Advances in biophysical feedbacks and the resulting stable states in tidal flat systems

Abstract

<p indent="0mm">As an integral part of the coastal wetland system, tidal flats play an important role in maintaining the health of coastal ecosystems, resisting natural coastal disasters, and blue carbon sequestration. Tidal flats can provide suitable habitats for a variety of organisms; simultaneously, such organisms mediate hydrodynamic and sediment processes, hence affecting the geomorphic evolution and stability of tidal flat systems. In recent years, tidal flat systems have faced the comprehensive impacts of rapid human activities, frequent storm surges, and reduced sediment availabilities, which have aggravated tidal flat erosion. Severe erosion is accompanied by a critical biomass loss, especially under complex biological-physical interactions. Tidal flat degradation is often amplified, threatening the stability of tidal flat systems. The ultimate goal of ecological protection and restoration of tidal flat systems is to establish a self-sustaining method with less artificial assistance. To date, only a few ecological restoration projects in China have considered the integration of biological and physical processes in tidal flats. The most important reason is the lack of understanding of the basic laws of natural construction and operation of the systems. Therefore, understanding and giving way to the self-organization process driven by the biophysical interactions in tidal flat systems have become an important scientific and technical necessity in the ecological protection and restoration of tidal flats. First, we summarized the biophysical effects of salt marsh vegetation and benthic microalgae on the hydrodynamic and sediment movement process of tidal flats. Salt marsh plants and benthic microalgae change the hydrodynamic and sediment processes through wave dissipation and sediment stabilization. Conversely, the changing physical conditions affect the growth and development of salt marsh and benthic microalgae. Second, we qualitatively analyzed the bistability and catastrophic shift of a tidal flat system driven by the salt marsh-sediment and benthic microalgae-sediment feedback, both containing some similarities and differences. Specifically, driven by biophysical feedbacks, both the salt marsh- and benthic microalgae-sediment systems have alternative stable states; with the increased bottom shear stress, both systems shift from a state characterized by high biomass and sediment deposition to that with low biomass and sediment deposition; the bistable range of the salt marsh-sediment system is wider, and the system has a greater hysteresis, indicating that once degraded, it is more difficult for the salt marsh system to be restored to its original state. Third, we discussed the self-organized bio-geomorphic characteristics. The scale-dependent feedback between organisms and physical environments results in spatial patterns. The self-organized patterns have been widely found in a tidal flat ecosystem, for example, circular salt marsh clusters, salt marsh cliffs, and patterned mudflats characterized by the microalgae-covered hummocks alternating with water-filled hollows. Self-organized patterns can improve the primary productivity and sediment deposition of tidal flats, thereby enhancing the stability of ecosystems, which is of great significance for the ecosystem function of tidal flats. Finally, this paper proposed the critical scientific problems that still need to be solved in the future, such as the continuous field observation at the system level, the regulation of biological interspecific interactions under physical conditions, and the quantitative simulation of catastrophic shift thresholds in the tidal flat bio-geomorphic system.