini, Orthocladiinae, Chironomini, and Tanyphodinae. Heptagyini and Podonominae were not common. Tanytarsini were the dominant component of the fauna with Corynocera sp. as the most abundant species during pre-Polynesian times (before 1000 yr BP). The abundance and the composition of the fossil chironomid taxa have fluctuated markedly over the last 6000 years. These fluctuations could be partly correlated with changes in the stratigraphy of the sediments in the core. Layers of highly minerogenic sediment contained the lowest numbers of remains whereas high abundances were found in the sections of the core with the greatest proportion of organic matter. It is suggested that major shifts in the structure of the chironomid community have been mainly caused by changes in the hydrology and inflows of the lake, and the rate and type of sedimentation, in addition to variations in lake productivity.     

Observations on Neotropical Rainpools (Bolivia) with emphasis on Chironomidae (Diptera)

Nine rainpools in the savanna of northern Bolivia were examined using a random, semi‐quantitative sampling method. Both the macrophyte and insect assemblages showed that the longevity of a pool tended to be the dominant influence on the colonizing biota in the Tropics. Twenty‐four families of insects were recorded, including seven Heteroptera, five Odonata and five Diptera. The highly ephemeral (A‐)pools, which were dominated by beetles, differed from longer‐lived waters (B‐ and C‐pools). The composition of the insect assemblages in the B‐ and C‐pools formed a continuum, and the Chironomidae became more numerous and diverse with increasing water body longevity and habitat complexity. Out of the 20 recorded chironomid genera, three uncommon larval types are presented (Chironomini Gen X, "Tanytarsus”; (b), "Tanytarsus”; (dark L.o.). The study area is among the least researched in Bolivia, so many insects, and the chironomids in particular, are suspected as being newly discovered species.     

Chironomids as water quality indicators in the River Mignone (Central Italy)

Chironomids of the River Mignone (Central Italy) were studied in order to examine their community structure and their relationship to some common biological indices (Biotic Score, Extended Biotic Index, Indice Biotique de Qualité Générale) utilizing the total macroinvertebrate fauna (chironomids generally at family level) for water quality assessment in rivers.A total of 36 taxa belonging to Tanypodinae (2 taxa), Orthocladiinae (22 taxa) and Chironominae (12 taxa) was collected at seven stations four times during a year. The results support the importance of the chironomid identification in water quality assessment studies in rivers, and demonstrate the usefulness of the factorial correspondence analysis as ordination technique based on both qualitative and quantitative chironomid data to determine environmental quality gradients.The value as bioindicator of some taxa was discussed, and chironomid assemblages were related to biological water quality of the river according to functional feeding groups.This study forms part of a larger Research project supported by MPI and CNR grants and is aimed at the biological water quality assessment of the river and at the analysis of the total macrobenthic community.     

The response of Chironomidae (Diptera) to a long-term exclusion of terrestrial organic matter

We examined the effects of a seven-year detrital exclusion on chironomid assemblages in an Appalachian headwater stream. We hypothesized that litter exclusion would lead to a reduction in all chironomids at both the subfamily and generic levels because organic matter serves as both food and habitat in these headwater streams. Tanytarsini total abundance and biomass significantly declined after litter exclusion. Before litter exclusion, Tanytarsini average abundance was 4271 ± 1135 S.E. m⁻² and 625 ± 98 after litter exclusion. Biomass was 3.57 ± 0.96 mg AFDM m⁻² before litter exclusion and 1.03 ± 0.9 after exclusion. In contrast, Orthocladiinae abundance and biomass did not change because a psammanophilic chironomid, Lopescladius sp., and other Orthocladiinae genera did not decline significantly. Overall chironomid taxa richness and diversity did not change as a result of litter exclusion. However, Canonical Correspondence Analysis (CCA) of genus-level biomass did show a clear separation between the litter exclusion stream and a reference stream. Separation of taxa between the two streams was due to differences in fine (r ² = 0.39) and coarse (r ² = 0.36) organic matter standing stocks and the proportion of small inorganic substrates (r ² = 0.39) present within a sample. As organic matter declined in the litter exclusion stream, overall chironomid biomass declined and the chironomid community assemblage changed. Tanytarsini were replaced by Orthocladiinae in the litter exclusion stream because they were better able to live and feed on biofilm associated with inorganic substrates. Handling editor: K. Martens     

Environmental controls on the distribution and diversity of lentic Chironomidae (Insecta: Diptera) across an altitudinal gradient in tropical South America

To predict the response of aquatic ecosystems to future global climate change, data on the ecology and distribution of keystone groups in freshwater ecosystems are needed. In contrast to mid‐ and high‐latitude zones, such data are scarce across tropical South America (Neotropics). We present the distribution and diversity of chironomid species using surface sediments of 59 lakes from the Andes to the Amazon (0.1–17°S and 64–78°W) within the Neotropics. We assess the spatial variation in community assemblages and identify the key variables influencing the distributional patterns. The relationships between environmental variables (pH, conductivity, depth, and sediment organic content), climatic data, and chironomid assemblages were assessed using multivariate statistics (detrended correspondence analysis and canonical correspondence analysis). Climatic parameters (temperature and precipitation) were most significant in describing the variance in chironomid assemblages. Temperature and precipitation are both predicted to change under future climate change scenarios in the tropical Andes. Our findings suggest taxa of Orthocladiinae, which show a preference to cold high‐elevation oligotrophic lakes, will likely see range contraction under future anthropogenic‐induced climate change. Taxa abundant in areas of high precipitation, such as Micropsectra and Phaenopsectra, will likely become restricted to the inner tropical Andes, as the outer tropical Andes become drier. The sensitivity of chironomids to climate parameters makes them important bio‐indicators of regional climate change in the Neotropics. Furthermore, the distribution of chironomid taxa presented here is a vital first step toward providing urgently needed autecological data for interpreting fossil chironomid records of past ecological and climate change from the tropical Andes.     

Chironomid (Diptera: Chironomidae) communities in six European glacier-fed streams

1. A study on glacial stream ecosystems was carried out in six regions across Europe, from Svalbard to the French Pyrenees. The main aim was to test the validity of the conceptual model of 38 with regard to the zonation of chironomids of glacier‐fed rivers along altitudinal and latitudinal gradient. 2. Channel stability varied considerably, both on the latitudinal and altitudinal scale, being lowest in the northern regions (Svalbard, Iceland and Norway) and the Swiss Alps. Water temperature at the upstream sites was always <2 °C. 3. There was a prominent difference in taxonomic richness between the Alpine and the northern European regions, with a higher number of taxa in the south. In all regions, the chironomid community was characterized by the genus Diamesa and the subfamily Orthocladiinae. Of a total of 63 taxa recorded, two (Diamesa bertrami and Orthocladius frigidus) were common in all the regions except Svalbard. 4. On the basis of cluster analysis, seven distinct groups of sites were evident amongst glacial‐fed systems of the five regions (Pyrenees excluded). This classification separated the glacier‐fed streams on geographical, latitudinal and downstream gradients. 5. Canonical Correspondence Analysis (CCA) of environmental variables was carried out using 41 taxa at 105 sites. Slope, water depth, distance from source, water temperature and the Pfankuch channel stability index were found to be the major explanatory environmental variables. The analysis separated Diamesinae and typical upstream orthoclads from the other chironomids by low temperature and high channel instability. 6. In all six regions, Diamesa was present closest to the glacier. Within 200 m of the glacier snout, other genera of Diamesinae were found together with Orthocladiinae. Pioneer taxa like Diamesa species coexisted with later colonizers like Eukiefferiella minor/fittkaui in relatively unstable channels. 7. The longitudinal succession of chironomid assemblages across altitudinal and latitudinal gradients in glacial streams followed the same pattern, with similar genera and groups of species. The general aspects of the conceptual model of 38 were supported. However, Diamesa species have wider temperature limits than predicted and other Diamesinae as well as Orthocladiinae colonize metakryal habitats.     

The response of Chironomidae to sediment pollution and other environmental characteristics in urban wetlands

1. We investigated the distribution of chironomid taxa in urban wetlands in the greater Melbourne area, Australia, to test if their distribution was influenced by sediment pollution and other environmental variables. 2. For identification of the Chironomidae, DNA markers generated via polymerase chain reaction–restriction fragment length polymorphism of cytochrome c oxidase sub unit I (COI) were validated against morphology and reference specimens for more than 5000 chironomids representing over 80 species. DNA‐based identification generally concurred with morphological separation, but also indicated the existence of cryptic diversity in some genera. 3. Non‐metric multidimensional scaling (NMS) and canonical correspondence analysis (CCA) showed chironomid assemblages were structured among wetlands and could be linked to several habitat characteristics. However, Chironomidae assemblages were only weakly linked to sediment pollution. 4. Logistic regressions identified potential bioindicators of sediment pollution. Riethia stictoptera, Tanytarsus inextentus, Coelopynia and Chironomus ‘februarius’ were negatively associated and Chironomus duplex was positively associated with sediment pollution. Thresholds for the pollution sensitivities of specific species were mostly similar to those established with previous microcosm tests. 5. Several other environmental factors influenced the distribution of specific chironomid taxa. Salinity, substratum type and submerged and riparian vegetation were particularly important. 6. We conclude that specific chironomid taxa rather than assemblages have potential as bioindicators of sediment pollution provided their ecological preferences are considered and their pollution sensitivities are characterized using multiple methods. The integration of DNA‐based techniques should facilitate accurate and rapid identification of bioindicators species.     

A molecular phylogeny of antarctic chironomidae and its implications for biogeographical history

The chironomid midges Belgica antarctica, Eretmoptera murphyi (subfamily Orthocladiinae) and Parochlus steinenii (subfamily Podonominae), are the only Diptera species currently found in Antarctica. The relationships between these species and a range of further taxa of Chironomidae were examined by sequencing domains 1 and 3–5 of 28S ribosomal RNA. The resulting molecular relationships between B. antarctica and E. murphyi, within Orthocladiinae, were highly supported by validation analyses, confirming their position within Chironomidae, as generated by classical taxonomy. Within Podonominae, P. steinenii from the Maritime Antarctic was more closely related to material from sub-Antarctic South Georgia than to material from Patagonia. Taking advantage of the availability of a molecular substitution rate calculated for this gene in Diptera, a dating of divergence between our study taxa was tentatively established. The divergence dates obtained were 49 million years (Myr), between B. antarctica and E. murphyi, and 68.5 Myr between these species and the closest Orthocladiinae taxon tested from Patagonia, suggesting that B. antarctica and E. murphyi were representatives of an ancient lineage. As both are endemic to their respective tectonic microplates, their contemporary distribution is, therefore, likely to have been shaped by vicariance rather than dispersal.     

Chironomidae from marine coastal environments near St. John's, Newfoundland, Canada

Chironomid larvae and adults were collected from rock pool and intertidal sites between 1990 and 1994 on the exposed Atlantic coast and in more sheltered bays near St. John's. From several thousand specimens collected, 16 chironomid taxa were identified. Two were from intertidal habitats and the remainder occurred in rock pools. In intertidal habitats Halocladius variabilis was abundant in summer among clumps of fine filamentous algae, e.g. Pilayella littoralis. The previous Nearctic records of this chironomid were from Hudson Bay and Greenland although it is widespread in northern Europe. The other intertidal species, Telmatogeton japonicas, was recorded from one site, a rock outcrop on a beach in the mouth of a small stream. This Holarctic species has not been reported previously north of New York on the Atlantic coast. The rock-pool communities were dominated by Psectotanypus dyari, Cricotopus sylvestris, Psectocladius sordidellus gp, Orthocladius dubitatus, Chironomus riparius, and Tanytarsus sp. These taxa are known to tolerate sites with elevated salinity and/or conductivity and thus are present in polluted sites inland. The other eight chironomid species were rare and most were found at only one site. An exception was the Metriocnemus sp. collected in very low numbers at three sites and also collected previously during a survey of rock pools for larval mosquitoes in the same area. The current study shows that a diverse coastal chironomid community is present in the cold ocean habitats of eastern Newfoundland. The community is made up of both arctic and temperate faunal elements with widespread distributions not confined to coastal habitats. Ranges of temperature, salinity, conductivity and pH for the common species occurring in the rock pools are provided. The results suggest that coastal rock pool habitats could be a natural laboratory for studies on the evolutionary ecology of pollution tolerance in chironomids.     

Community structure of larval Chironomidae (Diptera) in a backwater area of the River Danube

1. The larval chironomid community was studied quantitatively in three cross-sections of an impounded area of the River Danube between July 1985 and July 1986. 2. Sixty-six species were identified, most of which belonged to the subfamily Orthocladiinae, but species of the tribe Chironomini were numerically dominant. 3. The sections differed distinctly in terms of species number and diversity; a tendency towards increasing species richness and diversity was observed in the upstream sections of the impoundment. Marked fluctuations in these two parameters characterized the area close to the dam; the temporal oscillations in diversity values differed significantly between the sections (ANOVA: P<0.05). 4. Clear differences were found between sampling sites in terms of community resilience, the minimum eigenvalue of the community matrix defined as the rate of return of a disturbed system. Longer return times after ‘disturbances’ could be expected at sites further upstream from the dam. 5. Temporal persistence of species composition was estimated for each section with Kendal's correlation coefficient from 1985 to 1986. Sites further upstream of the dam illustrated a significantly greater temporal species persistence over time and a smaller degree of fluctuation, compared to the dam area (ANOVA: P<0.05). A significant negative relationship was found between overall persistence and cumulative monthly water discharge, at the dam site (P<0.01). 6. Classification of the larval chironomid community gave evidence of generally low spatio-temporal overlap between species in the impoundment area.     

Changes in fossil chironomid remains along a depth gradient: evidence for common faunal thresholds within lakes

Many environmental variables that are important for the development of chironomid larvae (such as water temperature, oxygen availability, and food quantity) are related to water depth, and a statistically strong relationship between chironomid distribution and water depth is therefore expected. This study focuses on the distribution of fossil chironomids in seven shallow lakes and one deep lake from the Plymouth Aquifer (Massachusetts, USA) and aims to assess the influence of water depth on chironomid assemblages within a lake. Multiple samples were taken per lake in order to study the distribution of fossil chironomid head capsules within a lake. Within each lake, the chironomid assemblages are diverse and the changes that are seen in the assemblages are strongly related to changes in water depth. Several thresholds (i.e., where species turnover abruptly changes) are identified in the assemblages, and most lakes show abrupt changes at about 1–2 and 5–7 m water depth. In the deep lake, changes also occur at 9.6 and 15 m depth. The distribution of many individual taxa is significantly correlated to water depth, and we show that the identification of different taxa within the genus Tanytarsus is important because different morphotypes show different responses to water depth. We conclude that the chironomid fauna is sensitive to changes in lake level, indicating that fossil chironomid assemblages can be used as a tool for quantitative reconstruction of lake level changes.     

Distribution of chironomids (Diptera) in low arctic West Greenland lakes: trophic conditions, temperature and environmental reconstruction

1. Surface sediment samples of subfossil chironomid head capsules from 47 lakes in southern West Greenland were analysed using multivariate numerical methods in order to explore the relationship between chironomid assemblages and selected environmental variables. The study lakes are located along a climate gradient ranging from coastal maritime conditions near the Davis Strait to a continental climate near the margin of the Greenland ice sheet. 2. High‐resolution surface water temperatures were measured through the summer season using automatic data loggers in 21 of the study lakes. The mean July surface water temperature (1999) ranged from 7.3 to 16.5 °C in the data set. 3. In all lakes, a total of 24 chironomid taxa were recorded; Micropsectra, Psectrocladius, Chironomus and Procladius were the dominant genera. There was a strong correlation between the trophic variables [total nitrogen and total phosphorus (TN, TP)] and temperature, and in redundancy analysis (RDA) the three variables explained almost equal significant amounts of variation in the chironomid data (19.8–22.3%). However, temperature lost significant explanatory power when the effect of TN was partialled out in RDA. 4. The lakes were classified using two‐way indicator species analysis (TWINSPAN ) into eight groups defined by temperature, trophic variables, salinity (conductivity) and lake‐morphometric data. Fourteen chironomid taxa showed significant differences in percentage abundances among groups, with Heterotrissocladius, Micropsectra, Ablabesmyia and Chironomus as the most robust group‐indicator taxa. Forward selection of taxa in multiple discriminant analysis was used to fit chironomid assemblages into lake groups. Using only eight taxa, 95% of lakes were correctly classified at a second TWINSPAN division level (four groups) and 85% of lakes at a third division level (eight groups). 5. This study showed that there is considerable potential in using subfossil chironomid head capsules as paleoenvironmental indicators in both short‐ and long‐term (down‐core) studies of lake ontogeny and palaeoclimate conditions in West Greenland. However, because of the strong correlation between temperature and trophic variables, a quantitative reconstruction of lake‐ and habitat‐type is recommended, in combination with direct reconstruction of single variables such as temperature.     

Spatial and temporal patterns in the fish assemblages of individual pools in a midwestern stream (U.S.A.)

Synopsis The composition and consistency of fish assemblages in 14 adjacent pools (6–120 m long) of a clear-water, limestone and gravel creek in midwestern U.S.A. were quantified in eight snorkeling surveys over 19 months, to establish a baseline of natural variation in the system at this scale. The fauna of the stream was dominated numerically by minnows (Cyprinidae), sunfish and black bass (Centrarchidae), and topminnows (Fundulidae). The pool fish fauna of the total 1 km reach (including all 14 pools) was highly consistent throughout the study, despite two major floods. Assemblages in individual pools generally were consistent, but there was more variation within pools than at the scale of the entire reach. Throughout the study, most individual pools remained within discrete subsets of the total occupied multivariate space in a principal components analysis based on fish species abundances. Sunfishes (Lepomis spp.) and bass (Micropterus spp.) were more consistent in their distribution among pools than were minnows (Cyprinidae) or a topminnow (Fundulus). There were 25 significant correlations in occurrence of species pairs among stream pools, out of 91 possible comparisons of the 14 most abundant taxa in the reach. Many pools contained assemblages either dominated by large centrarchids or by abundant cyprinids and juvenile centrarchids, but intermediate assemblages also were observed. The dynamics of distribution of fish species and fish assemblages among individual stream pools are likely influenced by a combination of species-specific behaviors and habitat selection, predator constraints on use of individual pools by small fishes, riffles as size-selective barriers to fish movements between pools, dispersal of young-of-the-year fishes, and abiotic phenomena like floods. Individual stream pools appear to be discrete habitat units for fishes, and do represent an appropriate scale for biologically meaningful studies of fish assemblages or their effects on streams.Department of Zoology, University of Oklahoma     

Temporal and spatial distribution of chironomid larvae and oligochaetes in two Dutch storage reservoirs

Long-term changes in distribution and taxonomic composition of chironomid larvae and oligochaetes in two water storage reservoirs in The Netherlands were studied. A succession among the chironomid species was observed. In the last 10–15 years chironomid densities varied. Compared with other lakes chironomid densities were high. Densities of chironomids were higher in the littoral zone than in the profundal zone. The opposite was found for oligochaetes. Densities and composition of the chironomid fauna in the two reservoirs were similar.Procladius, Tanytarsus andHarnischia dominated at all depths. However, pupal exuviae samples showed some differences in chironomid taxonomic composition between the two reservoirs. Orthocladiinae, rarely found in bottom samples, abounded in pupal exuviae samples.