The roles of biological factors in coastal sediment transport: A review

Abstract

<p indent=0mm>Coastal sediment transport and geomorphological evolution are important scientific focuses in the research field of land-ocean interaction. The feedback between various factors such as hydrodynamics, extreme climate conditions, biological effects, human activities results in complex ecological and environmental characteristics and poses a great challenge to the development and protection of coastal areas. Closely related to the evolution of coastal geomorphology, the stability of coastal sediment is mainly affected by sediment properties and associated dynamics. Traditionally, it is considered that the change of sediment characteristics and dynamics is greatly driven by physico-chemical factors such as particle size distribution, water content and mineral composition and so on. However, the impact of various active biological factors is generally neglected. Over the past decades, with the deepening understanding of coastal ecological environment, more attention has been paid to the critical roles of biological factors in coastal systems. A large amount of field observation data and laboratory-controlled experiments demonstrated that organisms inhabiting coastal areas contribute a lot to sediment erosion, transportation, deposition, consolidation processes in direct or indirect ways. For example, the cohesion between sediment particles could be increased by microorganisms through secreting extracellular polymeric substances (EPS) so as to enhance sediment stability; salt-tolerant vegetation has an ability to improve the stability of the sediment bed by capacity of sediment retention and reducing hydrodynamic force; the structure of sediment bed could be destroyed by coastal benthos through feeding and burrowing, which would lead to the failure of the bed. On the one hand, these multiple biological effects endow sediment particles with “biological characteristics”, gradually modifying sediment characteristics and sediment dynamics; on the other hand, they try to strengthen or weaken external erosive forces by remolding sedimentary or dynamic environment. Finally, a more stable or unstable coastal biological sedimentological system different from the traditional physico-chemical sedimentological system is formed as a result of the interaction between these diverse biological factors. This enhances the cognition of traditional coastal sediment dynamics, and relevant research is gradually becoming a research hotspot. Understanding the roles of biological effects in coastal sediment dynamics and evolution of coastal geomorphology is helpful for enriching theory of coastal sediment dynamics, promoting the intersection of coastal dynamics, sediment dynamics and ecological environmentology, and promoting the development of the research field “biomorphodynamics”. Focusing on the role of biological factors in the coastal sediment dynamics, this paper reviews the state-of-the-art research on the distribution characteristics of salt-tolerant vegetation, benthos and microorganisms in the coastal area, their contribution to coastal sediment movement as well as the underlying mechanisms. At present, the research on the influence of biological factors on the coastal sediment dynamics is still in the initial stage, and many of the research conclusions are relatively loose, mostly qualitative at macro-level, and a complete theoretical framework has not yet been established. The mechanism of the interaction between coastal sediment and biology, as well as the establishment of simulation models of sediment dynamics and morphological evolution will be the focus of future research.