Living organisms can have a major impact on their environment. In particular in intertidal areas of estuaries and coasts, where high densities of organisms are found, impacts on various physical characteristics are known. The objective of this project is to investigate the influence of biogeomorphological processes to macro-mega (whole estuary) scale sediment balance. Full description Sediment deposited in the Wadden Sea originates from the rivers and the sea. The exchange of water between the Wadden Sea and the North Sea is the driving force of sediment exchange between both water systems. The processes that define the resulting sediment balance for different seasons and between years are physical, chemical and biological. The influence of biological processes on the sediment balance and sediment distribution within the Wadden Sea and between Wadden Sea and North Sea is unknown and possibly relevant. In the last decade research on the influence of ecosystem engineering organisms on morphodynamics has been initiated. This research has focused on the micro-meso scale of individual mudflats. Some data suggest that biological processes are having an impact at the macro-mega-scale of whole estuaries, through increased roughness, altered sediment stability and changed sediment composition and -distribution. At present, these macro-mega scale influences of organisms are not researched thoroughly. The objective of this project is to quantify the influence of species that alter their physical environment (ecosystem engineers) on the macro-mega scale sediment balance and sediment distribution. This will be realized by studying the relationships between spatial distribution and temporal variability of these species, the values of physical parameters that are affected and the resulting change in sediment fluxes, that constitute the sediment balance. To achieve this, existing field data will be analysed, a mathematical model will be developed and laboratory experiments will be executed.