Saltmarsh change

Salt marshes are among our most valuable coastal ecosystems, providing habitat for wildlife, protection against storms, and capturing carbon. Yet, these precious landscapes are highly sensitive to change. Recent focus has been on the threat sea level rise poses to marsh survival. However, our research indicates another critical player in determining whether marshes grow or disappear: sediment supply.

To understand these dynamics, we looked at over 150 years of historical maps and aerial photographs across 25 estuaries and around 100 salt marshes in Great Britain. We found a striking geographical pattern: marshes in the north and west had expanded significantly over time, while those in the south and east were shrinking.

What explains this contrast? The answer lies in sediment – the mud and sand that marshes need to grow. Sediment is delivered to marshes by rivers and tidal movements. Where sediment supply is plentiful, marshes can build upwards and outwards, maintaining their presence even as seas rise. But if sediment supply diminishes, marshes become vulnerable to erosion from waves and storms.

This study identified sediment supply as the most significant factor affecting the long-term growth or erosion of marshes across Britain. The researchers discovered that northern and western marshes benefited from ample sediment delivered by stronger tidal flows and moderate wave action, allowing these marshes to steadily expand.

In contrast, marshes in southeast Britain faced reduced sediment inputs. With less sediment available, these marshes have struggled to keep pace with erosion from increasing wave action and rising seas. Notably, human interventions, such as coastal construction or invasive species, had limited long-term effects compared to sediment availability. Temporary expansions due to invasive marsh plants or engineered sea defences eventually gave way to the overriding influence of sediment supply.

This research challenges the traditional focus solely on sea level rise, highlighting sediment supply as a crucial factor in coastal management. In fact, marshes may continue to vertically accrete (build up) even while losing ground at their edges, demonstrating that vertical growth alone does not fully reflect marsh health.

Globally, sediment supplies to coastlines are declining due to factors such as damming rivers, which traps sediments upstream. The implications are clear: without sufficient sediment, marshes worldwide risk substantial erosion, compromising their resilience to sea level rise and climate change.

Managing sediment supplies effectively might therefore be key to preserving salt marshes and the myriad benefits they offer. Strategies such as restoring sediment flow through rivers or careful coastal engineering projects could enhance marsh resilience, providing a nature-based solution against the impacts of climate change.

This study makes a compelling case for monitoring and managing sediment as a vital, yet often overlooked, component of coastal conservation. After all, the future of our coastlines might literally depend on mud and sand.