Heterogeneity in Esthwaite Water, a Small, Temperate Lake : Consequences for Phosphorus Budgets.

MacKay, Eleanor B (2011) Heterogeneity in Esthwaite Water, a Small, Temperate Lake : Consequences for Phosphorus Budgets. PhD thesis, Lancaster University.

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Abstract

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' importance 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 to 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.