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THE EFFECT OF CHANGED WATERFLOW AND TEMPERATURE ON THE MAYFLY BAETIS RHODANI IN THE REGULATED RIVER AURLAND, WESTERN NORWAY Gunnar G. Raddum and Arne Fjellheim, Museum of Zoology, Univ. of Bergen, N-5007 Bergen, Norway. A full year cycle of the benthic invertebrate fauna of the lowland part of the Aurland watershed was investigated in 1966/67. During the period 1970 to 1983 the watershed was built out for hydropower production. After regulation water discharge and temperature schemes changed, one part of the river with low flow and increased summer temperature and one with high flow and reduced summer temperature. Effects of the regulation on the benthic fauna was investigated the season 1988/89 by using the same methods as during the work carried out in 1966/67. Baetis rhodani was the dominant macrobenthic species both before and after regulation. In the part of the river with high reduction in waterflow the species increased between 10 and 20 times in abundance after regulation. The reason for this seems to be increased temperature, reduced drift of larvae and increased amount of organic material in/on the bottom after regulation. In the part of the river with high discharge the abundance increased 2 to 5 times after regulation. In this part the temperature was reduced during summer, resulting in a reduced and delayed summer generation, compared with the preregulation situation. Older larvae was accumulated in the downstream stretches, while young and small larvae dominated in the upper parts. Biomass and production of B. rhodani increased 2 to 5 times after regulation. EFFECTS OF INCREASED DISCHARGS ON THS YEARLY PRODIICTIVITY OF BENTHIC INVERTEBRATES IN A RSGULATSD RIVER. Arne Fjellheim, Jarle Haavardstun and Gunnar G. Raddum, Museum of Zoology, University of Bergen, N-5007 Bergen, Norway Benthic animal densities, biomass and production were studied in a weir basin and an adjacent riffle area in a regulated West Norwegian river in 1988 and 1989; two years of high differences in discharge. As water from a large part of the catchment area normally is transferred to a power station in an other watershed, the discharge at the study site is normally low. In 1989 heavy precipitation and a mild winter climate caused surplus production of hydroelectric power. Consequently large amounts of stored water had to be released as overflow at the dam. On a yearly basis the discharge at the study site was 4.8 times higher in 1989 while peak values amounted to 12.0 mss"t and 54.0 mgs't in 1988 and 1989, respectively. The change in flow regime resulted in reduced biomass and faunal changes. The biomass reduction was especially evident for the chironomids, which dominated the benthic macrofauna. In the weir basin large species like Chironomus melanotus, Stictochironomus pictulus and Micropsectra groenlandica were heavily reduced, while some of the smaller rheophilic species, like Nilotanypus dubius increased. An increase in the biomass was also recorded for other rheophilic insect larvae, like the stoneflies Leuctra jusca, L, hippopus, Capnia pygmaea and the mayfly Baetis rhodani. On the other hand lentic caddis larvae like Apatania spp. and Oxyethira spp. were reduced. The faunal changes is explained by altered physical environment, destruction of lentic habitats and reduction of organic materials in the sediment. EFFECTS OF GROUNDWATER ABSTRACTION ON THE ECOLOGY OF CHALK STREAMS IN EASTERN ENGLAND. Melanie Bickertonl, Geoff Pettsl, Patrick Armitagez, and Emmanuel Castellal. 1Freshwater Environments Group, International Centre of Landscape Ecology, Department of Geo~raphy, Loughborough University of Technology, Leicestershire, LE11 3TU. Inst. Freshwater Ecology, River Laboratory, East Stoke, Wareham, Dorset, BH20 6BB, UK. Abstractions from groundwater have been recognized as having significant effects on the flows of many Chalk streams in southern and eastern England, and schemes to augment flows are being considered. This paper examines the invertebrate faunas of three such streams: the Pang, Rhee and Wissey, and assesses the between-river and between-site differences in relation to a range of environmental variables. The rivers studied have catchment areas of between 100 and 280 km2. Eight invertebrate samples were collected by three-minute kick-sample from each river; two marginal cells and two main-stream cells from each of two sites: one near the head of the river severely stressed by flow abstraction and the other at the first downstream site experiencing near normal flow. Each invertebrate sample was related to a range of environmental variables: velocity, depth, substrate characteristics, and macrophyte cover. Significant deferences in environmental variables were recorded between sites, not least in discharge and associated hydraulic conditions. However, differences of these variables between the pair of sites on each river are not consistent, notably substrate characteristics and macrophyte cover. Nevertheless, TWINSPAN analyses clearly distinguish between the faunal characteristics of the two sites in each case. Combined analyses of the 24 sample sets indicate that there are general similarities between the three streams, sharing a set of faunal groups that decrease in the impacted site. However, there are also important differences which reflect the variations of the environmental variables. Ecological profiles of the most important species common to the three rivers were constructed based on 204 sample datasets form rivers throughout the UK, using three environmental variables: percentage of sub-2mm sediment in the substrate, macrophyte cover, and Froude number. Centered profiles were developed from raw profiles by calculating the difference between the frequency of one species in one given class and the frequency of that species in the total set of samples. Strong profiles are shown for both Ephemeroptera end Molluscs, emphasizing the influence of local factors in determining the direction of macro-invertebrate response to flow abstraction.