Is your salt marsh in the zone?

Scientist James collecting plants in a Massachusetts marsh, part of the Plum Island Ecosystems Long Term Ecological Research site

Scientist James collecting plants in a Massachusetts marsh, part of the Plum Island Ecosystems Long Term Ecological Research site

The activities are as follows:

Tides are the rise and fall of ocean water levels, and they happen every day like clockwork. Gravity from the moon and sun drives the tides. There is a high tide and a low tide, and the average height of the tide is called the mean sea level. The mean sea level changes seasonally due to the seasonal warming and cooling of the ocean, and annually due to the melting of glaciers and a long-term trend of ocean warming. The scientific evidence shows that sea level is rising faster now than it has in the past. As the climate continues to warm, it is predicted that the sea level will continue to rise.

Salt marshes are plains of grass that grow along much of the ocean’s coast worldwide. They grow between mean sea level and the level of high tide. Marshes flood during high tide, and are exposed to the air when the tide goes out. The health of a salt marsh is determined by where it sits relative to the tide (the “zone”) – a healthy marsh is flooded only part of the time. Too much flooding and too little flooding are unhealthy. Scientists wanted to know, can salt marshes keep up with current rates of sea level rise or will they loose their balance between high and low tides once sea levels are higher? How can this be measured?

A picture of James’ “marsh organ” which holds plants at different elevations relative to mean sea level. He gave it that name because it resembles organ pipes!

A picture of James’ “marsh organ” which holds plants at different elevations relative to mean sea level. He gave it that name because it resembles organ pipes!

In the 1980s, scientist James Morris began measuring the growth of marsh grasses. He was surprised to discover that marsh grass growth was rising and falling every year, depending on mean sea level, and that there also was a long-term trend of increasing plant growth. Plant growth was greatest in years when the annual mean sea level was unusually high. He wanted to know whether marsh plants could continue this growth and handle rising sea levels. He thought that the long-term trend of increasing plant growth was symptomatic of the marsh surface not rising fast enough to keep up with increasing rates of sea-level rise. If this is the case, plants are growing the most in years where mean sea level is high to stay above the water surface. To test this, James measured elevations of the marsh surface every year and compared this with the elevation of mean sea level. He also made these same measurements in another marsh to determine if the patterns originally observed could be repeated.

James devised a method of growing a marsh at different elevations within the vertical range of the tides. He invented a device that he called the “marsh organ” which is made from PVC pipes that stand at different elevations and are filled with marsh mud and planted with marsh vegetation. The marsh organ design allowed him to experimentally control the elevation. He then measured the growth of the vegetation in the pipes. If higher water levels stimulated growth, then pipes at lower elevations should support more plant growth than pipes higher elevations.

Featured scientist: James Morris from the University of South Carolina

Additional teacher resource related to this Data Nugget: Jim has created an interactive salt marsh model called the “marsh equilibrium model”. This online tool allows you to plug in different marsh levels to explore potential impacts to the salt marsh. To explore this tool click here.

To read more about Jim’s research on “tipping points” beyond which sediment accumulation fails to keep up with rising sea level and the marshes drown, click here.

There are two publications related to the data included in this activity:

  • Morris, J.T., Sundberg, K., and Hopkinson, C.S. 2013. Salt marsh primary production and its responses to relative sea level and nutrients in estuaries at Plum Island, Massachusetts, and North Inlet, South Carolina, USA. Oceanography 26:78-84.
  • Morris, J.T., P.V. Sundareshwar, C.T. Nietch, B. Kjerfve, D.R. Cahoon. 2002. Responses of coastal wetlands to rising sea level. Ecology 83:2869-2877.
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