High Tide

Posted July 23rd, 2009 by John Callender


Linda was the one who taught me to pay attention to the tide. Low tide is her favorite time to check out the rocks at Tar Pits, or walk the beach to the marsh entrance at Sand Point. If the tide is high, she’s not really interested.

But high tide is a great time to visit the marsh. I took a walk there last weekend, and timed it to coincide with maximum high tide. It was a 6.3; that is, the water’s surface was 6.3 feet above Mean Lower Low Water (MLLW). A 6.3 isn’t as high as it gets; at new moon a couple of days later (a couple of days ago, now, as I write this), the tide got up to 7.2.

At 7.2 pretty much all the low marsh habitat, which is dominated by pickleweed (Salicornia virginica), is underwater. Here’s a shot I got last weekend of some pickleweed taking its saltwater bath:


It’s a neat trick for a plant: being able to live in the open air, while also being able to survive immersed in seawater for hours at a time. The high marsh plants, like saltbush, ragweed, mugwort, and sagebrush, can handle a lot, but immerse them in salt water and they’re history.

Because of the tide, the marsh’s plant communities are vertically stratified, and once you learn to look for it it’s really obvious. Going from lowest to highest, the marsh’s major communities are:

  • Eelgrass beds – Below the lowest of low tides, in the bottoms of the channels that wind through the marsh, is eelgrass. These are strictly aquatic plants.
  • Mud flats – A little higher, in the intertidal zone, are the mud flats. Not much in the way of visible vegetation lives here, but there’s lots of decaying detritus. There are also microorganisms that feed on it, and lots of invertebrates, and aquatic vertebrates (like fish) and terrestrial vertebrates (like shorebirds) that take turns exploiting the flats as the water rises and falls.
  • Low marsh – This is the area where the pickleweed reigns supreme. Most of the time this community is above the waterline, but twice a day the high tide soaks its lower reaches, and twice a month (at the time of new and full moon) the high tide goes all the way to the top, killing any would-be invaders from the high marsh, and maintaining the boundary, as level as if it were layed out by a surveyor, between the two communities.
  • High marsh – A wider assortment of plants, tolerant of the high salt levels in the marsh soil, but incapable of actually being immersed.

There’s something else that happens during the highest tides in the marsh: Aquatic predators (like fish) invade the inundated area, picking terrestrial insects off the pickleweed stems. I’d love to see that.


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