Chapter 5 - Tidal flat transitions signaled by self-organized algal patterns Version 13

To understand the establishment of vegetated coastal wetlands, we combined mathematical modelling and a field study to investigate the emergence of algal-covered sedimentary ridges that constitute distinct spatial patterns on tidal flats. Strikingly, these patterned bedforms appear to facilitate the establishment of saltmarsh vegetation. We measured plant seedling densities in the field, which showed that seedling densities are higher on elevated bedforms colonized by the filamentous algae Vaucheria. Based on these field measurements and earlier laboratory experiments, we developed a mathematical model under the hypothesis that the bedform patterns are self-organized due to algal-induced scale-dependent feedbacks (filaments increase sediment strength, creating elevated ridges where algal growth is further improved; algal growth is inhibited adjacent to ridges due to scour and inundation). Model simulations confirmed this hypothesis, as they reproduced the wide range of observed Vaucheria patterns, varying from no cover to parallel ridges and from branching networks to full algal cover. Moreover, we used the model to explain the observation that large mudflat areas remain topographically smooth and bare (i.e., no algal mats nor plants) for years despite the development of algal-covered bedform patterns elsewhere on the mudflat at similar elevation. We found that the scale-dependent feedbacks make both the configurations with smooth mud and with algal bedforms highly resilient to changes in environmental conditions, such that these two configurations can coexist as alternative stable state under a range of mudflat elevations. Furthermore, we showed that changes in the spatial complexity of the algal pattern indicate imminent tipping points that lead to ecosystem collapse or establishment. Our study sheds light on processes that might be highly relevant for wetland restoration and conservation projects. Such projects could benefit from ecosystem engineers like Vaucheria and the signaling value of their spatial patterns.