Saturday, July 15, 2017

The Fitzgerald Lab: Salt Marshes and Sediments

Hey y’all! I’m Carina Terry and this summer I’m working in Duncan Fitzgerald’s lab. I’m a rising sophomore in the College of Arts and Sciences. I started off as a physics major, but after a semester in the BURECS program, I decided to switch to earth and environmental sciences.

The Fitzgerald lab is a perfect internship for my major. We’re studying salt marshes; their composition, dynamics, and most importantly, how we can save them from impending sea level rise. The marsh we’re looking at now is the Great Marsh right here in New England. This marsh has a high occurrence of slump blocks – large pieces on the edge of the marsh that are slowly separated from the rest of the marsh until they reach a tipping point and cave into the tidal creek. The loss of these blocks isn’t great for the marsh, as it means the marsh is eroding even more quickly.
Tidal creek with slump blocks at high tide 
Many theories in the world of marsh studies state that erosion increases when wave power increases. This is applicable to many other cases of marsh erosion, but Professor Fitzgerald doesn’t believe high wave power is the cause of our marsh’s slump blocks. The blocks that we observe are found fairly far upstream in the tidal creek – far enough that the waves can’t get that powerful because the channel is fairly narrow. Since high wave power doesn’t seem to be the cause of the slump blocks, we have to figure out what is.
To solve this question, we test the marsh in several ways. After driving about an hour north of Boston, we park our truck and then hike out into the marsh. This hike usually takes about 10 – 20 minutes, depending on which part of the marsh we’re examining. Occasionally we have to crash our way through forest and brush, which can be a bit challenging, but it’s worth it – when we make it to our marsh site, the view is beautiful.

When we reach the tidal creek, someone has to climb down into the creek. (We have to make sure we go to the marsh at low tide, so the water is shallow enough that we can do this – we don’t want anyone to take an unpleasant swim!) After they’ve climbed down, they use a special syringe to take samples down the side of the creek bed. We do this for about 3 or 4 sites a day, then head back to the lab.

Once we’ve got the samples back in the lab, we first have to dry them for several days. When they’re dry we weigh them, which will allow us to calculate their bulk density. Then we split the samples in half and begin the rest of our analysis.
Dried marsh samples
The first half are assessed for organic content. We crush the samples up into a fine powder using a mortar and pestle, then place the crushed samples into crucibles. After weighing the crucibles to find the initial mass, we place them into the furnace. Then, at around 5 or 6 pm, one of us has to come back to turn the furnace on. The samples are burned at 550 degrees Celsius (1022 degrees Fahrenheit!) for 16 hours; we come back in the morning to turn the furnace off and let them cool, then weigh them once again. Since all the organic material was burned away in the furnace, this tells us what percentage of the sample was composed of inorganic sediments and what percentage was organic.

Mortar and pestle
The other half of the samples are analyzed for biomass. First, we use a sieve to separate the sediments from the vegetation in the sample. The vegetation is dried and then weighed by itself, to find what percent of the sample was composed of vegetation. The sediments are split in half; one portion is dried and weighed, while the other is stored in a vial for future use. Eventually, these samples will be sent to a lab in Louisiana, where the size of the sediment grains will be analyzed and recorded.

Sieves and brush
Clearly, a lot of the analysis we do is related to the mass and composition of the marsh: what percentage is inorganic, how much vegetation is present, and how big the sediment grains are. We analyze these factors because we suspect they could have a correlation to how stable the marsh is. Once we have collected all our data, we will look for trends and try to find which of these factors have the largest effect on marsh stability. Then, when we know the important factors, we can work on restoring the marsh and fortifying it against rising sea levels.

The salt marsh is an important ecosystem; it serves many functions, including sheltering young fish and insects and removing carbon from the atmosphere. It’s very important that we work to keep it from being destroyed due to climate change. And, of course, we wouldn’t want to lose this beautiful view.

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