Julie Hope – CBESS (2012

Biological stabilisation of mixed and cohesive sediments

My PhD project is tied to the NERC funded COHBED project (COHesive BEDforms), which together aim to investigate the influence of biological processes on the behaviour of cohesive and non-cohesive intertidal sediments. Understanding the physical and biological influences on sediment stability is increasingly important with regards to coastal erosion and management, however many sediment transport models are still based on abiotic sediments.

Figure 1: a) visible biofilm on the surface of a mixed sediment in the Eden estuary b) Low Temperature Scanning Electron Micrscopy (LTSEM) image of a biofilm illustrating the microbial community and EPS matrix surrounding grains c) LTSEM image of microbial community of diatoms and cyanobacteria in a matrix of EPS.

It is well established that microbial assemblages found on the sediment bed and the extracellular polymeric substances (EPS) they secrete (Figure 1) can significantly stabilise sediment. A greater understanding of these processes and the role of bioturbation and feeding of larger organisms is required in order to better predict the behaviour of intertidal sediment.

Figure 2: COHBED partners measured biological and physical characteristics of different sediment beds by deploying instruments on rigs in order to capture information during high tide and sampling during low tide.

Field surveys include (Figure 2) an interdisciplinary field campaign in the Dee estuary, Liverpool involving all COHBED partners; University of St Andrews, NOC Liverpool and Plymouth, Hull, Bangor universities. This campaign allowed an array of physical and biological measurements to be taken from the sediment bed and water column with the continuous recording of sediment bed and suspended sediments during tidal immersion. The data from this campaign is currently being processed.

This compliments a long term field campaign in the Eden estuary, Scotland. The relative influence of biological and physical factors are measured across all seasons and under different tidal conditions. Sediment samples are collected using the contact core method to flash freeze surface sediments in situ, allowing the biological characteristics of the sediment, including microbial biomass (Chlorophyll a) and EPS (carbohydrate concentration) to be related to sediment stability and several other factors.

In addition to the field studies, laboratory flume experiments are carried out using mixtures of clean sand, clay and xanthan gum (as a proxy for EPS). This allows us to investigate the influence of EPS on the behaviour of different sediments under flow without the complexity of natural systems. Increasing complexity will be added to these experiments to investigate the effects of bioturbation and feeding behaviours and experiments with natural mud will add contextual information.