Tag Archives: Blue Carbon


Seagrass Wrack in the Salt Marsh – Blessing or Curse?

2-Minute Video: Seagrass wrack kills part of the marsh, but do its benefits outweigh the destruction?

Our videos to date have centered on biodiversity in the marsh and how it can make a marsh stronger against disturbances. As we see in this video, at least one type of disturbance might actually promote genetic and/ or species diversity.
Dr. Randall Hughes FSU Coastal & Marine Lab/ Northeastern University
This snake was found in a seagrass wrack experiment in the Saint Joseph Bay State Buffer Preserve. Blue crabs were often found taking shelter in their experimental plots as well.


This time of year if you look around salt marshes in our area, you’ll probably see a strip of dead plant material, or “wrack”, resting on top of the salt marsh plants around the high tide line. Look closer, and you’ll see that it’s mostly made up of seagrass leaves that have either been sloughed off naturally (seagrasses produce lots of new leaves in the summer and shed the old ones) or, occasionally, uprooted by boats driving through shallow seagrass beds. Look even closer (say, by picking it up), and you may just find a harmless marsh snake (or worse, a cottonmouth!) – in our experience, they like to hang out in the cool, moist areas under the wrack.

So is this wrack “good” or “bad” for the salt marsh? As with many things in life, the answer depends on your perspective. If you’re a snake or other critter that likes the habitat provided by the wrack, then it’s probably a good thing. On the other hand, if you’re one of my crew who finds that snake, and particularly if you’re Robyn who REALLY doesn’t like snakes, then it’s most definitely a bad thing. Or, if you happen to be the plant that the wrack settles on top of for long periods of time, then it’s a bad thing, because many of those plants die. But, if you’re a seed that is looking for a good spot to germinate in the marsh, then the bare spot created by the wrack is likely a good thing.

Bare spot left in salt marsh left by seagrass wrack.Last fall, David and I teamed up with Dr. Peter Macreadie from the University of Technology Sydney to find out how the bare “halos” created when wrack mats smother the underlying marsh plants influence the marsh sediments. It turns out, these bare areas store less carbon in the sediments than the nearby vegetated areas, which makes them less valuable as “sinks” for carbon dioxide. But as I mentioned earlier, the bare areas can also serve as a good spot for new plant species (or new genotype of a given species) to start growing, potentially increasing the overall diversity of the salt marsh. And as the seagrass wrack decays, it can provide valuable nutrients to the marsh sediments that support future plant growth. So what is the net outcome of all these good and bad effects?

We decided to do an experiment to answer that very question. As Ryan and Meagan will attest (along with almost everyone else in our labs who we enlisted to help us), this was a very labor-intensive experiment. First, we had to figure out how much wrack is typically in a given area of marsh. Then, we had to collect a lot of wrack, weigh it, assemble it into bags that could be “easily” moved to our experiment, and add it to cages that would help hold it in place. We’re talking ~1.5 tons of wrack picked up and moved to various spots!

FSU Coastal and Marine Lab technician Megan Murdock spin dries seagrass wrack for an experiment at the Saint Joseph Bay State Buffer Preserve.To make matters even more interesting, we had to soak the collected wrack in water to make sure it was all the same wetness, and then spin it around in mesh bags (think salad spinner on a very large scale) for a set amount of time to make sure we could get a consistent weight measurement on each bag. Anyone driving past the SJB Buffer Preserve in early September of last year must have wondered what craziness we were up to! And since we were interested in whether the length of time the wrack sits in one place influences its effects, or whether the number of times that wrack sits in a particular area matters, we moved all of this wrack around in our cages every 2 weeks for 3 months to mimic the movement of natural wrack by the tides. And then we measured everything we could think of to measure about the marsh.

We’re still going through all the data to determine the net outcome, but as expected, whether the wrack is a blessing or curse depends on who you are:

  • Juvenile blue crabs seem to like hanging out in the wrack (which is a much nicer surprise to find than a snake, even when they are feisty!)
  • Fiddler crabs also appear to like the wrack, with greater burrow numbers when wrack is present.
  • Contrary to our expectation that wrack would kill cordgrass and allow other plant species to recruit into the marsh, it looked like cordgrass actually did better in the wrack cages!
  • Sea lavender, a marsh plant with very pretty purple flowers, does not do so well when covered with wrack (Learn more about sea lavender and its relationship with mussels).

More to come once all the data are analyzed…


Music in the piece by Philippe Mangold.

Randall snorkels in a seagrass bed in Saint Joseph Bay Peninsula State Park. Photo by Dr. Peter Macreadie. Peter is a researcher from the University of Technology, Sydney, who is visiting Randall and David.

What Have Seagrasses Done For Me Lately?

Episode 6: Blue Carbon Where the Stingray Meets the Horse Conch

At the beginning of September, Randall and David had a visit from Dr. Peter Macreadie of the University of Technology, Sydney.  In this video, Randall takes Dr. Macreadie for a snorkel in St. Joseph Bay.
Dr. Randall Hughes FSU Coastal & Marine Lab

IGOR chip- habitat 150IGOR chip- filtration 150
We now focus our attention to seagrasses, which as it turns out, often don’t get a lot of attention, at least in comparison to other marine habitats like coral reefs or even salt marshes.


In part, this lack of attention is due to the fact that seagrasses typically live completely underwater, except at very low tide, and so they are not as noticeable as marshes are. In addition, seagrasses often occur in shallow estuaries not known for their great visibility (and thus not as ideal a location as coral reefs for snorkelers or scuba divers). And, although I disagree, some people just don’t find them very pretty.

Last week as I was starting to think about this post, there was a small uptick in the number of media articles related to seagrasses, at least in Australia. The increased interest was in response to a proposal by the Environment Minister, Tony Burke, to require greater seagrass protection from mining and development projects (read more in this article from the Brisbane Times). As justification for the increased financial burden on companies, Mr. Burke cited the many benefits that seagrasses provide. And just what are those?

Scallop in St. Joseph BaySeagrasses (like salt marshes and oyster reefs) provide habitat for many, many fishes and invertebrates. Studies have found that the number of animals living in seagrasses beds can be an order of magnitude higher than the number living in adjacent coastal habitats. Many of these animals rely on the seagrass beds as a “nursery” that protects them from predators until they grow larger. And lots are recreationally and commercially important species that we like to eat. (Scallops, anyone?)

Seagrasses are also incredibly productive plants, sometimes growing more than 1cm per day, and rivaling our most productive crop species like corn. Because a significant portion of this plant material (particularly the roots and rhizomes below ground) stays in place once the plants die, seagrasses can also serve as important ‘carbon sinks’, or buried reservoirs of carbon. In fact, a recent study estimates that the carbon stored in the sediments of seagrass beds is on par with that stored in the sediments of forests on land!

Although lots of the productivity of seagrass beds makes its way underground, some of it does get eaten. Major consumers of seagrasses include urchins and fishes, as well as the more charismatic dugongs, manatees, and sea turtles.

Spider Crab in St. Joe BaySeagrasses (like salt marshes) also play an important role in reducing nutrients that run off from land into the water. Unfortunately, these nutrients can also lead to the loss of seagrasses, by promoting increased growth of algal “epiphytes” that grow on the blades of the seagrasses themselves. When there are not enough small fishes and invertebrates around to eat these algae, they can overgrow and outcompete the seagrass, leading to its decline. And when the seagrasses become less abundant, the animals that rely on them are also often in danger.

The Big Bend and Panhandle of Florida are home to expansive seagrass beds that also often go unnoticed. But they contribute to the productivity, diversity, and beauty of this area in many ways, as anyone who has been scalloping recently has surely realized!

Here is a quick guide to the animals featured in the video above:
0:40 Horse conch and sea urchin joined suddenly by a stingray
1:41 Juvenile pinfish
1:18 Two shots of a bay scallop
1:33 Sea urchin
1:49 Pen shell clam covered in sea stars (2 shots)
1:56 Horse conch
