title: Developing a novel method for quantifying the long-term phosphorus budget at Crose Mere, UK, using the lake sediment geochemical record
Creators: name: Moyle, Madeleine
Publisher: name: University of Liverpool Repository
Year: 2022

description: Effective management of lakes and their catchments is key to reducing excess nutrient inputs and reversing the global deterioration in water quality and overall ecosystem health. Restoration of these systems requires the development of suitable methods to understand and assess the drivers of these impacts, and to better quantify the scale of more recent (post 1850 AD) increases in lake water nutrient concentration. Reference values play a central role in the management of these nutrient-impacted lakes, representing the lake system conditions that would be expected in the absence of significant human impact. Palaeolimnological records have made a valuable contribution to defining these reference values. Much of this work has focused on fossil assemblages in the sediment record, particularly diatoms but also plant macrofossils and fossil invertebrates. These have been used for understanding changes in past community structures and for reconstructing (pre-)historic nutrient levels using ecological transfer functions. However, there are fundamental gaps in our understanding that limit how lake sediment geochemical phosphorus (P) records could be used to contribute towards the broader understanding of changes in past and present lake water quality. Here, a methodological framework for reconstructing past lake water total phosphorus (TP) concentrations based on the sediment geochemical record is developed. This comprises a sediment P model that is based on existing mass balance principles. The newly developed model is applied to a set of published sediment P records, assembled as part of this project, to examine the variability and probable controls over catchment-scale changes in Holocene P across the Northern Hemisphere. The individual site trajectories of reconstructed Holocene mean landscape P yield and lake water TP are found to vary systematically. Three distinct traits are apparent, with differences attributable to landscape development history, in turn driven by climate, human impact and other local factors. This synthesis is the first attempt to reconstruct values for average landscape P yield and lake water TP concentration for the Northern Hemisphere over the Holocene. The approach shows that long-term knowledge of individual site development is essential to understanding present day terrestrial P cycling and lake water phosphorus enrichment. The findings from the synthesis provide the context for an in-depth examination of the long-term P record at the case study site of Crose Mere, a small eutrophic lake in Shropshire (UK). Crose Mere was identified as a priority site by Natural England, the PhD project partners, as poor water quality at the site is a threat to its protected ecological status. The site is studied using a combination of lake and catchment monitoring, multi-proxy lake sediment analysis (geochemistry, bulk sediment composition, fossil pigments, pollen), and modelling using mass balance and export coefficient approaches. Application of the sediment P model to the recent sediment record from Crose Mere produces a lake water TP record that agrees strongly with with both monitored TP concentrations and an independent published diatom inferred TP record, providing further evidence that mean lake water TP can be inferred from sediment P records. Extending the approach back further in time, the newly-dated Holocene sediment record from Crose Mere shows P enrichment from ca. 9000 BP, with firm evidence human impact on lake water TP concentrations from at least ca. 6600 BP. The reconstructions suggest the lake may have become eutrophic as early as the Iron Age, reaching TP concentrations comparable with those in the present day following a significant shift in vegetation cover, the introduction of cattle, and increased erosion in the catchment. The clear link between human activity and the increase TP demonstrates that the widely held view that Crose Mere is naturally eutrophic can be rejected. This also highlights the need for a millennial scale perspective truly capture pre-disturbance lake conditions. A comprehensive monitoring programme shows that the lake P budget balances, suggesting that the major sources of P in the catchment have been identified. Together with the sediment evidence this forms an empirical basis for quantifying the P budget under a range of land use scenarios using an export coefficient model. The modelling approach combined with the long-term perspective gained from the sediment record suggests that it may not be possible for lake water TP to return to pre-disturbance levels. However, it may be possible to achieve water quality targets at Crose Mere if the direct contribution from cattle and the human contribution via septic systems are removed. Overall this thesis shows that lake sediment P records can make an important contribution to the continued development of palaeolimnology as a tool for informing lake management decisions, by providing the long-term perspective needed to fully contextualise present day lake status.