Heterogeneity in Esthwaite Water, a Small, Temperate Lake: Consequences for Phosphorus Budgets
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Heterogeneity in Esthwaite Water, a Small, Temperate Lake: Consequences for Phosphorus Budgets Eleanor B. Mackay BSc. MSc. Lancaster Environment Centre Lancaster University Submitted to Lancaster University for the Degree of Doctor of Philosophy August, 2011 ProQuest Number: 11003693 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 11003693 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 Declaration I declare that this thesis is entirely my own work and has not been submitted for a higher degree at another institution or university. < Signed Date. \3rA\../2pi 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. Acknowledgements Trying to express my gratitude in words to the people who have helped and supported me through my PhD seems somehow inadequate when I think back over all of the hours of work (not to mention tea and cake) which have got me to this point. I would like to start with my supervisors, Ian Jones, Andrew Folkard and Philip Barker who have provided me with unending support, guidance, patience and encouragement throughout the process, not to mention the paid work to keep my finances on track! They have given me a fantastic grounding in the resilience required to do research work, particularly when there is just one more thing... I am also grateful to the Faculty of Science and Technology at Lancaster University for funding the PhD project. Another important thank you goes to the Centre for Ecology and Hydrology at Lancaster, particularly the Lake Ecosystem Group for trusting me with your facilities and equipment. In particular, I would like to thank Clive Woods, Kathryn Hockenhull and Mitzi de Ville for training me in the ways of phosphorus and chlorophyll a analysis. It is also vitally important that I thank all of the volunteers, who out of the goodness of their hearts helped me with my fieldwork, so thank you to Liz, Fanghua, Tom, Brian, Becky M, Becky J, Helen, Sam, Josh, Andy, Ruth, Jon, Jen C, Jen R, Emma, Dan, Mitzi, Steve, Rosie and Tracy. Finally, I would like to thank all the brilliant people in the Lake Ecosystem Group who kept me smiling when everything seemed like a struggle. In particular, Steve Thackeray and Mitzi De Ville, for just being fantastic. I must also pay a special thanks to my parents who have supported me through this work, even though it was time I got a proper job and Clym who has completely supported me, tolerated the angst and ensured that I have had a life outside work. This thesis was completed despite the fact that the sun was so often shining outside. iv Table of Contents Declaration ...............................................................................................................................i Abstract ...................................................................................................................................ii Acknowledgements ...............................................................................................................iv List of Figures ......................................................................................................................vii List of Tables ......................................................................................................................viii List of Papers ........................................................................................................................ ix Chapter 1 Introduction ...........................................................................................................1 1.1 Overview ............................................................................. 1 1.2 Aim and objectives .....................................................................................................3 1.3 Structure of thesis .......................................................................................................4 Chapter 2 Literature Review .................................................................................................6 2.1 Importance of lakes and freshwaters .........................................................................6 2.2 Water quality policy and eutrophication ...................................................................8 2.3 Phosphorus .................................................................................................................11 2.4 Consideration of heterogeneity ............................................................................... 24 2.5 Summary ....................................................................................................................33 2.6 Study Site ...................................................................................................................33 Chapter 3 Spatial heterogeneity in a small, temperate lake during archetypal weak forcing conditions ............................................................................................................... 37 3.1 Abstract ......................................................................................................................37 3.2 Introduction ............................................................................................................... 38 3.3 Methodology .............................................................................................................42 3.4 Results........................................................................................................................51 3.5 Discussion ................................................................................................................. 58 Chapter 4 Contribution of sediment focusing to