Anthropogenic influences on riverine ecosystems have led to numerous impacts that have compromised fish communities. Large European lowland rivers have been amongst the most impacted aquatic environments globally. However, paradoxically, these environments have also been amongst the least studied, due the inherent difficulties of studying fish in large, turbid, dynamic environments. Common bream Abramis brama (L.) populations are amongst those impacted by reduced lateral and longitudinal connectivity that are a consequence of modifications to large lowland rivers in Europe. Thus common bream in the lower River Witham, Lincolnshire, UK, were selected as a case study to examine how fish utilize these river systems. A literature review was carried out to assess the influence of human intervention on riverine ecosystems and their impacts on fish communities. In addition the use of telemetry techniques for studying fish populations was reviewed. An ecological assessment of the lower River Witham was carried out using data available from The Environment Agency monitoring and hydrological systems and data specifically collected during this study. The history of this highly modified lowland river was reviewed to give perspective, timescale and context to the degree of modification that has been undertaken on the river and its consequences. Chemical and biological water quality indicators showed improvements in the last twenty years. Fisheries data identified three species that had become locally extirpated in the lower river since the 1800s. The river is now roach dominated, but common bream are still present, possibly due to available lateral spawning habitats that provide surrogates for, and are functionally similar to, the lentic floodplain waterbodies of natural riverine ecosystems. Hydroacoustic surveys show that fish communities are aggregated and favour the upstream half of the lower river where the channel is more heterogeneous. Recent changes in river character have made traditional fishery assessments by seine netting less effective. Data collected during fish tracking studies are most valuable when the tagged fish are behaving naturally, thus the effects of tag attachment should not impact on the behaviour or well being of the fish. Acoustic telemetry was used to investigate the impacts of tagging, surgical inter-peritoneal implantation, and translocation of common bream. Fish were recaptured following surgery and the incision site photographed to assess healing, which was clean in all cases. All recaptured fish appeared to be behaving normally as they were part of large shoals of fish. On five separate occasions fish were tagged when other previously tagged fish were present allowing their behaviour to be compared in terms of the distances moved and linear range; there were no significant differences between the two groups indicating the tagging procedure to have no detectable impact on the short-term behaviour of the fish at the resolution of the tracking undertaken. Three recaptured fish were translocated ~35 km downstream; these fish appeared to exhibit homing behaviour, returning to the capture site between 6-24 days. Their level of activity (in terms of distance moved) did not significantly differ from non-translocated fish, but their linear range was larger. The distribution and habitat use of adult common bream was assessed by tracking their longitudinal and lateral movements over forty-three months, producing a dataset of over 3.1 million detections. A positive relationship between activity and temperature was revealed, with common bream moving greater cumulative distances during the warmer months and occupying a greater longitudinal proportion of the main channel. The occupancy of tributaries was related to temperature, with common bream entering shallow tributaries during rising temperatures in the spring, whilst a deeper, slow-flowing tributary was used more frequently during the cooler autumn/winter months. During the autumn, occupancy of this deeper tributary was positively related to flow rate, suggesting that common bream use this as a refuge from high main channel flow. The home range concept is at the centre of theoretical models to explain the spatio-temporal behaviour of a wide range of animals including fish. There are lots of different ways of expressing home ranges, including areas, but since rivers are principally linear systems, most range is reflected linearly. Home ranges were calculated for 100% and 90% of locations. Seasonal differences were identified with fish occupying larger home ranges during the spring and summer opposed to autumn and winter. Artificial water level manipulations, management actions for the control of flood risk, affected home range size with fish occupying smaller ranges at reduced winter levels than at increased summer levels. Analysis of when and how often individuals within a population interact with one another provides a method to study the social organisation of animals with the potential to reveal ecologically significant aspects that would otherwise have remained hidden. Network analysis and social network theory were used to examine the social interactions of the tracked fish. The extent of sociality appears to follow a normal distribution, such that there was no evidence of discrete classes of social/non-social fish. Fish were more social immediately after tagging and less social as time progressed, as such little evidence could be found to support the hypothesis that these groups of fish remained in each others’ company for extended periods. However, more active individuals were found to be more socially connected, but home-range size did not significantly affect sociality. The functions of these behaviours are discussed along with potential management and rehabilitation strategies for the lower River Witham and other heavily modified lowland rivers in order to meet good ecological potential/status under the Water Framework Directive (2000/60/EEC).

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External phosphorus loadings on a range of freshwater Sites of Special Scientific Interest (SSSIs) thought to be affected by eutrophication were predicted utilising export coefficients. The effect of such loadings on lake trophic status was evaluated using the Vollenweider-OECD eutrophication model. Estimates of the relative contribution of phosphorus from various sources enabled the selection of possible reduction strategies on a site specific basis. The effect of reduction strategies on trophic status was predicted with the aid of the model. It was established that diffuse agricultural losses of phosphorus are the most common source of enrichment. However, consented discharges of sewage efHuent appear to affect a significant number of sites. Phosphorus in urban runoff is a notable source for lakes situated in less rural areas. Agricultural point sources significantly influence a small number of lakes. A critique of the methodology concluded that the use of separate export coefficients for organic and inorganic sources may be useful for identifying appropriate management strategies, but that the scientific basis for such an approach is dubious. In addition, the employment of agricultural returns for data on livestock levels may introduce an unacceptable degree of error into the calculations. The Vollenweider-OECD model appears to predict the trophic status of the lakes under assessment reasonably well, but there is a need for a reliable method of ascertaining loading reduction objectives. A review of current legislation and policy applicable to the alleviation of eutrophication of freshwater SSSIs encompassed laws relating to nature conservation, to water quality, and to agricultural extensiflcation. It confirmed that legislation which directly addresses the problem is nonexistent, but that certain laws may be applied in a piecemeal manner. In general, the form of nature conservation protection adopted in this country is not designed to prevent deterioration of water quality. An aspect of eutrophication control which may prove to be the most problematical in legislative teniis is the regulation of diffuse agricultural sources of phosphorus. Proposals for changes in law and policy on this issue included the establishment of a catchment-wide scheme, specifically designed to reduce diffuse agricultural lossesof phosphorus, and targeted at eutrophicated SSSIs.

Current applications of the critical loads concept are geared primarily towards targeting emission control strategies at a national and international level. In the UK maps of critical loads for freshwaters are available at 10km2 resolution based on a single representative site in each grid square. These maps do not take variations of water chemistry within mapping units into account and are therefore of limited use for application to non-mapped sites. This thesis describes the development of an empirical statistical model, which uses nationally available secondary data, to predict freshwater critical loads for catchments lacking the appropriate water chemistry information. A calibration exercise using data from 78 catchments throughout Scotland is described. Water chemistry for each catchment has been determined and each catchment is characterised according to a number of attributes. Multivariate statistical analysis of these data shows clear relationships between catchment attributes and water chemistry and between water chemistry and diatom critical load. The key variables which explain most of the variation in critical load relate to soil, geology and land use within the catchment. Using these variables (as predictors) in a regression analysis diatom critical load could be predicted across a broad gradient of sensitivity (R2adj = c. 0.8). The predictive power of the model was maintained when different combinations of explanatory variables were used. This accords the model a degree of flexibility in that model paramaterisation can be geared towards availability of secondary data. There are limitations with the model. These relate to the nature of the predictor variables and the ability of the model to predict critical loads for more sensitive sites. Nevertheless the ability of the model to differentiate between sensitive and non-sensitive sites offers considerable scope for environmental managers to undertake national inventories of catchment sensitivity and specific assessments of individual catchments.