SEMINAR: Physics-biology feedbacks in estuarine fine sediment dynamics

An increase in human activities in river catchments is resulting in increased muddiness of estuaries in many regions of the world. This increased muddiness has management implications, such as -increased turbidity and decreased quality of life for the human population, -storing pollutants (e.g. heavy metals) for decades to centuries and then liberating them when the mud is eroded or dredged, -changing some coasts from sandy to muddy, which is a significant and usually permanent environmental change -modifying the net nutrient budgets of estuaries - transferring seaward the occurrence of Harmful Algae Blooms if the light regime becomes the limiting factor in the estuary In recent decades, much of the knowledge of fine sediment (mud) dynamics, and its modelling, came from the engineering community. Modelling mud dynamics by engineers seemed so ‘simple’, the belief was that mud was just a messy fluid and that its behaviour could be modelled by adding a few equations for erosion and deposition to models calibrated in laboratory experiments with the belief that the models could then easily be ‘fine-tuned’ against some field data for the collection of which ingenious probes were designed. Experience has shown that these engineering models are unable to reproduce much of the field observations for muddy estuaries. A reason for this failure is that these models largely neglect the biology and chemistry, which has a major influence in controlling: • the settling of mud flocs • the resuspension of settled mud • the dewatering (consolidation) • the patchiness • the nutrient dynamics • the fluidization of mud by waves • the dynamics of tidal creeks

Pollutants in turn can modify these biological feedback processes and lead to another trajectory for the ecosystem health of estuaries. We quantify, using the LOICZ estuarine nutrient budget model modified for mud, the role of mud in the nutrient dynamics. Mud and muddy waters must be studied as a living body, not just a messy fluid as originally seen by engineers. This highlights research priorities to advance the knowledge of mud dynamics by quantifying the physics-biology links.