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Biomechanics of the Sediment Erosion, Transport, Deposition and Consolidation cycle

Final Activity Report Summary - BIOMECH (Biomechanics of the Sediment Erosion, Transport, Deposition and Consolidation cycle)

Sediment erosion and transport is critical to the ecological and economical health of the coastal zone. In the years to come, the beaches and coasts will be exposed to increasing hydrodynamic forces and higher frequencies of storm events due to climate change. There is now a consensus that bio-stabilisation -namely the effect of living organisms on sediments- is important in reducing sediment erosion. Pioneer works suggests that organic glues secreted by many invertebrates and bacteria (known as 'extracellular polymeric substances' - EPS) promotes bonding between sediment particles.

Although the urge to incorporate biological variables in sediment erosion and transport models is reflected in the increasing numbers of paper over the last years, there are still significant gaps in fundamental understanding of the role of living organisms in biostabilisation (e.g. EPS binding characteristics due to different producers or abiotic factors such as light, nutrients, salinity). This project concerns the effects of microbial polymers on sediment stability by addressing the engineering potential of marine bacteria which have been neglected so far in this context.

In an initial experiment, it was shown that a natural benthic bacterial assemblage stabilised the test sediment. Nutrient addition to bacterial cultures resulted in higher bacterial cell numbers, higher EPS concentrations and sediment stabilisation as compared with nutrient-depleted bacterial assemblages. Unlike previous studies, sediment stabilisation was more closely associated with the bacterial EPS proteins than with carbohydrates. It also became clear that a more sensitive device was needed to determine an early increase in sediment stability. Thus, we further developed an electromagnet to capture special particles added to the sediment. The strength of attachment of these particles was measured reflecting the binding forces within the sediment.

With the help of this sensitive device, bacterial stabilisation of the substratum could be shown after only one day. In this second experiment, the bacteria stabilised the sediment over the course of five weeks much more than other marine microbes such as unicellular algae. This experiment helps us to understand the general importance of bacteria for holding sediment in place in the natural world. In a third experiment, the combination of bacteria and microalgae resulted in the highest sediment stabilisation. The results suggest that the highest binding capacity in the mixed assemblage (bacteria and microalgae) is due to interaction of complimentary EPS secreted by bacteria and microalgae. In a fourth experiment, the bacterial stabilisation was shown to increase with salinity (from 5 to 35 PSU) along with shifts in bacterial assemblages and consequently, their EPS secretion. The results have implications for the binding capacity of microbial communities in different habitats, ranging from freshwater and estuaries to marine waters.

In the last months, two further experiments have been performed to investigate the influence of bacterial and microalgal EPS as well as the effects of different salinities on the characteristics of the eroded sediment flocs and thus their impact on lateral transport and deposition of the eroded flocs. In this context, the eroded floc sizes were measured as well as their floc strength and settling velocity using conventional methods (video and photography) and laser holography, but these data are currently under evaluation.

There is now a consensus that the natural biota often provides the important ecosystem function of 'biostabilisation' for depositional habitats. The improved understanding of this functional capacity is necessary to improve models of sediment dynamics and optimize coastal management strategies. The current studies showed that bacterial assemblages should not be neglected when considering microbial sediment stabilisation / flocculation and that a change in abiotic conditions (here represented by nutrients and salinity) can significantly affect the composition, the EPS secretion and thus the stabilisation potential of bacterial assemblages.