Algal‐Induced Biogeomorphic Feedbacks Lay the Groundwork for Coastal Wetland Development

Abstract

Ecosystem establishment under adverse geophysical conditions is often studied within the "windows of opportunity" framework, identifying disturbance-free periods (e.g., calm wave climate) where species can overcome establishment thresholds. However, the role of biogeophysical interactions in this framework is less well understood. The establishment of saltmarsh vegetation on tidal flats, for example, is limited by abiotic factors such as hydrodynamics, sediment stability and drainage. On tidal flats, raised sediment ridges colonized by algal mats (Vaucheria sp.) appear to accomodate high densities of plant seedlings. Such ridges were previously found to have higher sediment strength than substratum without algae. Here, we investigate whether these measurements can be explained by geophysical factors only, or that biological (Vaucheria-induced) processes influence tidal marsh establishment by forming stabilized bedforms. We performed two experiments under controlled mesocosm conditions, to test the hypotheses that (a) Vaucheria grows better on elevated topographic relief, that (b) the binding force of their algal filaments increases sediment strength, and that (c) Vaucheria consequently creates elevated topographic relief that further facilitates algal growth. Our experimental results confirm the existence of this algal-induced biogeomorphic feedback cycle. These findings imply that benthic algae like Vaucheria may contribute significantly to tidal marsh formation by creating elevated and stabilized substratum. This suggests biogeophysical feedbacks can "widen" the windows of opportunity for further ecosystem establishment. Our results could be useful for the design of managed realignment projects aimed at restoring the unique ecosystem services of coastal wetlands, such as habitat biodiversity, carbon sequestration potential and nature-based flood defense.Plain Language Summary Densely populated coastlines are exposed to flood risks due to sea-level rise and storms. Tidal marshes, sandy or muddy coastal plains colonized by plants, form a natural buffer zone that reduces flood risks. However, tidal marshes only form when plants manage to establish on unvegetated coastal plains known as tidal flats. Waves and currents wash away young plants and inundation by salt water limits plant growth. Hence, firmer sediment and higher sediment elevation are geophysical factors that increase plant survival. However, the biological processes affecting plant establishment are less well understood. In the field, we found plants concentrated on elevated sediment hummocks colonized by Vaucheria algal mats. We performed laboratory experiments, simulating tidal flat conditions, to investigate the role of these algae. We found that Vaucheria algae grow better on elevated hummocks and that hair-like algal filaments strengthen the sediment. We showed that sediment strengthening by Vaucheria creates elevated sediment hummocks, thus self-reinforcing algal growth. Our findings imply that, despite being small, algae like Vaucheria may promot...

Early indicators of tidal ecosystem shifts in estuaries

“…Hence, once initially small elevation differences have developed, the patterns tend to intensify through physical and biogeomorphic feedbacks 45,46 (Fig. 1) and ultimately prime the tidal flat for vegetation establishment 32,33,36 .…”

“…Effect of elevation & micro-topography on vegetation establishment. Next, we performed an analysis to demonstrate the facilitative role of micro-topography in vegetation establishment that differentiated it from the impact of tidal inundation (similar to an analysis performed in Fivash et al 33 ). In this approach, we identified newly established vegetation by comparing adjacent years of vegetation data, generally spaced 1-3 years apart.…”

“…Presently, studies aiming to predict the occurrence of intertidal vegetation rely mainly on the topographic elevation and local hydrodynamic forces 22,30,31 . However, small-scale morphological features on the tidal flat known as micro-topography have been found to create habitat uniquely suitable to the establishment of pioneer vegetation, in small-scale studies 32,33 . When micro-topographic features appear, the normally smooth surface of the tidal flat is restructured into a repeating pattern of meter-scale ridges and runnels, with a vertical relief usually no more than 10 cm 34,35 (Fig.…”

“…The establishment of vegetation in association with tidal flat microtopography has been the focus of a growing number of studies 32,33,39,40 . But to date, most of these studies focus on either de-embanked areas (polders that have been converted back to wetlands by being reintroduced to tidal inundation), or experiment with artificially created microtopography in order to develop applications for wetland restoration.…”

“…This has had the unintended consequence that the widespread occurrence of these patterns in natural settings has been downplayed and their role in large-scale natural vegetative expansion events, overlooked. Yet micro-topographic patterns may be a setting uniquely capable of driving the transition from bare to vegetated tidal flats through the widespread facilitation of marsh seedlings, rather than the more common, but much slower clonal propagation (as discussed in Fivash et al 32 & van de Vijsel et al 33 ).…”