This study sought to determine the potential of sedimentary cladoceran assemblages for inferring changes in shallow lake ecological structure and function. The first stage of the study aimed to isolate and quantify the main structuring forces of the contemporary zooplankton community. Zooplankton populations were sampled from 39 mesotrophic to hyper-eutrophic shallow lakes (<3 m average depth) in the U.K. and Denmark. Additionally, a suite of environmental variables, including: fish, macrophytes, algal crop, nutrient concentrations and water transparency were analysed. A variety of numerical techniques including constrained ordination and logistic regression were employed to determine whole community and individual species response to the environmental variation within the data set. The sedimentary remains of zooplankton were then enumerated from the surface sediments of the 39 'training set' lakes. These data were compared with the contemporary counts. Despite differential preservation of sedimentary remains, the main factors structuring the assemblages were the same in both data sets, zooplanktivorous fish density (ZF) and submerged macrophyte abundance (MA). Furthermore, these factors displayed a similar degree of influence on both assemblages. A multivariate regression tree (MRT) was used to calibrate sedimentary cladoceran assemblage response to ZF and MA. The resultant model places a site in one of five groups based on cladoceran assemblage and corresponding to a range of values of ZF and MA. These groups represent distinct forms of ecosystem structure and function varying from sites with clear water, abundant macrophytes and low ZF to turbid, phytoplankton dominated systems with high ZF. Sedimentary cladoceran assemblages from dated cores for two sites, Felbrigg Lake (Norfolk, England) and Kenfig Pool (Glamorgan, Wales) were analysed. The cladoceran inferred ZF and MA indicated that both sites have undergone dramatic changes in ZF and MA. Comparison with plant macrofossil data and historical records at both sites indicated the veracity of the model.

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Eutrophication through phosphorus enrichment of lakes is potentially damaging to lake ecosystems, water quality and the ecosystem services which they provide. Traditional approaches to managing eutrophication involve quantifying phosphorus budgets. An important shortcoming of these approaches is that they take little account of the inherent heterogeneity of lakes. Furthermore, most studies of lake heterogeneity have been carried out in large lakes, a situation which reflects neither small lakes' imp0l1ance in biogeochemical cycling nor their significant contribution to the global sum of lake environments. This thesis reports investigations into heterogeneity in Esthwaite Water, UK, a small, temperate, eutrophic lake. The overarching aim of the work was to improve understanding of phosphorus fluxes and budgets in this type of environment. Heterogeneity, and its governing physical mechanisms were elucidated in the lake's surface waters and bed sediments. In addition, the effects that this heterogeneity had on phosphorus supply to phytoplankton from the main stream inflow and internal sediment sources were examined. The research is presented as four studies, which address surface water heterogeneity, bed sediment heterogeneity, and phosphorus supply via inflowing streams and from internal sediment storage. Significant heterogeneity was found in the surface water, despite the lake being small and the physical forcing, relatively weak. Assumptions about the physical processes contributing to sediment heterogeneity based on models of large or shallow lakes were found not 10 be applicable. Taking account of bed sediment heterogeneity was found to be important for the accurate calculation of burial rates of both total phosphorus and organic carbon, as ignoring it led to discrepancies up to 110%. Soluble reactive phosphorus (SRP) dispersal in the shallow transition zone at the mouth of the main inflowing stream, and was found to be the result of not only hydrological dilution but also biological uptake. Ignoring SRP heterogeneity in this zone led to up to an 18% underestimate of lake-wide averaged concentration during the growing season. SRP pathways in the lake were both spatially and temporally heterogeneous, resulting in large seasonal and inter-annual variations in phosphorus supply. Internal and external supplies were of similar magnitude during the summer but internal anoxic sources dominated in the late summer and autumn. Inter-annual variation in the hypolimnetic build-up of phosphorus associated with differences in lake stability and mixing strongly affected the internal phosphorus supply. Overall, the thesis concludes that spatial and temporal heterogeneity is a characteristic of this lake at many scales, despite the relative weakness of the governing physical forcing, and that it affects significantly not only the nature of the lake at specific locations, but also lake-wide averaged parameter values. Specifically, different phosphorus sources have distinctly different patterns of variability, which need to be taken into account when calculating phosphorus budgets. Finally, the importance of particular physical processes for phosphorus budgets is likely to differ between large and small lakes owing to the influence of basin morphometry and therefore understanding derived about these budgets in large lakes cannot simply be assumed when considering small lakes.

The focus of this research was to confirm or identify patterns in plankton biomass and composition as a function of total phosphorus (TP) concentration in Northern Irish lakes. Herbivory, lake thermal structure, and predation effects on lake trophy, plankton biomass, and community structure have been investigated in forty lakes sampled in 2005 and 2006. Mid-lake epilimnetic samples were collected and analysed using standard water chemistry analytical procedures. Surface phytoplankton samples, and zooplankton vertical haul samples were collected and preserved. Plankton were enumerated, sized, and identified. Data was transformed, and relationships were identified with LOWESS algorithms, and tested using linear regression and general linear modelling. Phytoplankton biovolume was a less useful measure of algal biomass than chlorophyll a (Chi) concentration in nutrient-algal biomass trend analysis. Total nitrogen (TN) concentration had less predictive power than TP concentration. Plankton biomass increased along the TP gradient, however there was no trend in the zoo: phytoplankton ratio along the TP gradient. The biovolume of chlorophytes and cryptophytes increased along the TP gradient. The contributions of Oaphnia sp. and cyclopoids increased along the TP gradient, whereas calanoid contribution decreased. The body length of Oaphnia sp. increased along the TP gradient. Zooplankton volume increased with phytoplankton biovolume, specifically chlorophyte biovolume. Concentrations of TP and Chi, and zooplankton volume increased with lake depth. The trophic state of shallow mixed lakes was higher than that of deep stratified lakes. Algal biomass was not significantly different between shallow mixed and deep stratified lakes. There was a greater contribution of cyanobacteria to total phytoplankton in deep stratified lakes than in shallow mixed lakes. There was no lake thermal structure or herbivory effect on the TP-Chl relationship. The presence of fish did not alter the size structure of zooplankton; however the presence of Chaoborus sp. shifted the size structure towards larger-sized species.

Improved understanding of the impact of catchment processes on freshwater carbon cycling would aid prediction of carbon flux attributed to changes in climate or land use. Carbon transfer through terrestrial and aquatic environments can be traced using stable carbon isotopic analysis (δ13C). A refined method of δ13C, free from host effects, is the utilisation of diatom-bound carbon (δ13Cdiatom)-

Eastern England is mostly rural with extensive arable farming. It is one of the driest areas of the UK, receiving approximately two thirds of the national rainfall average. The rivers in this region are generally considered sluggish with high nutrient content. This study examined the sources and seasonal pattern of phosphorus concentrations in these lowland rivers. Biological effect of phosphorus (measured as soluble reactive phosphorus, SRP) was investigated in algae; specifically the response of diatom species and periphyton biomass to elevated levels of phosphorus. The rivers in this part of the UK are routinely monitored by the Anglian Region of the Environment Agency. Routine monitoring data for SRP were used to identify high and low phosphorus rivers. Overall, concentrations ranged between <10 g 1-1 to over 10,000 g 1-1 SRP with values significantly skewed towards the lower end of this range. For 1991-5, 215 of sampled river stretches had 100 g 1-1 SRP or below, 49% between 100-500 g l-1 and 30% between 500-1000 g l-1. The ratio of nitrogen to phosphorus (total oxidised nitrogen to soluble reactive phosphorus by mass) for the upper reaches of eight rivers indicated that 83 % of sites on these rivers during summer months were potentially phosphorus-limited when the boundary ratio of 10:1 was applied. Effluent and geology were found to influence instream SRP concentrations. Rivers on chalk geologies had significantly lower phosphorus concentrations than those on other geologies. A significant relationship between effluent load and instream load was found in the small headwater streams of the River Welland and eight other rivers within the Region.