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Eutrophication leads to salt marsh deterioration - October 30, 2012

It has been thoroughly documented that many different anthropogenic and natural causes contribute to the decay of salt marshes; however a recent study revealed that eutrophication, the input and accumulation of excessive nutrients in the water, can lead to even more rapid deterioration of these valuable wetlands.

A study done by the Plum Island Ecosystem Long-Term Ecological Research Program (PIE-LTER), supported by the National Science Foundation (NSF), found that adding nitrogen and phosphorous to an isolated salt marsh area resulted in the local ecosystem’s decay and ultimately ended with most of the marsh area being washed out by tidal action.

At first this study seems counterintuitive; by adding more nutrients to the marsh one would assume the local vegetation would thrive which in turn would attract more animals. As it turns out, adding significantly more nutrients overloads the marsh system and results in a much less productive ecosystem.

The study, conducted at Plum Island in Massachusetts over a nine-year span, was designed to observe and monitor a salt marsh system over a long time period with nutrient enrichment levels identical to levels seen around large, densely populated and developed areas. The study was done using two salt marshes, one being a control where no extra nutrients were added and the second was the subject area where the researchers could periodically add more nutrients to obtain the levels they wanted. The purpose of the study was to see the effects of eutrophication on salt marshes over a prolonged period of time and compare this with other studies done on eutrophication of salt marshes and other wetlands.

This study was the longest of its kind, which was ultimately beneficial because initially the health of the marsh looked better, but once the study reached its fourth year it became clear that the marsh system was actually in decline.

Other short term studies had showed the eutrophication of salt marshes and other wetlands can lead to greater production from plants but also an increase of monoculture by plants adapted to take advantage of greater nutrient levels.

An example of this is Phragmites australis or Common Reed, which out-competes other plants when there is an abundance of nutrients available in the system that would not normally be there without anthropogenic sources. As the Phragmites spreads across the marsh it eliminates native marsh grasses like Spartina spp which leads to a monoculture salt marsh with less biodiversity. This leads to a decline in overall health of the marsh system and can lead to sediments being washed away due to a change in the ecosystem dynamics and hydrology.

In the study done by PIE-LTER, the research shows that during the first few years of the study the marsh system appeared to be doing well, however it was a false positive. At first the marsh grasses were growing taller and were greener, just like with normal nitrogen rich fertilizer; however because of the extra growth the plants were not putting down enough roots and rhizomes. This led to the grasses eventually becoming too tall for their own root systems and eventually the plants on the marsh’s bank fell into the water due to tidal action.

Once the plants on the bank were lost the sediments they were holding were washed away by the tidal action and the edge of the marsh became mudflat instead of a productive wetland area.

After the marsh bank was washed away, the rate of deterioration of the salt marsh increased dramatically. The marsh bank area is accustomed to having waves and tidal action acting on it however once you get to the high tide line and above the plants and soil are less adapted to those extra stressors. By the sixth year of the experiment, the researchers noticed large cracks in the upper marsh areas above the bank.

These cracks were larger and more numerous than in the control marsh, which lacked the excess nutrients. Once these cracks had formed in the upper regions of the marsh it was only a matter of time before natural causes eventually triggered large chunks of the marsh to slide down towards the water leaving behind large un-vegetated mud flats.

The study showed that approximately 2.5 more chunks of marsh fell into the water from the entirety of the subject marsh compared to the entire control marsh. All told, the salt marsh with excess nutrients was much less productive and less stable after the nine year study when compared to the control marsh.

Ultimately, this study could be one of the best examples of why we need to regulate our terrestrial environment better if we want to continue to have a healthy marine and estuarine system. Fish, bivalves, and other aquatic animals rely on healthy marshes for shelter, food and places to spawn. Fertilizers, especially those rich in nitrogen and phosphorus, and improperly treated sewage are major contributors to eutrophication in the marine system.

By limiting the amount of fertilizer used, selecting fertilizers with less nitrogen and phosphorus and by implementing and enforcing strict sewage treatment regulations we can hopefully reduce the amount of excess nutrients being added to these marsh systems. If we can do this we can keep them healthy and stable which in turn will keep our fisheries strong and our waterways clean and safe for future generations.


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