Invertebrate drift in the regulated River Tees, and an unregulated tributary Maize Beck, below Cow Green dam

SUMMARY. The Cow Green dam was completed in the summer of 1970 and invertebrate drift was sampled below the dam and in an adjacent tributary, Maize Beck, on thirty-one occasions between July 1970 and September 1973. Drift was sampled by pumping river water through a filter. The intake was placed in Maize Beck for the first sample and in the Tees for the second, and so on alternately for the rest of the sampling period. Nets were used on ten occasions, nine of these in winter months and once when the pump broke down. A total of ninety-five taxa were recognized, of which eighty-six occurred in Maize Beck and seventy-one in the Tees. The Tees fauna was dominated numerically and in terms of biomass by a large population of micro-crustaceans originating in the reservoir. Hydra and Naididae also formed a large proportion of the Tees drift but contributed little to the biomass. Ephemeroptera were most abundant in Maize Beck samples. Diptera were abundant in drift catches in both streams with simuliid larvae most numerous in Maize Beck and chironomid larvae most numerous in the Tees. The greatest drift densities of the benthic fauna were observed between April and October; the mean number of organisms per 10 m³ were seventy-three in Maize Beck and 144 in the Tees. The mean densities in winter were very low, respectively two and seventeen per 10 m³ in the two rivers. There was no significant difference between the mean levels of the total bottom fauna (numbers and biomass) in the drift in the two rivers during the period April-October, but vrtnter biomass was significantly greater in the Tees. In July 1970 micro-crustaceans represented 29% (14 per 10 m³) of total drift numbers and 3% (0.7 mg wet-weight per 10 m³) of the biomass, whereas in 1973 they represented 99% of both the numbers (37 670 per 10 m³) and weight (2.2 g wet-weight per 10 m³). The relation between benthos and drift was examined. In the drift Plecoptera and Baetidae were more abundant in Maize Beck than in the Tees. Only Chironomidae and Nais spp. were more abundant in the Tees, In the benthos the density of Plecoptera and Baetidae was not significantly different in the two rivers, but all other groups with the exception of Simuliidae occurred at greater densities in the Tees. The proportion of baetids present in the drift was greatest in Maize Beck. No such difference was demonstrated for total fauna. Diel rhythms were observed in baetids and simuhids with densities greater in night catches. Nocturnal peaks of these organisms were less pronounced in the Tees. Chironomid larvae showed no diel changes in abundance. Significant diel changes in the mean weights of individual animals were not detected in baetid nymphs or chironomids. Micro-crustaceans showed no nocturnal peaks of abundance. Preliminary observations on the quality and quantity of seston caught in drift samples between April and October showed great differences between the rivers. In the Tees the bulk ofeach sample consisted of algal filaments derived from the river and micro-crustaceans from the reservoir. In Maize Beck algae were un-common and the sample was composed of peat and mineral particles. Data are presented on seston output at different discharges.     

The food web in the lower part of the Seine estuary: a synthesis of existing knowledge

The Seine estuary illustrates the alterations to estuaries due to human activities heavy releases of pollutants of various origins and significant morphological changes beginning in the middle of the 19th century. The intertidal mudflat surface has been seriously reduced (< 30km²) since the channels of the Seine River came under management. While the role of the Seine estuary in the dynamics of the eastern English Channel ecosystem is recognized as important, the biological characteristics of the estuary remained relatively unknown until the 1990s. Biological diversity was progressively impoverished from the polyhaline zone to the oligohaline zone. In spite of a heavily contaminated environment, the macrobenthic and planktonic fauna of the Seine estuary remains similar to those of other northeastern Atlantic estuaries. The fauna exhibit clear contrasts between areas with very high abundance and others with very low abundance. The pelagic fauna, especially the copepod Eurytemora affinisand the shrimp Palaemon longirostris, are more abundant in the Seine estuary than in other estuaries. Diversified and abundant, Abra alba-Pectinaria koreni and Macoma balthica benthic communities occur, respectively, in the outer and inner parts of the estuary. In subtidal flats, benthic fauna is especially poor in terms of specific richness, abundance and biomass. Paradoxically, considering the high abundance of prey, fish are particularly scarce. Two food webs have been identified. In the oligohaline zone, where turbidity is maximum, the food web is exclusively planktonic, due to dredging that prevented benthic fauna from settling. In the polyhaline zone, fish populations that feed particularly on benthic fauna benefit from low turbidity and high oxygen concentrations. So, in spite of heavy organic and metallic contamination and human activities, the Seine estuary remains a highly productive ecosystem, which provides a nursery for marine fish and feeding grounds for migratory birds. A global management plan appears to be necessary in order to guarantee that the Seine estuary continues to function as it currently does.     

A quantitative study of the invertebrate fauna of the River Tees below Cow Green Reservoir

Monthly samples were taken in the River Tees below Cow Green dam from four sites differing in flow conditions, by means of trays containing stones from the river, which were immersed for a period of 1 month. Information on the distribution, abundance and biomass of the benthos of the River Tees is given with additional data on the life-histories of some of the more common species, during the period May 1971 to May 1972. Of the seventy-two taxa found, fourteen made up 95% of the total numbers. Hydra, Polycentropodidae, and the molluscs Limnaea peregra and Ancylus fluviatilis were the most abundant animals in the slower-flowing water (10–26 cm s^?1) and Simuliidae, Orthocladiinae, and Baetis rhodani dominate the riffle (50–75 cm s^?1) fauna numerically. Some organisms, Nais spp., Caenis rivulorum and Baetis scambus favoured intermediate flows of 20–60 cm s^?1. The slowest-flowing water contained the largest number of taxa and the site with the fastest flow supported the least. A mean monthly weight per square metre of 14·56 g (95% limits from 7 to 31) was calculated using data from all four sites. The frequency distribution of nymphal size-classes is given for B. scambus, C. rivulorum, Ephemerella ignita, and Brachycentrus subnubilus, and the association of certain nymphal size-classes with a particular site is analysed. The composition and distribution of the fauna is discussed. It is suggested that the relative richness of the fauna in terms of biomass is attributable to the organic enrichment of the river following the building of the Cow Green dam. The regulation of the flow has allowed the dense growth of algae and mosses, and the development of large molluscan populations, and the reservoir itself provides a rich source of food, particularly zooplankton and phytoplankton, for the river benthos.     

Time scales of change in the San Francisco Bay benthos

Results from multi-year investigations in the San Francisco Bay estuary show that large abundance fluctuations within benthic macroinvertebrate populations reflect both (1) within-year periodicity of reproduction, recruitment, and mortality that is not necessarily coincident with seasonal changes of the environment (e.g., the annual temperature cycle), and (2) aperiodic density changes (often larger than within-year fluctuations) following random perturbations of the environment.Density peaks of the small, short-lived estuarine invertebrates that comprise the vast majority of individuals in the bay's relatively homogeneous benthic community normally occur between spring and autumn depending on the species, in large part a reflection of reproductive periodicity. However, because mild winters permit reproductive activity in some of the common species throughout much of the year, other factors are important to within-year density fluctuations in the community. Seasonally predictable changes in freshwater inflow, wind and tidal mixing, microalgal biomass, and sediment erosion/deposition patterns all contribute to observed seasonal changes in abundance. For example, the commonly observed decline in abundance during winter reflects both short-lived species that die after reproducing and the stress of winter conditions (e.g., inundation by less saline, sediment-laden water and the decline in both planktonic and benthic algal biomass — a direct source of food for the shallow-water benthos). On the other hand, data from several studies suggest that observed recruitment and mortality may in fact be the migration of juveniles and adults to and from study sites. For example, the common amphipod Ampelisca abdita apparently moves from shallow to deep water, or from up-estuary to down-estuary locations, coincident with periods of high river runoff in winter. Growth of individuals within the few studied species populations is also highly seasonal, and appears to be coincident with seasonal increases in the abundance of planktonic and/or benthic microalgae.Two multi-year studies have shown that, in addition to within-year periodicity, major restructuring of the benthic community can occur as a result of anomalous (usually climate-related) perturbations of the benthic habitat. For example, during wet years freshwater-intolerant species disappear from the upper part of the estuary and from shallow areas of the bay. During a two-year drought these same species colonized the extreme upper end of the estuary in large numbers. Other aperiodic perturbations include localized instances of sediment erosion or deposition and algal mat accumulations that greatly depress abundance. Additionally, there is evidence (observations that the clam Macoma balthica establishes large populations only when the amphipod A. abdita is not abundant) that species interactions can contribute greatly to interannual variations. Thus, while community composition may change little over the long term, year-to-year predictability of species abundances is low.     

Downstream changes in the composition,numbers and biomass of bottom fauna in the Tees below Cow Green Reservoir and in an unregulated tributary Maize Beck,in the first five years after impoundment

Changes in composition, numbers and biomass of benthic fauna of the Tees below Cow Green Reservoir and an unregulated tributary Maize Beck were followed between 1972 and 1975 and pre- and post-impoundment conditions were compared. Species diversity was lowest just below the dam and numbers and biomass were highest 240 m downstream of the dam. Faunal densities increased in the Tees after impoundment but in Maize Back no major changes were observed.Freshwater Biological Association, Cow Green UnitFreshwater Biological Association, Cow Green Unit     

环境污染对小清河口大型底栖动物多样性的影响

利用2008年7月小清河口和黄河口各8个站位的底栖动物调查资料,运用多种多样性指数对两河口底栖动物群落多样性特征进行了比较,并运用Spearman相关分析分析了14个环境因素和底栖动物群落多样性特征的相关关系。结果显示,小清河口共有大型底栖动物4门6纲17目35科37属38种,平均分类差异指数为85.7,分类差异变异指数为585.6,黄河口共有大型底栖动物6门9纲24目42科45属48种,平均分类差异指数为89.3,分类差异变异指数为435.8。小清河口的多样性指数(H’)仅约为黄河口的1/3。Spearman相关分析表明,H’与铵盐、硝酸盐、铜和汞含量呈负相关性,说明底栖动物群落多样性随着富营养化程度的提高而相应降低,营养物质氮的大量输入和重金属的污染对小清河口大型底栖动物群落产生了一定负面影响。ABC曲线法分析显示,小清河口底栖动物的丰度优势度曲线与生物量优势度曲线相交,表明小清河口底栖动物群落处于中度干扰状态;BOPA指数显示,小清河口8个站位中有4个站位受到不同程度的环境污染,生活污水和工业废水的排放可能对小清河口底栖动物生态特征带来巨大影响,加强工业和生活污水排放管理,强化清洁生产,加强工业污染源的治理力度,对恢复小清河流域水质、改善生态环境具有重要意义。     

MANAGING THE ST. LUCIA ESTUARY—MESSAGES FROM THE MUD

Summary

The infaunal macrobenthic community of the St Lucia estuary has been exposed to natural disturbance in the form of salinity fluctuations arising from floods and hypersaline conditions, and human-induced interference arising from dredging and beam trawling. The responses of the benthos to these disturbances have been recorded between 1983 and 1994 and are discussed in the light of management options for the estuary. A simple benthic sampling strategy is suggested to monitor the important benthic species.     

Changes in the ecology of the Norfolk Broads

The Norfolk Broads have recently shown a marked loss of both floral and faunal diversity. To detect any recent changes and to provide a baseline for future research, twenty-eight broads were visited in 1972–73. Samples of aquatic flora and benthic fauna were taken and a number of physical parameters were measured. In the private broads a good inverse correlation between chlorophyll-a concentration and Secchi disc readings was found, suggesting that phytoplankton was largely responsible for the turbidity of the water. The relationship did not hold for the public broads where erosion and the stirring of sediments by motor launches is intense. Eleven broads were completely devoid of macrophytes. A macrophyte flora was well developed only in Upton and in the five Thurne broads and in these latter there was evidence of a recent loss in diversity. The sudden loss of formerly abundant species of macrophyte has been recorded frequently from the broads. The benthic fauna was poorly developed in most broads and was dominated by Limnodrilus hoffmeisteri and/or Potamothrix hammoniensis. Valvata piscinalis was the only regularly occurring gastropod in the benthos. Most of those broads with a diverse benthic fauna also had an abundant growth of macrophytes. Possible reasons for the recent changes in the ecology of the broads are discussed.     

Fossil plankton depth zones

Varied evidence from the known depth correlated distribution of benthic shelly facies communities in the Ordovician and Silurian clearly shows that graptolite taxonomic diversity increased from the nearshore regions to the shelf margin regions, as well as into the deeper portions of epicontinental basinal areas. We then consider a method for determining the depth distribution of individual graptolite taxa. This method depends on correlating the first appearance of graptolite taxa as one departs from shoreline in terms of the underlying benthos, arrayed in benthic assemblages. The first appearance departing from shoreline corresponds with the upper depth limits of each taxon in terms of the underlying depth indicated by the underlying, associated benthos. The depth of maximum abundance for each taxon should correspond to that point in the increasing relative abundance of that taxon where relative increase in abundance ceases. The lower depth limit of each taxon should correspond to that point on the relative abundance curve where increase in total relative abundance ceases.Using information about the stratigraphic ages indicated by the graptolite faunas, zone-by-zone, combined with that provided by the communities present within the associated and formerly underlying benthic assemblages should provide a more refined stratigraphic zonation than that obtained without combining both.     

Composition and seasonality of benthic invertebrates,and drift in the Naro Moru River,Kenya

A survey was carried out to establish the nature and composition of the benthos along the Naro Moru, a tropical river in central Kenya using artificial substrate baskets, from November 1986 to October 1987. A clear longitudinal zonation existed for Diptera and Ephemeroptera which were the major benthic taxa. Maximum colonization took place after ten days of exposure. Seasonal variations in abundance were also observed. All taxa collected from the bottom samples were also collected in the drift samples, but the percentage composition of the benthos showed variations with that of the drift. Simulium sp. dominated the benthos whilst Baetis spp. dominated the drift. There was a positive correlation between drift rate and benthic fauna density.     

The effects of mine drainage and organic enrichment on benthos in the river nent system,Northern Pennines

A preliminary survey of the benthic fauna of sites on the Nent system affected by elevated zinc levels, acid water and organic enrichment is described. Data on faunal composition are presented for 17 sites. A clustering technique was applied to the taxa/site matrix to demonstrate the degree of association between sites varying in zinc concentration. Chandler biotic scores were found to be lowest at sites affected by high zinc levels and/or dense growths of the alga Stigeoclonium tenue. The role of elevated zinc levels and algal growth in determining the diversity and abundance of benthos is discussed.     

Spatial Patterns of Benthic Macroinvertebrates in Intertidal Areas of a Southern European Estuary: The Tagus, Portugal

This study characterizes the composition and spatial distribution patterns of the benthic macrofauna in the intertidal mudflats of the Tagus estuary, western Portugal. A total of 68 species, more than 226,000 specimens with a total wet weight biomass of approximately 1170 g were identified in 380 sites. The species Streblospio shrubsolii, Cyathura carinata, Tharyx sp., Hydrobia ulvae and Tubificids were the most common and abundant. Scrobicularia plana strongly dominated the biomass. The invertebrate macrofauna of the Tagus estuary shows similarities to what is known from other temperate mudflats. The diversity of species, their overall abundance and the ratio of Molluscs plus Crustaceans to Polychaete species corroborate the distinctiveness between temperate and tropical mudflats and sandflats. The spatial distribution of the fauna reflects the sediment characteristics but the relationship between the environmental and the biological data is not as strong as obtained for sublittoral areas. This relationship diminishes from the sublittoral shelf to sublittoral estuarine areas, showing minimum values in this study, suggesting that such a relationship is less straightforward as natural disturbance increases. Nevertheless, a mixture of grain-size, elevation (inundation time) and particular habitats (relic oyster beds) form the best explanatory factors for the spatial distribution patterns of the intertidal benthic macrofauna of the Tagus estuary.     

Benthic, Drifting and Marginal Macroinvertebrate Assemblages in a Lowland River: Temporal and Spatial Variations and Size Structure

Aquatic macroinvertebrates living in anastomosing lowland rivers use different habitats and respond differently to the hydrological regime. In this paper, the structure and composition of benthic, drifting and marginal macroinvertebrate assemblages are analyzed in the lowland river Ctalamochita (Córdoba, Argentina). The assemblages were studied in an annual cycle; a comparison among the composition of benthos, drift and marginal fauna was carried out; and size structure of the assemblages was characterized. Samples were obtained from two sites: a rural and an urban site. In total 73 taxa of aquatic macroinvertebrates were collected. Benthos was characterized by Chironomidae and Oligochaeta; marginal fauna was mainly constituted by Coleoptera, Heteroptera, Decapoda, the Trichoptera Nectopsyche sp., Ephemeroptera and Odonata. Drifting assemblage was composed by macroinvertebrates from local and remote upstream benthos, and from the marginal zone. Marginal fauna diversity was higher than benthos and drift. Total biomass of the assemblages pooled together was relatively equitably among size classes. Larger size classes consisted of organisms from the marginal zone whereas the smallest ones were composed by benthic and drifting organisms. In the study area there is habitat partitioning in the lateral dimension of the river. Marginal fauna was more diverse due to the asymmetry of transport and deposit processes, which generate a heterogeneous habitat in the bankside. The relation between fine substrate and high current velocity determines an unstable habitat in the central channel, which makes colonization by benthic macroinvertebrates difficult.     

Baseline study of soft bottom benthic assemblages in the Bay of Santander (Gulf of Biscay)

The Bay of Santander constitutes one of the most important estuarine areas in northern Spain (Gulf of Biscay) which has been significantly stressed by sewage discharges. However, an improvement in the environmental quality of the bay is expected once the new sewer system comes into operation. The evaluation of such an evolution needs to be contrasted with the current situation, established in specific baseline studies. For this purpose, a field study was carried out on the soft bottom assemblages of the estuary in spring of 1998. At each station, the benthic fauna abundance, grain size, organic matter, carbon, nitrogen, phosphorus and heavy metals concentrations were analysed. The community structure and the physical and chemical characteristics of the sediment reflected the higher impacts both near the points of discharges and in the low water exchange zones, whereas those areas showing higher tidal flows presented an acceptable quality. High heavy metal concentrations for Cd, Hg, Pb and Zn indicated contamination of industrial sources in several areas of the Bay. This information allows to establish a reference situation in order to improve the design of a specific monitoring program for this area.     

Seasonal and interannual variation in distribution and population abundance of the shrimp Crangon franciscorum in San Francisco Bay

Shrimp are an important component of the San Francisco Bay biota, both as predators on benthic fauna, and as a food source for predatory fish. Of three common species in the bay, Crangon franciscorum is the most abundant. The bay is predominantly a nursery area for maturing shrimp of this species. During the main reproductive period in the early spring, ovigerous females and planktonic larvae are in most years centered outside the bay in the nearshore ocean, although both are also present in the bay. Juveniles move into both the southern reach and the northern reach shortly after settling, and landward-flowing bottom currents are possibly instrumental in this migration. The seasonal cycle of shrimp abundance in the bay, dominated by this spring immigration of newly settled juveniles, is characterized by a progressive migration of the growing shrimp up the estuary coincident with upstream penetration of higher salinity water during summer. Differences in abundance and distribution between the years 1980, 1981, and 1982 suggest that the level of river discharge and accompanying salinity regime are important controlling factors in the distribution, recruitment levels, and subsequent survival and growth of C. franciscorum in the San Francisco Bay.     

黄河山东段河岸带春季大型底栖动物群落结构与多样性

河岸带是河流生态系统与陆地生态系统相互作用的生态交错带,具有明显的边缘效应和丰富的生物多样性。近年来,由于受人类活动(如水利工程、堤坝和农业生产等)的干扰,黄河下游河岸带生态系统受到严重破坏。为了解黄河下游河岸带大型底栖动物的群落结构现状和影响因素,于2015年5月对黄河山东段河岸带8个断面的底栖动物群落进行了系统调查,采用相对重要性指数、物种多样性指数和均匀度指数等方法分析该区域大型底栖动物的群落特征。调查期间共采集到37种底栖动物,其中,环节动物3种,软体动物7种,水生昆虫27种。底栖动物物种组成主要以水生昆虫为主,尤其是摇蚊幼虫。该段河岸带大型底栖动物的平均密度为(2 129±436)ind/m2,平均生物量为(2.20±0.75)g/m2。就空间分布而言,各断面底栖动物的种类数、优势种、现存量和多样性均呈现出明显的空间分布差异,且呈现出不规则波动,这表明河岸带受到严重的人为活动干扰。综合分析表明,各断面的河岸带形态(如宽度和生境类型)和水生昆虫的繁殖行为是影响黄河山东段河岸带底栖动物空间分布的主要因素。     

New data from Oman indicate benthic high biomass productivity coupled with low taxonomic diversity in the aftermath of the Permian–Triassic Boundary mass extinction

A new Early Triassic marine fauna is described from an exotic block (olistolith) from the Ad Daffah conglomerate in eastern Oman (Batain), which provides new insights into the ecology and diversity during the early aftermath of the Permian–Triassic Boundary mass extinction. Based on conodont quantitative biochronology, we assign a middle Griesbachian age to the upper part of this boulder. It was derived from an offshore seamount and yielded both nektonic and benthic faunas, including conodonts, ammonoids, gastropods and crinoid ossicles in mass abundance. This demonstrates that despite the stratigraphically near extinction at the Permian–Triassic Boundary, Crinoidea produced enough biomass to form crinoidal limestone as early as middle Griesbachian time. Baudicrinus, previously placed in Dadocrinidae, is now placed in Holocrinidae; therefore, Dadocrinidae are absent in the Early Triassic, and Holocrinidae remains the most basal crown‐group articulates, originating during the middle Griesbachian in the Tethyan Realm. Abundant gastropods assigned to Naticopsis reached a shell size larger than 20 mm and provide another example against any generalized Lilliput effect during the Griesbachian. Whereas the benthic biomass was as high as to allow the resumption of small carbonate factories, the taxonomic diversity of the benthos remained low compared to post‐Early Triassic times. This slow benthic taxonomic recovery is here attributed to low competition within impoverished post‐extinction faunas.     

A biological trait approach to assess the functional composition of subtidal benthic communities in an estuarine ecosystem

Within transitional/estuarine environments ‘ecosystem functioning’ has been mostly investigated with “traditional” taxonomic analysis, based on the taxonomic composition of benthic invertebrate communities. However, ‘ecosystem functioning’ depends also greatly on the functional characteristics (biological traits) of organisms.It was a priori suggested that the biological traits of the subtidal benthic invertebrate communities within an estuarine environment would respond to the high variability of environmental pressures (natural and human induced) within this type of ecosystem.For this study, traditional taxonomic analysis (species richness, species density and Shannon–Wiener diversity) as well as biological trait analysis were used together for the first time to investigate the response of the subtidal benthic invertebrate communities to the environmental pressures within the Mondego estuary (Portugal).Biological trait analysis, in addition to traditional taxonomic analysis provided a more comprehensive understanding of the functioning within this type of ecosystem. Some of the most important outcomes are: (i) the trait “salinity preference” was the most important trait that distributed the species along the estuary, (ii) the central part of the estuary appeared to be under higher environmental stress levels than the other areas, as suggested by a dominance of some “opportunistic” traits (e.g. small short-lived species), (iii) the ratio between functional diversity (FD) and Shannon–Wiener diversity (H′) indicated lower functional redundancy at the upper reaches of the estuary. Our results, suggest that the ratio (FD/H′) might be a helpful tool to visualize this functional attribute and could potentially be applied to different communities from distinct environments. Using the traditional taxonomic analysis alone, this last functional aspect would not be detectable. Therefore, the inclusion of biological traits analysis is recommendable for estuarine ecological studies.     

A survey of stream invertebrates in the Cow Green basin (Upper Teesdale) before inundation

In 1967 a programme was initiated by the Freshwater Biological Association to study fish populations within the proposed Cow Green reservoir basin and in the Tees downstream of the dam, before and after impoundment. This paper describes the result of a supporting study on aquatic invertebrates covering the pre-impoundment period 1967–70. The benthic faunas of six streams in the reservoir basin, the Tees below Cauldron Snout and Maize Beck, a tributary ofthe Tees below the dam, were studied. Species list are presented for each habitat and changes in seasonal and annual abundance are discussed. All areas sampled lie at altitudes between 440 and 550 m O.D. and are situated amongst moorland and limestone grassland. Conditions in the streams ranged from slow-flowing peaty reaches to small streams with moss-covered bottoms and larger stony rivers and streams with relatively unstable bottoms. Samples were taken in riffles and pools using the ‘kick’ method wherever possible. An attempt was made to quantify kick-sample catches by comparing them with shovel-sample catches which cover a known area of stream bottom. It was found that 10 5 kicks gave a catch equivalent to the populations of 1 m², giving a population density of about 1200 animals/m for the reservoir-basin riffles. Over 120 taxa were recorded, 100 of which were at the species level. In the reservoir-basin streams, 116 taxa were found with seventy-one in Maize Beck and fifty-six in the Tees below Cauldron Snout. Ephemeroptera were the most abundant group in the reservoir basin and Maize Beck faunas with Rhithrogena semicohrata, Heptagenia lateralis and Baetidae being the most abundant forms, although Ecdyonurus spp. especially E. dispar were much more common in Maize Beck. In other groups Leuctra spp. and Gammarus pulex were very common. In the Tees below Cauldron Snout Limnaea peregra, Chironomidae and Baetidae formed the bulk of the fauna and Plecoptera were uncommon. Amongst the reservoir basin streams Weelhead Sike supported the largest number of species and species groups (eighty-one), and all the streams had forty-nine or more taxa represented. Information on seasonal changes in the numbers of those species or species groups composing 90% or more of the total fauna is presented. Faunal density was high in May with Ephemeroptera, particularly Ecdyonuridae, and the plecopteran Leuctra inermis being the most abundant forms. In August, the numbers of animals appeared t o fall and common members of the community were Baetidae, Diptera, Leuctra fusca and Ecdyonuridae. In October, Ecdyonuridae particularly R. semicolorata, were most abundant. A comparison ofthe bottom fauna of riffles and pools was made and more animals were found in riffles than in pools. The effect of gravel extraction on the bottom fauna of the Tees was examined. A severe drop in the numbers of animals was observed after extraction. Diptera were the first group to return to their pre-disturbance density. Elminthidae and Annelida were worst affected and slowest to recover. The fauna of the area is discussed and possible reasons for its relative species richness are put forward. Habitat diversity and chemical richness appear to be the most likely reasons for the relatively large number of species found.     

The impact of an introduced bivalve (Corbicula fluminea) on the benthos of a sandy stream

1. The effect of an exotic, burrowing bivalve ( Corbicula fluminea ) on the benthic fauna of a sandy-bottomed stream was assessed by field and laboratory experiments. Corbicula differs from other freshwater, non-native bivalves in that it both filter- and pedal-feeds and thus has the potential to influence the streambed community in different ways.
2. In the field, cages were used to vary the abundance of Corbicula in the streambed. Increasing abundance of Corbicula was negatively associated with the abundance of benthic bacteria and flagellates but had no apparent effect on other protists or meiofauna.
3. In the laboratory, we compared the effect of Corbicula on the benthic community when the bivalves were able both to filter- and pedal-feed with that when they were able only to filter-feed. Bivalves restricted to filter-feeding were placed in sediment lacking benthic fauna and organic matter, then the benthos in nearby natural sediment was compared with the community present when Corbicula was able to move freely through the sediment and both to filter- and pedal-feed. Corbicula able to pedal-feed were again associated with a decreased abundance of benthic flagellates and bacteria, as well as diatoms.