Saturday, November 26, 2011

Green, fading to black

One of the pleasures of summer along our shores is to wade at the low, low tides, knee-deep in the eelgrass beds, watching the swirl of the deep green blades reaching up to the surface and floating there. We watch our step carefully; among the stalks, good-sized, maybe toe-pinching crabs scuttle. On the long leaves, if we look closely, we can see large green isopods and small snails. Sculpins and other small fish dart away at our approach, no matter how stealthily we tiptoe.

Most of what lives in these green forests, though, we don't see. Kozloff, in his chapter on sandy beaches, lists, among the inhabitants:
  • On the leaves: the tinies; diatoms and other microscopic plants, invisible alone, but in mass, forming a brownish "fur" or long threads; bacteria, protozoa, algae, microscopic worms, crustaceans, copepods.
  • The small critters: amphipods, including skeleton shrimp, hydroids, flatworms, ribbon worms, jellyfish, anemones, chink snails, sea slugs, including Melibe leonina, two species of isopods.
  • On the mud, sand, roots: bubble shell snail, clams, sea stars, sea cucumbers, five crabs; Dungeness, red, two spider crabs, and helmet crabs.
Another list adds sponges, bryozoans, spiral tube worms. I would add the limpets and their cargo. There are many more, and we must not forget the fish.
So important is Zostera's role in this food cycle that estimates reveal that more than 20 species of commercially valuable fish species feed in these eelgrass meadows at some point in their lives. (R.I. Sea Grant Fact Sheet)
 That's the summer.

Summer eelgrass, waiting for the return of the tide.

In the fall, the eelgrass begins to die back. Stormy seas rip up the plants by their roots, and propel them toward the shore. The waves, coming and going, roll them up in dense masses, metre-long tough leaves wound and knotted around each other, and push them onto the beaches. There, they gradually die and blacken, always pushed further up the beach by the next tide's eelgrass roll-up, until they form great mounds of black, sharp-smelling gunk.

Drying eelgrass, caught on driftwood log. Boundary Bay.

But their useful life isn't over yet! First, while they are drifting ashore, Canada geese congregate to eat the green stems that they usually can't reach.

Geese and gulls, watching the eelgrass come in. White Rock.

I collect handfuls for my aquarium critters from the fresh rolls, disentangling the greenest, the ones with bits of root still attached. In the winter, the low tides usually fall well after dark; I can't wade out to what's left of the eel-grass beds. I'm glad it comes to me, instead.

In summer, any eelgrass I harvest is well checked over for other life. I always find limpets, usually quite a few snails, occasionally an eel-grass isopod or two. There may be amphipods along the leaves, often polychaete worms in the roots, and always the fuzzy diatom communities.

In the fall and winter, the eelgrass looks bare until I run a finger down a leaf; then I notice the hard, raspy bryozoan communities, the thickening where a cryptic limpet is attached, and maybe an isopod will transfer his clingy feet to my finger. All of these go into the aquarium where I plant the eelgrass.

I don't bring home the black stuff. Some people do; I have seen gardeners with wheelbarrows and big barrels on the shores of Crescent Beach and Boundary Bay, filling their containers (once a pickup truck bed!) with nutritious mulch for the winter garden.

But I do roll it over, to watch hundreds of beach-hoppers leap every which way. (I remember, back when Laurie didn't know me quite so well, I called him over to see. "Look!" I said, and turned over a heap of dead eelgrass. He wasn't prepared; the hoppers ended up on his legs, and he jumped back, annoyed. He got over it, and now is as enthusiastic as I am. Sometimes more.)

Inside that rotting mass, worms, fungi and bacteria work at reducing it to fine dust, which will sift slowly out to sea, returning the nutrients to their source for another round. In the meantime, tiny black flies swarm over the surface, feeding and breeding in the relative warmth from the decomposing grass. Nothing ever gets wasted.

Bryozoan colony on dying eelgrass.

In my aquarium, fall eelgrass only lasts a couple of weeks before it dies and turns black. I doesn't look good, and eventually will disintegrate and clog the filters, so I remove it. But first, I make sure I'm not removing live animals. Most of the limpets have moved on to greener pastures; so have the isopods and amphipods. But there are still patches of bryozoans and hydroids.

I wondered, the other day, whether the bryozoan colonies were still alive, so I brought them out to a tray of shallow water and examined them under a microscope. All the tiny, glassy barrel shells were empty; the makers were gone.

Each little tube was the home of one zooid.

If you were seeing these photos live, you would also see tiny specks of light moving between the shells, barely visible transparent threads weaving back and forth, white dots that seem to have legs scuttling across the green spaces. The bryozoans have died, but the colony is still very much alive. And the spiral tube worms are still busy catching the specks and threads for supper.

Bryozoan colony with feeding tubeworm.

I replaced the eelgrass in the aquarium. These industrious hunters and gatherers deserve their full life span.


  1. Professor Kozloff ... A whole darn list of books to add to my collection. I need to learn my Puget Sound ecology

  2. Nothing goes to waste in nature. The closeups of the Bryozoans are superb. Say hi to Laurie for me. - Margy

  3. Kozloff; I wouldn't be without his book. And Lamd and Hanby's "Marine Life of the Pacific Northwest" is well worth getting, too.

    Thanks, Margy. And I'll tell Laurie.


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