Depositional dynamics and vegetation succession in self-organizing processes of deltaic marshes

Abstract

Global deltaic marshes are currently facing a multitude of pressures, including insufficient sediment supply, rising sea levels, and habitat loss. Consequently, unraveling the internal regulatory mechanisms within deltaic marshes is of paramount importance. Here, we harness years of observational data and high-resolution numerical models to uncover depositional dynamics and vegetation succession in self-organizing processes of deltaic marshes. Our findings indicate that the colonization of salt marsh vegetation triggered a robust phase of growth in the initial stages of river deltas formation. However, as vertical accretion intensifies and inundation decreases, the delta is driven towards a state of critical slowing down due to insufficient sediment supply. We have captured a pivotal turning point in the evolution of deltaic marshes. In accordance with the critical submergence threshold we have established, when the inundation time of deltaic marshes exceeds 0.97 h/d, these salt marsh platforms sustain a higher annual growth rate. Conversely, when the inundation time of deltaic marshes falls below 0.97 h/d, the interannual accretion rate continues to decrease. Our research reveals that, in the absence of human disturbances, the deposition rate in deltaic marshes transitions from growth to decline. During this period, the delta undergoes an interesting succession of pioneer salt marshes (Suaeda salsa) and high-elevation salt marshes (Phragmites australis). Even without reductions in sediment input due to human activities, the vertical deposition rate within deltaic marshes will still shift from acceleration to deceleration under the influence of this internal negative feedback regulation. This adaptive capacity of marshes may foreshadow that when observing a slowdown in vertical accretion on deltaic marsh platforms, it cannot be solely attributed to reductions in sediment input caused by human activities.