A hydrological switch drives the transition from saltmarsh to reedland ecosystem

Coastal peatlands are vital freshwater biogeomorphic ecosystems that accumulate organic material and store carbon over millennia. Despite their crucial role in carbon accumulation, peatland initiation under brackish or even saltwater conditions is still poorly understood. Geology highlights that transitional reed-marsh phases often precede the hydrologically isolated ombrotrophic peatlands [2] suggesting biogeomorphological shifts in the saltmarsh feedbacks. This study investigates how Phragmites australis (reed), a precursor species for coastal peatlands, establishes on saltmarshes causing new interactions at the local scale that generate a hydrological switch that alters the landscape.

Methods: First, based on a saltmarsh development numerical model (SFERE) [1] we simulated the interactions between vegetation, hydrology, and sedimentation processes during the reed establishment and expansion. Model results were analyzed calculating hydrological metrics (topographic wetness index, residence time) to assess the new system state drainage conditions. Our simulations were compared with field observations from reed-established and saltmarsh areas of Saeftinghe. Field data included vegetation as well as elevation remotely sensed time series to assess the
clogging influence of reed vegetation on the tidal creek system and salinity measurements to identify the reduction of tidal influence within reed patches.

Results and conclusions: Our combined theoretical and empirical results highlight that a hydrological switch triggered by the positive feedback between reed invasion, reduced creek network complexity, and reduced saltwater incursion, drives the transition from saltmarsh to reed-land ecosystems. This process could be a key factor for the establishment of peatlands in the past, as well as for carbon accumulation in reed-based peatlands in the future.
Model results (Figure 1) show that accumulation of roots and organic matter by the reed vegetation reduces the complexity of the creek network, and leads to a higher, more homogeneous freshwater landscape that further facilitates reed growth. The flattened topography and reduced drainage density of the landscape eventually increase the residence time of flow and the surface wetness propensity.
These findings align with the field observations and sequential airborne laser altimetry from Saeftinghe. There is persistent reed expansion, and within the reed patches the significant creek elevation increase leads to altered hydrological conditions and reduced surface soil salt content.