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Assessing the Carbon Sink Potential and Impacts of Global Change on Intertidal Seagrass Meadows in Central Southern England Mari

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Assessing the Carbon Sink Potential and Impacts of Global Change on Intertidal Seagrass Meadows in Central Southern England Mari Assessing the carbon sink potential and impacts of global change on intertidal seagrass meadows in central southern England Mariana do Amaral Camara Lima A thesis submitted in part fulfilment of the requirements of the University of Brighton for the degree of Doctor of Philosophy March 2020 The University of Brighton Abstract Seagrass meadows provide a multitude of ecosystem services, including the capacity to sequester carbon within their sediments and biomass. However, seagrass research in the UK is still in its infancy, with very few published data on sediment carbon stocks, and no reports of sediment accretion and carbon sequestration rates within intertidal seagrass meadows. In addition to their carbon sink potential, seagrass meadows are also important habitats for commercially important and endangered fish species in the UK. However, frequent reports on the decline of their health and extent, have raised concerns on the efficiency of current protection and management projects, especially in the face of global climatic change. Therefore, this research provides evidence of the importance of intertidal seagrass meadows in England for climate change mitigation and the need for more robust conservation strategies for UK seagrass meadows, through these objectives: (i) to establish a regional carbon storage profile by determining aboveground and belowground biomass and carbon content of the sediments. Results showed that seagrass meadows from central southern England form significant carbon stocks, comparable to other global regions. In addition, this study also demonstrates the variability in sediment carbon stocks, sediment characteristics, above-ground biomass, shoot density, and below-ground biomass, in the form of roots and rhizomes, between the studied sites, and between seagrass meadows and neighbouring, un-vegetated, sampling points, highlighting the need for site specific assessment. (ii) to establish relationships between carbon storage and environmental factors to promote the understanding of the features that influence seagrass carbon sink potential. Results showed that the main factors significantly related to seagrass sediment carbon stocks were: elevation in relation to mean sea- level, pore water sulphates, pH and salinity; and sorting coefficient, grain size, proportion of mud and dry bulk density. Moreover, sediment characteristics such as dry bulk density, sorting coefficient and proportion of mud, can be grouped as the factors that act in conjunction to explain the bulk of the variation in sediment carbon stocks. (iii) to establish a geochronology to identify how organic carbon sequestration has fluctuated over time in relation to reported precipitation, storminess, and sea level. Results showed that there was significantly higher sediment i accretion and carbon sequestration rates during periods with extreme weather events, suggesting that future climate change is likely to impact intertidal seagrass meadows and their role as blue carbon sinks. This study has shown that intertidal seagrass meadows in central southern England are essential providers of carbon storage benefits, comparable to estimated global sediment carbon stocks, while also providing robust evidence on the influence of environmental factors and direct or indirect human activities, such as climate change events, on their carbon storage and sequestration potential, essential for the development of effective governance and management of these ecosystems. ii Contents List of Figures ..................................................................................................................vi List of Tables .....................................................................................................................x Acknowledgements ........................................................................................................xi Author’s Declaration .....................................................................................................xii 1 General Introduction................................................................................................1 1.1 Research Context................................................................................................. 1 1.2 Research Aim and Objectives ........................................................................... 4 1.3 Research Approach ............................................................................................. 5 1.4 Thesis structure ................................................................................................... 7 1.5 Nomenclature......................................................................................................... 8 2 Literature Review ......................................................................................................9 2.1 Global Climate Change ......................................................................................... 9 2.2 Climate Change and Marine Environments...................................................... 11 2.3 Carbon Cycling and Coastal Vegetated Environments .................................. 13 2.3.1 Blue Carbon .................................................................................................. 17 2.4 Seagrass ............................................................................................................... 21 2.4.1 Seagrasses in the UK .................................................................................. 29 2.5 Summary .............................................................................................................. 31 3 Study Area and General Methods .................................................................... 32 3.1 Study Area ............................................................................................................ 32 3.1.1 Geology and Sedimentation ....................................................................... 39 3.1.2 Hydrology...................................................................................................... 40 3.1.3 Sampling Sites.............................................................................................. 44 3.2 General Methods ................................................................................................. 53 3.2.1 Statistical analyses ...................................................................................... 54 3.2.2 Carbon stocks Analyses ............................................................................. 56 3.2.3 Particle Size Analyses ................................................................................. 63 3.2.4 Pore Water Analyses ................................................................................... 65 3.2.5 Carbon sequestration rates ........................................................................ 66 4 Estimation of carbon stocks .............................................................................. 70 4.1 Preamble............................................................................................................... 70 4.2 Introduction.......................................................................................................... 70 4.3 Methods ................................................................................................................ 74 4.3.1 Sampling Sites.............................................................................................. 74 4.3.2 Field methods ............................................................................................... 76 4.3.3 Laboratory methods .................................................................................... 77 iii 4.3.4 Statistical analyses ...................................................................................... 81 4.4 Results .................................................................................................................. 82 4.4.1 Above-ground biomass ............................................................................... 84 4.4.2 Below-ground biomass ............................................................................... 86 4.4.3 Dry Bulk Density........................................................................................... 88 4.4.4 Particle Size Analysis .................................................................................. 89 4.4.5 Sediment Carbon Stocks ............................................................................ 91 4.4.6 Comparison between seagrass and un-vegetated sediment organic matter and carbon content ........................................................................................ 98 4.5 Discussion.......................................................................................................... 102 4.5.1 Living Biomass ........................................................................................... 102 4.5.2 Particle size and sediment density .......................................................... 105 4.5.3 Sediment Cstock............................................................................................ 106 4.6 Conclusions ....................................................................................................... 111 5 Environmental factors as predictors of carbon stocks in seagrass sediments ...................................................................................................................... 113 5.1 Preamble ...........................................................................................................
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