Logo Utrecht University

Brunings Lecture

2017

2017 – Watch the full lecture

Dr Sergio Fagherazzi, Geomorphology, hydrology, and coastal and marine geology, Boston University

Non-Linear dynamics determine the fate of salt marshes

The potential of salt marshes to serve as natural buffers against violent storms seems even more important in view of significant threats imposed by climate change, such as increased storminess and higher hurricane activity registered in the past decades. Unfortunately, Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. Sea level rise is often viewed as the main driver of salt marsh deterioration.

However, while salt marshes can reach equilibrium in the vertical direction, they are inherently unstable in the horizontal direction. Marsh expansion driven by sediment supply rarely matches lateral erosion by waves, creating a dynamic landscape. Recent results show that marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is higher than the input of sediment from nearby rivers or from the continental shelf.
We propose that the horizontal dynamics and related sediment fluxes are key factors determining the survival of salt marshes. Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Moreover, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans.

 

Dr. Tjeerd J. Bouma, Estuarine and Delta Systems, Royal NIOZ

Small-scale process as driver of large-scale dynamics in coastal vegetation

 

There is a growing desire to manage (and even create) coastal vegetation such as e.g. salt marshes, mangroves and seagrasses for coastal defense. Such application however requires in depth understanding of the dynamic horizontal extent (i.e., width) of these ecosystems. Especially understanding the factors affecting the minimum vegetation width is important.
This presentation will highlight how process-based studies can help to provide insight in which factors affect the long-term large-scale development of salt marsh and other coastal vegetation. Recently it was found that vegetation establishment can be described by the Windows of Opportunity theory. Having this mechanistic understanding enables us to develop means to restore coastal ecosystems. Moreover, it allows us to gain a basic insight in which factors determine the minimum-width of a salt marsh, and how dredging material may potential be used to initiate marsh growth.
Recent insights explaining that the short-term vertical sediment dynamics on the bare tidal flat is a key driver of the lateral vegetation dynamics, emphasizes that we should start with continuous monitoring of such sediment-dynamics. The vegetation response to the short-term vertical sediment dynamics can however be highly species specific, resulting in species-specific large-scale ecosystem dynamics. Experimental process-based studies remain of key importance for understanding ecosystem dynamics in addition to the rapidly developing earth observation techniques and modelling capabilities.