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The response of anaerobic prokaryotic processes and communities in Severn Estuary sediments to environmental change

Thomas, Shaun 2014. The response of anaerobic prokaryotic processes and communities in Severn Estuary sediments to environmental change. PhD Thesis, Cardiff University.
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The Severn Estuary in the south-western UK is one of the most tidally dynamic environments on the planet. However, despite this the sediments of the estuary remain relatively understudied with regards to their biogeochemical potential. The aim of this project was to investigate how the constantly changing sedimentary environment in the estuary, in which millions of tonnes of sediment are eroded and deposited over the tidal cycle, affects the prokaryotes within the sediments and the processes they control, and also to determine what effect environmental changes in the estuarine system might have on these processes. The study showed that the sediments of the Severn Estuary have high rates of sediment oxygen demand (SOD) indicating a high degree of organic matter (OM) degradation. However, the sediments have low rates of the anaerobic processes that are expected to dominate in shallow marine systems (e.g. sulphate reduction and methanogenesis), suggesting that most OM degradation must be linked to processes further up the redox cascade. The sediments also showed a lack of microbial guild depth zonation, with methanogenesis occurring above or alongside sulphate reduction. Both of these unusual factors can be linked to the regular re-suspension of the estuary’s sediments by tidal action, resulting in large-scale oxidation and mixing of the sediment column and the suppression of anaerobic processes while potentially stimulating aerobic and dysaerobic activity. This same mixing would also distribute guilds of organisms throughout the sediment, creating isolated pioneer populations with a general lack of competition. Re-suspension is also likely responsible for the high cell counts that persist to significant depth around the estuary, as the mixing of sediment and entrainment of OM would produce high and homogeneous cell profiles upon deposition, which in turn can be linked to the high SOD of the estuary’s sediments. Despite this dominance of aerobic and dysaerobic processes throughout most of the estuary some isolated sites do show increased rates of anaerobic processes, particularly at locations that have undergone significant environmental change. These include fluidised mud pools in the deeper areas of the estuary, salt marsh peat deposits at St Brides Wentlooge (especially within the “activated interface”) and Cardiff Bay, an anthropogenic lake and former mudflat environment which shows significant methanogenic potential. Overall this study has shown that the dynamic conditions in the Severn Estuary promote the activity of aerobic and dysaerobic prokaryotic groups over the anaerobic groups traditionally thought to dominate in shallow marine sediments. However, this promotion is not uniform across the estuary, instead varying with topography/bathymetry and the degree of sediment disturbance.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Q Science > QR Microbiology
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:52

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