Chapter 4 - Algal-induced biogeomorphic feedbacks lay the groundwork for coastal wetland development Version 9

Coastal wetlands provide valuable ecosystem services, including biodiverse habitats, carbon sequestration potential and nature-based flood defense. The establishment of saltmarsh vegetation and other key wetland species on bare tidal flats relies on windows of opportunity, which are traditionally linked to physical factors such as hydrodynamic conditions, substrate stability and drainage. However, observations on tidal flats suggest that plant seedlings settle in high densities on raised sedimentary ridges colonized by mats of primitive filamentous algae (Vaucheria sp.). Such algal-covered ridges were previously shown to have higher sediment strength than substrate without algae. Here, we investigated whether these measurements can be explained by physical processes only, or that biological (Vaucheria-induced) processes influence early saltmarsh establishment through the formation of stabilized bedforms. We performed two experiments under controlled mesocosm conditions, to test the hypotheses that i) Vaucheria grows better on elevated topographic relief, ii) the binding force of their algal filaments increases sediment strength, which consequently iii) maintains and creates elevated topographic relief on which algal growth is further facilitated. The existence of this algal-induced biogeomorphic feedback cycle was confirmed in our experiments. Our findings reveal that halophytic plants are not the only important ecosystem engineer in coastal wetlands, but that fast-growing, mat- forming species like Vaucheria also play a major role by creating stable and elevated sedimentary relief. Our study indicates that biota can widen the windows of opportunity for ecosystem establishment, which can be utilized to optimally design managed coastal realignment projects that aim at the restoration of coastal ecosystems.