Tidal wetlands – remarkably adaptable ecosystems under pressure

Industrialised society has under-valued the marshes and mangroves that line our coasts. Somewhat ironically this “ugly duckling of ecosystems”, provides increasingly important services such as storm protection and carbon capture.  Tidal wetlands are also well-adapted to rising sea-levels. Unfortunately, when combined with a suite of other pressures, we are pushing these hardy ecosystems to their limits.

Coastal wetlands are simultaneously some of the most vulnerable and most economically important ecosystems on Earth*. Marshes and mangroves protect coastal regions from storms, sequester carbon, transform nutrients and provide the organic matter and nursery grounds that support commercial fisheries.

Marshes and mangroves occupy about 20 million hectares worldwide, and have been a prominent component of coastal and estuarine landscapes for at least 4,000 years. Over this period, the sea level has risen in most regions of the world by more than 2 metres.

An example of mangrove wetlands in the Everglades. Photo by: Riandi.

An example of mangrove wetlands in the Everglades. Photo by: Riandi.

To survive in such dynamic environments, a marsh or mangrove must build soil elevation at a rate faster than or equal to the rate of sea-level rise. Tidal flooding is one source of mineral sediment. Long periods of inundation and wetland plants increase the rate of sediment deposition.

Vegetation slows water velocity and also tends to stabilize the relative elevation and seaward extent of wetlands.  Plants also add organic matter to the soil surface.

The question is, can coastal wetlands continue to adapt to an increasing rate of sea-level rise? Despite robust feedbacks, observations of wetland deterioration in places such as the Mississippi River Delta indicate some wetlands are reaching their limits.

Unfortunately sea-level rise isn’t the only pressure on today’s coastal wetlands. Direct human modification is by far the major cause of historical and contemporary coastal wetland loss. Conversion of wetlands into other land uses including agriculture and aquaculture claimed roughly 25–50% of the world’s coastal wetlands during the twentieth century alone.

This satellite view of Chesapeake Bay shows how coastal wetlands have been converted to farmland. Photo by NASA.

This satellite view of Chesapeake Bay shows how coastal wetlands have been converted to farmland. Photo by NASA.

Human activity also indirectly threatens the survival of coastal wetlands. Regions of the world with drastic wetland deterioration occur mainly where humans have accelerated subsidence rates and/or decreased sediment delivery rates to the coast.

In places like coastal Louisiana and Venice, groundwater withdrawal and artificial drainage of wetland soils contribute to rapid subsidence. This means 8 of the world’s 20 largest coastal cities experience relative rates of sea-level rise that greatly exceed any likely climate-driven projection.

Venice lagoon  - one coastal area suffering subsidence as a result of altered wetland processes. Photo by S. Linke.

Venice lagoon – one coastal area suffering subsidence as a result of altered wetland processes. Photo by S. Linke.

Humans are also reducing the amount of sediment input to wetlands. Dams and reservoirs capture about 20% of the global sediment load while reforestation and agricultural sediment-control practices reduce the amount of sediment produced.

Historical strategies for protecting coastal property have favoured use of structures such as dykes and sea walls. These also contribute to wetland loss through ‘shoreline squeeze’, in which erosion removes the wetland area at the margin and structures prevent the addition of area by migration onto adjacent uplands.

The future of tidal wetlands can be helped by informed decision making. Understanding the different drivers of wetland maintenance can lead to innovative solutions. For example, limited conversion of mangroves to shrimp ponds may be possible while maintaining certain ecosystem services such as wave attenuation .

The non-market value of wetland ecosystem services – such as their substantial capacity to store and retain carbon – is also being used to promote the conservation, restoration and creation of coastal wetlands.

*The full article on which this post is based is:

Tidal wetland stability in the face of human impacts and sea-level rise.  By  M.L. Kirwan, and J. P. Megonigal, Nature 2013, 504 (7478), 53-60.

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