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Acidification has strongly affected natural ecosystems due to the deposition of acidifying pollutants in geographical areas with low buffering capacity. Here we show both that taxonomic richness of benthic invertebrates (as a measure of stream health) and breakdown rate of leaf litter (a major process in stream ecosystems) in three naturally acid streams were not significantly different from richness and breakdown rates of three circumneutral control streams in the same area in northern Sweden. In contrast, a comparative data-set representing a gradient from circumneutral to acidified streams in France showed decreasing richness and litter breakdown rates (by 60% and 70% between pH 6.5 and 4.5, respectively) typical of acidified streams. The strong negative effects found in many freshwater systems may be a consequence of low capacity to adjust to rapidly changed conditions. In contrast, organisms in naturally acidic systems, such as streams of the boreal region in catchments with accumulating organic material and a geology poor in buffering substances, seem better adapted as a consequence of protracted exposure to low pH over evolutionary time. These results have implications for the management of streams and rivers in northern Sweden, where considerable efforts are spent on remediation without consideration of the fact that the natural state of many of these systems is acidic.

The concentration and transport of faecal pellets (FPs) produced by blackfly (Diptera: Simuliidae) larvae were estimated in a large, free-flowing river in the north of Sweden during 1997-1999. FPs were abundant from May through August and FP loads in transport peaked at 429 t dry mass d(-1) at a site in the lower part of the river in 1997. Daily transport at the same site, averaged over each study period (16 Jun.-18 Sep. 1997, 6 Apr.-10 Sep. 1998, and 21 Apr.-5 Aug. 1999). was estimated to be 93.7 t dry mass (3.7 t carbon), 47.5 t dry mass (1.9 t carbon and) and 69.2 t dry mass (2.7 t carbon), respectively. On a large scale, there was a downstream trend of increasing FP concentration and, during periods with greater discharge, sedimentation was reduced so that more material was exported from the river. Samples from six sites in a regulated river (into which our focal river flows) parallel to six sites in the unregulated tributary showed considerably lower FP concentrations in the regulated river, presumably because of much smaller blackfly populations as a consequence of habitat loss through damming. A survey of two other large, unregulated rivers in northern Sweden confirmed that these also carry large amounts of FPs. We conclude that the transformation of small suspended food particles into considerably larger FPs by huge populations of filter-feeding blackfly larvae is a major process in large northern rivers.

Effects of species number and identity on the breakdown rate of leaf litter were estimated in a laboratory experiment using leaf-eating insects, three species of Plecoptera, as detritivores. We found significant differences between the different species on this process in single-species experiment, but not when animal biomass was accounted for. When species were combined the effect of species identity was strongly reduced and rendered insignificant, whereas the number of species had a significant effect. This shows that rates of ecosystem processes may benefit from species richness even when all species belong to the same guild, which is in contrast to hypotheses predicting redundancy within guilds. Facilitation between species and negative interactions, where intraspecific interactions are greater than interspecific interactions, are two potential mechanisms which could explain increasing decomposition rates with species richness.

The morphology of the labral fans of blackfly larvae is intimately interrelated with the lotic habitats in which they live. Marked seasonal variations in the primary fan morphology of Simulium lundstromi were found in a natural population. In separate experiments, larvae of the overwintering and summer generations were reared at two different current velocities. For both seasons, larvae living in slow velocity (8 cm s -1 ) flumes developed larger primary fans with wider distance between individual rays, and longer stems than those exposed to fast current velocities (31-35 cm s -1 ). These morphological adaptations apparently serve to enable feeding at different flow regimes by balancing particle encounter rates with malfunction of the fans caused by the effects of high drag. However, larvae of both generations had low survival rates and long development times at slow current velocities compared with those in faster flow, because the morphological modifications could not compensate completely for the reduced delivery rate of food particles resulting from the low current velocity. The observed plasticity of fan morphology in response to variation in current velocity is consistent with the natural conditions found in the stream where the larvae lived. This suggests that phenotypic plasticity in blackfly larvae has evolved in response to the intrinsically heterogeneous nature of running water environments.

The influence of a common perlid stonefly predator, Diura nanseni, on the turnover rate of leaf material in leaf packs and their associated fauna was investigated in laboratory and field experiments in northern Sweden. In the laboratory experiment significant indirect effects of the predator were demonstrated upon leaf mass loss and the densities of two nemourid stoneflies, Nemoura avicularis and Protonemura meyeri. There was evidence of non-lethal effects on the two species expressed as increased wound frequencies and reduced condition. In the field experiment predator influence on leaf turnover rates was supported but there were no discernible effects on prey densities

The behaviour of spawning brook lampreys, Lampetra planeri (Bloch), was studied in a stream tank. Freeze-branding enabled recognition of individuals. Aggression was observed mainly at the beginning of the experiment between nest-building males and intruding males. In spite of widespread promiscuity there was a significant positive correlation in size between the partners, indicating an assortative mating. Males competed for matings and it is suggested that the most alert males are the ones most likely to mate with the females. Frequently, additional males coiled round the same female. "Satellite" males were often seen to circle rapidly round the tails of copulating pairs, probably fertilising eggs. Males took part in the spawning on average twice as long as did females. Females often mated for several days after their last eggs were spawned. The reason for this behaviour remains obscure. In another experiment large differences in partner size reduced the fertilization efficiency. Fertilisation was most successful when the female/male length ratio was 1.05-1.14.

Transport and availability of various size fractions of organic material along with bottom structure and benthic invertebrates, grouped with respect to feeding types, were examined along an 8 km long stream in southern Sweden. Transport of coarse particulate organic material (CPOM) was insignificant, while considerable amounts of fine particulate (FPOM) and dissolved organic matter (DOM) were carried downstream. DOM showed little variation and was constantly 2-3 times the energy of suspended FPOM. Export of DOM was 5 times as high as that of FPOM, total organic output being twice as high as leaf input. On a stony bottom shredders and scrapers dominated, whereas scrapers, deposit feeders and filter feeders were abundant where sedimentation was heavy. Downstream the section with most extensive sedimentation all categories including predators were well represented.

Artificial stones having identical surface area and complexity, but different specific densities, were placed in four Swedish stream riffles. The rate of movement of stones was assumed to be related to their density. Invertebrate colonization showed indications in agreement with the intermediate disturbance hypothesis although these were weak and not conclusive. More significant positive (22 and 38%) and negative (16 and 11%) correlations among pairs of the most common species were found in two of the streams than would have been expected by chance alone (2.5%) suggesting biotic interactions. In general, blackfly number positively correlated to species richness, and this correlation was probably related to the structuring effect of the cocoons of Prosimulium hirtipes which resemble moss. While some species (e.g. Eusimulium cryophilum and Polypedilium laetum) dominated the lightest artificial stones, others showed higher incidences on stones of intermediate (e.g. Agapetus ochripes, Hydropsyche siltalai and Polycentropus flavomaculatus) or high densities (e.g. Gammarus pulex and Rheotanytarsus spp.) These patterns may be interpreted as interspecific differences in colonization abilities or strategies.

The filtration rate of differently sized larval Lampetra planeri (Bloch) was estimated experimentally for a set of temperatures ranging from 2.5 to 250C, and for 9 different food particle concentrations at 15'C. The food material presented consisted of suspended yeast. The concentration was measured indirectly as turbidity. Large larvae had a higher absolute filtration rate than small ones and were less dependent on temperature. If respiration was considered, the most "economic" filtration was achieved below 10C. This is in accordance with the low mean annual temperature of typical Lampetra planeri streams. High ratio of filtration to respiration was seen to coincide with maximum food availability of the stream from where the larvae were collected. Low particle concentrations were compensated for by an increased filtration rate. In the interval between 30 and 100 mg dry weight of suspended yeast per litre, larvae increased their rates of filtration. The importance of Lampetra planeri larvae in depleting the water of particles was calculated to be minimal. Less than 1 -10 % of the water passing each metre of stream length was filtered by the lampreys.

In laboratory experiments nymphs of the predatory stonefly Dinocras cephalotes (Curtis), supplied with large individuals of the prey Baetis rhodani (Pict.), emerged earlier than nymphs fed with small individuals of the mayfly. Early emergence, within certain limits, may be advantageous to the stonefly. The stonefly nymphs did not select prey of the most favourable size, probably because of their hunting technique. Small predators were able to kill large prey, but frequently left heavily sclerotized parts of large mayfly nymphs. During the last instar feeding was initially low, but increased to a maximum after a few days. Feeding thereafter decreased slowly and almost ceased several days before emergence. A similar pattern, although not so expressed was noted in earlier instars. Generally, food intake was pulsed with maxima every 2.5-4 d.

Drift of blackfly larva populations was studied at three different sites. One of the sites was at a lake outlet. Some larvae, but always a small part of the total populations, were carried from the lake outlet site in the drift and the proportion drifting was similar to that at two other sites downstream where benthic abundance in contrast was considerably lower. Larvae in the drift were consistently smaller than those in the benthos; those colonizing clean stones in an experiment at the lake-outlet site being of similar size to those in the benthos. There was a proportional reduction in drift during the night through the period of study in July 1975 and, of the three species dominating the lake outlet aggregation, Metacnephia tredecimatum Edwards, hatching first, showed the greatest tendency to nightdrift. This was lower in the later hatching Schoenbaueria annulitarsis Zett., and still further reduced in Simulium truncatum Lundstr. which probably hatched from eggs after the other two species.