Abstract
AbstractEcosystem 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.
Highlights
Coastal wetlands, which are found worldwide and provide valuable ecosystem services (e.g.,Arkema et al, 2013; Barbier et al, 2008; Temmerman et al, 2013; Zhu et al, 2020), are under increased anthropogenic pressure (e.g., Kirwan & Megonigal, 2013)
We found that Vaucheria grew better on sediment with raised topographic relief than on sediment without relief or with lowered relief (Figure 4)
Biogeomorphic feedbacks originating from the interaction between physical and biological processes play an important role in shaping coastal wetlands
Summary
Coastal wetlands, which are found worldwide and provide valuable ecosystem services (e.g.,Arkema et al, 2013; Barbier et al, 2008; Temmerman et al, 2013; Zhu et al, 2020), are under increased anthropogenic pressure (e.g., Kirwan & Megonigal, 2013). Most of these findings come from observations and measurements under natural field conditions, rather than controlled experimental conditions It remains unclear whether Vaucheria merely follow the pre-existing landscape topography by growing better on elevated relief, where the sediment is better drained and more stable (e.g.,Carling et al, 2009; Gouleau et al, 2000; O'Brien et al, 2000; Williams et al, 2008), or that these organisms play an active role in the formation of stabilized bedforms and might be an important biogeomorphic process to consider in the development of vegetated wetlands. Both experiments were performed under controlled mesocosm conditions that simulate sheltered intertidal field conditions
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