Sunday, February 12, 2017

Home, Home 'Neath the Waves

Oh give me a home, where the buffalo roam,
and the deer and the antelope play,
and seldom is heard, a discouraging word,
and the skies are not cloudy all day

The classic American folk song Home on the Range has been conjuring the American prairies in our imaginations for generations. For many, the John Denver version is the seminal cover of the song. As he sings you can see feel the warm breeze rippling the tall grass and flowers. Giant bison wade peacefully through the vegetation while waterfowl float on clear ponds nearby.

The only pie more American than apple may be the ones these guys 
leave behind after eating all that grass.
Courtesy: Ard van der Leeuw via Flickr

"But wait a minute! This is a marine science blog! Why are we talking about grass-lands?" Well imaginary snarky reader, the ocean is teeming with its own prairies. Even though they haven't gotten the same kind of press as coral reefs or kelp forests they're absolutely essential habitats. And underwater prairies are every bit as idyllic and wild as their terrestrial compatriots.

Oh give me a home, where the manatees roam,
and the snails and the isopods play,
and often is heard, the cries of seabirds,
and the waters are calm in the bay.

"Stay away from Washington State. It's terrible. There's nothing beautiful
 or interesting to give you any reason to move here ever." -Washington State Motto
Courtesy: EcologyWA via Flickr

The dominant organism in any prairie, above or below water, is grass. Though we tend to lump every under water photosynthesizer together as "seaweed"; sea grass is remarkably unique. Other "seaweeds" are algae, which is a group of plant-like organisms that doesn't have roots, lacks veins for transporting nutrients, and makes spores instead of seeds. Sea grass is a true flowering plant, just like the grass on your lawn, except this is the only flowering plant able to survive under salt water!

Algae typically grows on hard surfaces because its holdfast needs something stable to cling to. Sea grasses need soft substrate, usually sand, where their roots dig in and actually stabilize the soil. They have what are called rhizomaceous (pronounced: ry-zo-may-shus) roots which means they spread out horizontally in a net and put out new shoots where there's enough light. Anyone who's pulled ivy by hand has experience with these kinds of roots. The rhizomes of sea grass cover so much ground and hold so much soil that they're a major protector of coastal shorelines from erosion. In the Caribbean, Western Atlantic, and Gulf of Mexico where they're called turtle (Thalassia testudinium) or manatee grass (Syringodium filiforme); sea grasses keep hurricanes from washing beaches used by sea turtles and tourists alike out to sea. Grasses on the American plains once did the same thing for the land by protecting the earth from the harsh prairie winds. Losing all that grass to agriculture caused the Dust Bowl of the 1930's.

 But tell me again how humans can't affect major changes 
on the environment.
Courtesy: US Department of Agriculture via Flickr

Only 200 years ago the tall and short grass prairies of the American West were rich with a diversity of animals that thrived on the variable habitat provided by the grasses. Insects of all kinds, burrowing rodents, birds and even terrestrial crayfish skittered and survived among the blades. Sadly most of these habitats have been lost to agriculture and suburbanization, but sea grass plains have avoided these disturbances and still host an incredible diversity of organisms.

Sea grass creates vertical habitat just like trees do in forests; so many species can grow on, in, and around the grass. One study in Denmark found more than 200,000 organisms of 86 types on a little more than 1,000 blades of grass. The folks doing the research even found entire tiny ecosystems on the blades. Diatoms, a kind of single-celled algae, grew in mats on the leaves. Isopods, a relative of crabs and shrimp, would walk along and graze on the algal mats. The tiny crustaceans were eaten in their turn by small anemones that also attached themselves to the grass.

We've recently discovered that these minuscule animals may actually benefit the sea grass as well. Sea grasses have male and female flowers just like terrestrial plants, and just like terrestrial grasses males release pollen that has to get into the female flower some way or another. On land, pollen is transported by the wind and by animals that travel between plant flowers. Until recently we assumed sea grass pollen was only transported by waves and currents. But we've found evidence that planktonic baby crabs, and nearly microscopic, bristly, worms are actually acting as pollinators for sea grass. These plankton visit male flowers where pollen grains get stuck on the appendages that normally help prevent the animal from sinking. Then they swim along to the next flower in search of food and inadvertently pollinate the female. I don't have the right permissions to post a picture of the pollen grains on the plankton, but you can see the incredibly cute pictures in the article at this link.

This species, called tape grass (Enhalus acoroides), is found in Asia.
The male flowers (white nubs) float way from their parent, and get 
caught in the female flower (yellow petals).  
Courtesy: Ria Tan at Wild Singapore via Flickr

The abundance of grass and wealth of small animals inevitably attracts larger animals to these rich feeding grounds. Like lions on the Serengeti or wolves in Yellowstone; seals, sharks, and other big predatory fish move through sea grasslands to consume the smaller predators drawn by the smorgasbord. Of course it wouldn't be a grassland without big grazers. North America and historically Europe have bison, Asia and Africa have rhinos and buffalo, Australia has kangaroos, and South America: llamas. The plains of the ocean: manatees, dugongs and sea turtles.  Sea grasses also bring in the massive assemblages of birds that are often seen on prairies. The coolest of these is probably the brant (Branta bernicla).

Courtesy: JimGain via Flickr

In Europe, brant are called Brent Geese, presumably because the name Brent is used exclusively to identify obnoxious frat bros in the US. They're an incredible relative of the more familiar Canada goose (Branta canadensis), and they cannot survive without sea grass. The Pacfic's subspecies, the black brant (B.b. nigricans) summers on the north slope of Alaska and the islands to the north where their chicks can develop relatively free from predators. As the year wanes the brant are chased south by the frigid nights of winter. But brant aren't like the many marathon flyers that breed in the arctic. Brant have such a big body relative to how small their wings are that they can't make a trip from Alaska to to the southern hemisphere in one go like a bar-tailed godwit. Brant have to take pit stops the whole way from Prudhoe Bay to Baja to rest and refuel, and what do they look for en route? Sea grass.

The sheer abundance of eel grass (Zostera marina), as we call it on the west coast, makes it the perfect resource to gas up on. Eel grass is found all along the migration route, and because the blades are coated in organisms, the herbivorous goose probably gets much need proteins and fats for energy. 

"Waiter, there's a bunch of bugs on my salad. <Chomp, chomp, chomp> 
Actually never mind, don't worry about it."
Courtesy: Jon. D. Anderson via Flickr

Brant are the ideal illustrator of how important and powerful something as commonplace as grass can be. A good sea grass prairie produces so much food that it can attract an entire population of animals. We believe that every single black brant stops at Izembek bay in Western Alaska, which has the largest eel grass bed in the world, on their way north and south each year. And just like their Canadian cousins that settle over the prairies of the US interior during the winter brant show us the soothing wildness of grasslands. It's just too bad John Denver didn't SCUBA dive.

Oh give me a home, where the manatees roam,
and snails and the isopods play,
and often is heard, the cries of seabirds,
and the waters are calm in the bay.

References:

Ganter, Barbara, "Sea Grass (Zostera spp.) as Food for Brant Geese (Branta bernicla): An Overview", Helgoland Marine Research, vol. 54, pg. 63-70, 2000.

DeAmicis, Stacey & Foggo, Andrew, "Long Term Field Study Reveals Subtle Effects of the Invasive Alga Sargassum muticum Upon the Epibiota of Zostera marina", PLoS ONE, September 14, 2015.

Thormar et Al., "Eelgrass (Zostera marina) Food Web Structure in Different Environmental Settings", PLoS ONE, January 11, 2016. 

Tussenbroek et Al. "Experimental Evidence of Pollination in Marine Flowers by Invertebrate Fauna", Nature, vol 7, 2016.
Accessed via: http://www.nature.com/articles/ncomms12980

  
















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