Thursday, April 16, 2009

The Eelgrass Story. Part I a: What is Eelgrass?

As promised (although quite belated) here is the first installment of The Eelgrass Story.

What is Eelgrass?

Eelgrass is a type of seagrass and seagrasses are aquatic angiosperms (flowering plants). They are called seagrasses because most of the plants resemble large grasses found on land but are confined to marine environments. In North America, eelgrass refers to the species Zostera marina.

Seagrasses in general have an extensive below ground network that comprises of roots and rhizomes and grass like leaves above the sediment.
The rhizome is the main below ground stem off of which roots and new shoots can grow. The roots anchor the plant in the sediment and they also absorb nutrients from the water in the sediment, similar to the roots of land plants. Growth of new shoots from the rhizome is called rhizomatous growth and is how the plant reproduces asexually (ie: without the need for fertilization). This type of asexual reproduction is the main mode of expansion of a bed. An entire bed can sometimes be composed of just one or two individuals!

Above the sediment, seagrasses can have leaves of various shapes and sizes, from flat to filaments to round leaves from lengths of a few centimeters to over a meter! Eelgrass leaves are flat and narrow and really do resemble grass.

Copyright © 2006 Mary Jo Adams; From Washington State University (

The blades can be long or short, this depends on the amount of light reaching them. So in shallow waters their leaves are short but in deeper or more turbid (ie: lots of particles or murky) water their leaves can be over a meter long! The leaves need to be close to the surface of the water to gather enough light for photosynthesis.
This is a bed in Prince Edward Island where the length of the leaves was on average 20 cm long and was found in water about 0.75 m deep at high tide.

This was my study site in Musquodoboit Harbour (MH), Nova Scotia. This site had the longest leaves always over a meter long, but the water depth was only 1.5 m....her you can see the tops of the leaves on the surface of the water as the tide is receding. This was very hard to paddle/swim through. So why were the blades so long here with the water so shallow....
This is one of the reasons...this is underwater in MH in June, there is so much sediment, phytoplankton (floating unicellular plants, hence the greenish colour) and tannins in the water that not a lot of light actually gets through to the bottom. Tanins are tanic acid which comes from decaying plant matter and gives freshwater its brown colour. The brown freshwater can form a lense on top of the seawater (because it is less salty it floats) or get mixed in with waves and wind. There is a limit though to how long the leaves can get, and if the water is turbid and deep they will likely not occur there because they don't get enough light.

***Mini experiment*** Mix salt with water in the bottom of a glass (2 tbsp for 1/2 a cup), then put some food colouring in some freshwater of the same temperature in another glass, then slowly pour it in on top of the salty should see a clear separation...leave it for a while and see if there is any mixing. Then mix it up with a spoon. This is how it works in the oceans where rivers meet the sea! The freshwater will continue to float on top of the seawater unless something mixes it up, like waves, wind, boats, tides, etc. ***

At this site in Taylor's Head Provincial Park in Nova Scotia, the water is more like 2.5 m deep at high tide, but the leaves are only 30 cm long on average....but look at the beautiful blue colour of the water (ie: not a lot of phytoplankton, etc.) and the clarity. The instrument you see in the photo was place there to measure the amount of light and quality (ie: what colours or wavelengths) that reach the seagrass....these were placed at all my sites and are Diego's babies (he made them!).
As an aside, at this site we had a "plague" of jellyfish (big and small). At one point, I looked up from my work and counted 25 that I could see in front of me...they were dive buddy and co-worked Jess and I got repeatedly stung on the only exposed skin we had...our lips!
Here are 4 large ones tangled on a line we had in the water. Their bell (the round part) was about 25 cm in these were pretty big and had very long trails of stinging tentacles! Here their tentacles are entangles in the line (I set them free) but some of the stingers stayed behind and stung our boat person Kate as she hauled up the line...this was a hilarious (and somewhat painful) day!

Now, back to eelgrass...Since they are flowering plants, they do produce small flowers to reproduce sexually! Some species of seagrass have both sexes on the same plant (monoecious), while other have seperate male and female plants (dioecious). In the case of eelgrass, from what I understand (correct me if I am wrong) they are on the same flower. The flowers look like small white/yellow chevrons (or v-shapes) on the leaves (see image below).

Image by: Josef Ackerman; From the webpage of the Botanical Society of America (

Similar to flowering plants that let the wind carry their pollen, seagrasses let the water transport theirs and the shape of the flowers encourages pollen settlement by altering the water flow
around it. Each flower produces one seed.

Copyright © 2006 Jan Holmes; From Washington State University (

Sexual reproduction is the mechanisms by which the plants can expand their range and potentially colonize new areas within estuaries and beyond. The seeds can travel tens to hundreds of kilometers. It is also an insurance against disease and other disturbances that can cause catastrophic loss of a bed. Many of the seeds fall within the bed and form what is called a seed bank. So, if large scale loss happens and conditions remain suitable (this is the most important part) then the seeds can recolonize and area. In the 1930's "wasting disease" reduced eelgrass populations along the Atlantic coast of North America and in Europe by 90%. This disease ended the traditional harvest of eelgrass for insulation in houses as well as for use in matresses and pillows. It took the beds until the 1960's to recover in many areas but in others they never recovered because the conditions in that area changed preventing recolonization. Wasting disease is cause by a slime-mould like organisms called Labyrinthula zosterae. It continues to affect eelgrass in North America and Europe but no events have been as catastrophic as in the 1930's.
Image of the symptoms of wasting disease. From: OceanLink (

We even saw evidence of the wasting disease at many of our study sites (see photo below and the one above with the short leaves)
. But this was on outer leaves (ie: the older ones that are going to be shed) indicating that it was not getting ready to take over and destroy the beds.
Like most diseases, they often stay at a background level in populations (and in this case the water too) and only affect the older weaker leaves unless another stressor affects the plants ability to fight the disease then it takes over.

Stay Tuned for The Eelgrass Story Part I b: Where is it found? For more fun and interesting facts about these very versatile plants! :)


  1. This is wonderful, Alli...thanks so much for the cool lesson on eelgrass! I'm sending ds#1 over to read this. He knows enough about land plants to have this be interesting to him - comparing the differences and all that. :)

  2. Thanks for the eelgrass lesson! I'm looking forward to part I b :)

  3. This is very interesting ~ so sorry about the jellyfish stings. I hear they can be quite painful and cause swelling. Hope your poor lips are back to normal, although maybe you like the Angelina Jolie look (LOL). Barb xo

  4. Hi, I'm an Dal M. Arch student that happened upon your site when I googled eel grass, after I was curious as to whatever became of Cabot's Quilt Sheathing. So it was a disease that killed off most of the population and then people just forgot about collecting eel grass even after the species recuperated in the 60s? CRAZY! I wonder if it can be harvested sustainability again these days. I have a picture of the the original sheathing taken from the old opera house in Lunenburg where there's ongoing reconstruction being done to the upper levels. I can email it to you if your interested ( Nice blog by the way....

  5. Alli,

    This is a great blog! I work at a California non profit organization, Orange County Coastkeeper, and I am currently developing a curriculum for our new Back Bay Eelgrass Project in which we will provide hands-on environmental science experience to local low-income junior and high school students while also working to restore eelgrass in Newport Bay.
    I would love to feature a couple of your photos, with your permission, both in our curriculum and on large informative boards that will be viewed by the public at our Back Bay Science Center.
    Please email me if you are interested and sorry for writing to you about this on your blog!

    Amanda Bird