The Eelgrass Story Part I b: Where is it Found?

Click here if you missed Part I a: What is eelgrass?

Where is Eelgrass Found?

There are nine different species of the genus Zostera which span the globe but are notably absent from the Indian Ocean, Gulf of Mexico and Caribbean waters as well as the coast of South America. The most widely distrubuted Zostera species is Zostera marina which can be found along all Northern temperate coastlines and even in some arctic zones like Groenland, around Iceland and northern European countries like Norway.

Global distribution of Zostera marina in purple. From The marine life information network of Britain and Ireland (marLIN) http://www.marlin.ac.uk/species/adult_distrib_Zosteramarina.htm

Zostera marina is the dominant species found along the coast and in estuaries of eastern North America, from Labrador in Canada to North Carolina in the United States. It occurs predominantly as monocultures called beds, especially in the more northern latitudes, this is definitely true in Canada and some Northern US states but can co-occur with other seagrass species in the more southern latitudes of its range. The green areas in the image are States and Provinces where Zostera marina is present (white = absent) along their oceanic coast.

Taken from United States Department of Agrculture Plants Database (http://plants.usda.gov/java/profile?symbol=ZOMA).

It can occur subtidally, which means that although the leaves can touch the surface at low tide, the entire plant is never out of the water. These were the types of beds I worked on. But it can also occur intertidally, which means the entire plant is out of the water at low tide, as seen in this photo from a West Coast population.
Image from James Douglas (http://jimbodouglass.blogspot.com/2007_11_01_archive.html)

Subtidally, they can occur to depths of up to 12 m but this depends on water clarity for the same reasons as variation in leaf length (discussed here). In Newfoundland and Labrador, the water is so clear that eelgrass can get enough light to inhabit depths of 12 m but it is more typically found shallower. In Nova Scotia, it is generally between 2-3 m.

Because they have an extensive root system, these plants live in soft sediment that ranges from mud to coarse sand and cobble. There are some species of seagrass that can attach to bare rock, but eelgrass is not one of them. The can also live in a range of wave regimes, from very protected like all my sites in New Brunswick and Prince Edward Island that were behind barrier islands or more wave exposed like my sites along the Atlantic coast of Nova Scotia.

They are not typically found very high up in estuaries where there is a lot of freshwater input, they can tolerate a range of salinities (5 to 35 ppt) but grow best in 20 -26 ppt. They can also tolerate temperatures from freezing to 35 degrees C. These are perennial plants, which means that they persist year round and only die back (ie: lose their leaves) when it gets too hot or are physically removed. In many areas in the northern part of its range, the shallows are covered in sea ice that moves around with the winds and tides. This scours away much of the leaf tissue and sometimes even the roots and rhizomes but if there are rhizomes left behind the bed will regenerate!

Sea ice in Nova Scotia. From Nova Scotia Photo Album Blog (http://www.novascotiaphotoalbum.com/blog/2009/03/watching-the-sea-ice-float-away/)

The same is true if it is too hot, which can often happen in the shallows of more southern parts of its range. The leaves will die off but the roots and rhizomes will remain to fuel the growth of the bed when the temperatures return to tolerable levels.

So as you can see, eelgrass is very versatile and resiliant! It can and does inhabit any area where there are suitable conditions (ie: enough light, soft sediment, etc.). This is a persistent and widely distributed feature of the coastal ocean. However, 60% of the world's population (3.6 billion) lives within 60 km of the coast and this is expected to increase to 75% (6.4 billion!) over the next 30 years. Also, 16 of the world's 23 mega cities (> 2.5 million inhabitants) are within the coastal belt and their populations are growing by 1 million people PER DAY (!!!) making coastal marine systems around the world the MOST IMPACTED by human activities.

How does this all fit together? What does this mean for eelgrass? What does this mean for us? Stay tuned for more installments of the eelgrass story!! :)