The missing ingredient: biological snot!

Our climate is changing and sea-level is rising but the impact this will have on our coasts and estuaries remains very uncertain. This is because our ability to predict how such systems will respond to changes in wave and flow forces are limited by an incomplete picture on how sediments, such as sand and mud, move within these environments. A paper out today in Nature Communications aims to improve these predictions.

One of the authors, a PhD student in Geography, Environment and Earth Sciences, Leiping Ye explains that “The movement of sediments along the coast and in estuaries and rivers is controlled by the dynamics of small scale ripples, like the ones you see at the beach when the tide goes out.”

Present predictions of such features rely almost completely on experiments in clean-sand, despite most environments – such as estuaries – being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) – that are generated by microorganisms.

The paper presents novel experimental data showing that the pervasive distribution of low levels of EPS within sediments is a key control on ripple formation and dynamics, with ripple development time doubling even with extremely small quantities of EPS. This effect is far stronger than for physical cohesion produced by clays, because EPS inhibit sand grains from moving independently. The results show that our present bedform predictors, that are used in models of the impact of climate change, are overly simplistic and require re-assessment.

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