Impacts of watercress farming on stream ecosystem functioning and community structure Shaun Cotter School of Biological and Chemical Sciences Queen Mary, University of London Submitted for the degree of Doctor of Philosophy of the University of London September 2012 1 Abstract. Despite the increased prominence of ecological measurement in fresh waters within recent national regulatory and legislative instruments, their assessment is still almost exclusively based on taxonomic structure. Integrated metrics of structure and function, though widely advocated, to date have not been incorporated into these bioassessment programmes. We sought to address this, by assessing community structure (macroinvertebrate assemblage composition) and ecosystem functioning (decomposition, primary production, and herbivory rates), in a series of replicated field experiments, at watercress farms on the headwaters of chalk streams, in southern England. The outfalls from watercress farms are typically of the highest chemical quality, however surveys have revealed long-term (30 years) impacts on key macroinvertebrate taxa, in particular the freshwater shrimp Gammarus pulex (L.), yet the ecosystem-level consequences remain unknown. Initial studies were at Europe’s largest watercress farm at St Mary Bourne, Hampshire, during the bioremediation of its complex wastewaters and changes to farm management practices. These widened to include larger scale spatiotemporal studies at other watercress farms. Detrimental ecological impacts at the start of the study were detected by the structural and functioning measures, but they did not respond to bioremediation. However, an increase in G. pulex abundance was detected, providing evidence of recovery in response to altered practices, which may be attributable to the cessation of chlorine use. The detrimental impacts were unique to the St Mary Bourne watercress farm and were not consistent across the other watercress farms in the study. Our results demonstrate the importance of integrated metrics of both ecosystem structure and functioning, to derive a more comprehensive view of aquatic ecosystems 2 and highlights the difficulties associated with extrapolating from laboratory studies in response to stressors. Acknowledgements. I am deeply indebted to a number of people and organisations for their support and guidance over the last few years, from the projects inception one afternoon in Winchester to the completion of this PhD. I am extremely grateful to my first supervisor Dr. Guy Woodward for his unerring patience, enthusiasm (knowledge on fire) and assistance throughout the life of this project, without which this would not have been possible. Thank you also to Prof. Alan Hildrew my second supervisor and also to Guy for their comments, advice and discussions on the direction and scope of the work undertaken to underpin and present this thesis. I am very grateful to Graham Roberts of the Hampshire and Isle of Wight Wildlife Trust who put me in touch with Vitacress salads and their Technical Director Dr. Steve Rothwell, which allowed the initial research idea to become reality and for the project to begin. I am thankful to Vitacress who also allowed me unrestricted access to their watercress farms and for their financial help in the early stages of the project. I would also like to thank Shirley Medgett of the Environment Agency (EA) for access to her invaluable local knowledge, professional expertise, EA laboratory services and for financial assistance over the fieldwork stages of this project. I have had a number of friends assist and accompany me on my early morning sojourns for the collection of samples and the deployment of sampling arrays, I send a special thank you to Chris Dyer, Sue Philbey and Dave and Fergus Crawley. I also 3 thank Charmaine Grieger for her advice and assistance with Photoshop and the proof reading of an early draft of this thesis. And saving the best to last, I am extremely grateful to my parents Dr. John Leon and Marian Cotter for always providing the right environment, for their belief and for being there… 4 Table of Contents. Abstract............................................................................................................................. 2 Acknowledgements...........................................................................................................3 Table of contents...............................................................................................................5 List of tables......................................................................................................................8 List of figures...................................................................................................................10 List of plates....................................................................................................................12 1. General Introduction.................................................................................................13 1.1. Allochthonous pathways...........................................................................................16 1.2. The relative importance of decomposers and detritivores. ......................................18 1.3. Autochthonous pathways…......................................................................................19 1.4. Ecosystem functioning measures..............................................................................23 1.5. Chalk streams and watercress farming…….............................................................23 1.6. The problem at St Mary Bourne and other watercress farms and its remediation...27 1.7. Aims and structure of the thesis. .............................................................................35 1.8. References................................................................................................................39 2. Study Sites..................................................................................................................51 2.1. The Watercress farms...............................................................................................55 2.2. Study site…………………………………………………………………………..58 2.3. St Mary Bourne watercress farm, bed layout and operational detail………………62 2.4. Temporal study, Chapter 6.......................................................................................63 2.5. Spatial study, Chapter 6…........................................................................................63 2.6. References…............................................................................................................76 5 3. General Methods..........................................................................................................................78 3.1. Physical and chemical analyses of stream water......................................................78 3.2. Biological sampling…..............................................................................................79 3.2.1. Functional measures…..........................................................................................79 3.2.2. Structural measures................................................................................................88 3.3. Data analysis and transformations............................................................................88 3.4. References................................................................................................................91 4. Watercress production alters stream ecosystem functioning and community structure………….........................................................................................................93 4.1. Introduction………..................................................................................................93 4.2. Methods……….……...............................................................................................94 4.3. Results......................................................................................................................94 4.4. Discussion...............................................................................................................112 4.5. References..............................................................................................................115 5. Stream ecosystem functioning and structure metrics to assess a surrogate wetlands bioremediation of complex discharges at Europe’s largest watercress farm ..............................................................................................................................119 5.1. Introduction............................................................................................................119 5.2. Methods..................................................................................................................121 5.3. Results....................................................................................................................122 5.4. Discussion...............................................................................................................141 5.5. References..............................................................................................................146 6 6. The temporal and spatial responses of ecosystem functioning and community structure to watercress farming across Hampshire and Dorset..............................150 6.1. Introduction…........................................................................................................150