Human activities have caused acidification of freshwater systems on a large scale resulting in reduced species diversity and ecological functioning in many lakes and streams. However, many naturally acidic freshwater systems have also been found, for instance in northern Sweden. In regions where such naturally acidic aquatic ecosystems have prevailed over evolutionary periods, species diversity and ecological functioning are not automatically impaired due to possible adaptation to the putatively adverse environmental conditions. I studied species diversity patterns and ecological functioning in anthropogenically acidified, naturally acidic, circumneutral, and limed streams to test the adaptation hypothesis and examine the ecological effects of variation in naturally acidic water chemistry. Species diversity was studied using benthic macroinvertebrates, while functioning was modelled using the decomposition rates of leaf litter. In accordance with the evolutionary species pool hypothesis, species richness was reduced more strongly in regions with anthropogenic than natural acidity when compared to circumneutral streams, supporting the adaptation hypothesis. In contrast, the patterns in ecological functioning along the pH-gradients did not differ between regions with anthropogenic and natural acidity, likely resulting from compensation: the biomass of tolerant taxa probably increased which thus rescued the loss in functioning otherwise mediated by the more sensitive taxa. Furthermore, the naturally variable acidic water chemistry clearly supported distinct macroinvertebrate assemblages, as was reflected in differing patterns of species diversity and ecological functioning. Such naturally acidic waters that were rich in dissolved organic carbon supported higher ecosystem process rates and lower species diversity than waters that contained little dissolved organic carbon. Upon liming naturally acidic streams microbial leaf decomposition increased, whereas shredding decreased along with changes in shredder abundances. The abundance of large caddisflies decreased, while the abundance of small stoneflies increased. The results suggest that various types of benthic macroinvertebrates with varying levels of adaptation and tolerance inhabited the hydrochemically variable naturally acidic streams. The distributions of macroinvertebrates in response to different pH levels and differences in acid quality and how these distributions translate into varying patterns of species diversity and ecological functioning are worthy of further investigation. This will likely improve our understanding of how such naturally acidic streams and their biota can be successfully managed.

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Chlorine stable isotope analysis (?37Cl ) means of stream water (- 0.95 ‰, n = 22), rainwater (- 1.51 ‰, n = 12), fog water (- 1.08 ‰, n = 7) and silicate mineral bound chloride (+ 0.13 ‰, n = 3) are used in an isotope mass balance approach to estimate sources of stream chloride. During summer-baseflow conditions, the chloride budget of two catchments in southwestern Nova Scotia is approximately 39 % from rainfall, 37 % from fog water and 24 % from rock/water interactions. The results of a significant source of geological chloride suggest the use of chloride in stream water as a proxy for marine derived sulphate may not be valid. This conclusion implies that anthropogenic sources of sulphate to acid sensitive ecosystems of southwestern Nova Scotia have been underestimated when chloride is assumed to be a conservative ion in the hydrological cycle.

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This Thesis presents the results of various studies on peat soils and organic surface horizons of peaty podzols derived from acidification-sensitive parent materials. It aims primarily at finding out which peat and organic surface horizon chemical properties correlate most significantly with acidifying deposition parameters, the effect that acidifying pollutant deposition has on growth of vegetation (<I>Calluna vulgaris</I>) and litter decomposition, and the fate of ammonia in peat soils. Two regional surveys were carried out throughout Scotland. Significant correlations were found particularly between the chemical properties of peat and the effective concentrations of the acidifying deposition components. The more significant correlations found with peat pH(water), rather than with pH(CaCl₂), point to the fact that the mobile anion effect is important. A method was established for estimating the pH of soil solution at field conditions, <I>i.e.</I> at a dilution factor of unity, and a close similarity was subsequently found between this estimated pH and the effective pH of deposition. It is suggested that this provides an excellent basis for pH prediction for ombrotrophic peats. Significant seasonal variations in peat chemistry were found, with peat pH(water), for example, varying by up to 0.4 pH units throughout the year. A 7-month pot experiment demonstrated significantly lower growth rates of <I>Calluna vulgaris</I>, and slower litter decomposition rates, at higher acidifying deposition inputs. Studies on the retention of NH₄⁺ in peats clearly show that there is only a limited capacity for biological immobilisation of NH₄⁺ at high input concentrations. More information is needed on the fate of NH₄⁺ inputs in the soil, and on the extent to which retention is via cation exchange and biological uptake, and this needs to be taken into consideration when setting critical loads for N.

An analytical method was developed for the experimental investigations. This included sampling, separation into fractions, clean-up and determination of the organochlorines in suspended solids and seawater. A sampling apparatus was designed and built in our laboratory, capable of sampling seawater up to 28 l, and separating the suspended particles prior to the extraction of the filtered water with organic solvent. The analytical method for the determination of chlorinated biphenyls (CBs) and a large number of organochlorine pesticides in sediments was readily available in our laboratory. In addition, a method for the determination of pentachlorophenol (PCP) in seawater and solids was developed. The complete analytical method for the determination of particulate and dissolved organochlorines in the seawater column, at the ng l^-1-pg l^-1 level, was validated. Small scale experiments were not able to replicate the environment, therefore, a large scale experiment was undertaken in a Loch in the NW of Scotland. Experimental bags, 60,000 l capacity, were attached to a floatable frame and filled with seawater, then contaminated sewage sludge was dumped on it. A sampling strategy was designed, and a set of samples was obtained to investigate the factors affecting the partitioning, and the kinetics of the partitioning, between suspended solids and seawater. The experimental data from the analysis of the samples were used to propose a simple mathematical model describing the changes in the concentrations of particulate and dissolved organochlorines over the time of the experiment. The mathematical modelling provided values for the desorption rates and the partition coefficients of the nonequilibrium for the individual organochlorines. These calculated parameters were used to elucidate the possible trends related to physico-chemical properties of the organochlorines, and to propose the process affecting the organochlorine-particle interactions: intra-organic matter diffusion.

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