Salt marshes are important ecosystems that form a dividing line between land and sea. They are typically structured as a mud or sand flat that is colonised by salt-tolerant plants that can cope with being submerged twice a day, such as grasses, herbs and low shrubs. Their positioning forms an effective natural barrier that protects coastal cities from storm damage. They also have important environmental functions, supporting fisheries and sequestering carbon: essentially, storing carbon dioxide away in an immobilised form that prevents it from contributing to climate change. In fact, salt marshes are second only to tropical rainforests for the speedy rate at which they can do this. Salt marshes also form the cosy homes of numerous wildlife species, including fish, birds, crustaceans, worms and insects.
The loss of coastal salt marshes has proceeded rapidly in recent years, with global estimates of disappearances between 25-50%. Rising sea levels are generally known to contribute to this loss, but more specific local reasons are less clear. One new study analysed if an influx of nutrients, consistent with agricultural fertilisers leaching into water sources, could contribute to this process.
Over a period of 9 years, the team added nitrates twice a day during high tide into a test marsh landscape (the Plum Island Estuary, Massachusetts, USA) and observed how the overall ecosystem changed. This addition of extra nutrients, a process known as eutrophication, led to salt marsh integrity being undermined: more organic matter decomposed, plant water content was higher, plant roots grew less and some plants fell over. The combination of these changes led to tidal currents being able to degrade the low marshland, and without this low marsh support, the integrity of the high marsh subsequently failed.
This research suggests that as the human population continues to rise and food production increases to compensate, efforts must be made to protect salt marshes, which provide important ecological benefits.
You can read the original research article here.
Deegan LA, Johnson DS, Warren RS, Peterson BJ, Fleeger JW, Fagherazzi S, & Wollheim WM (2012). Coastal eutrophication as a driver of salt marsh loss. Nature, 490 (7420), 388-92 PMID: 23075989