By mid-summer, heaps of luxuriant, slippery seaweeds heavily clothe the lower reaches of Oregon’s rocky intertidal.

What we call “seaweeds” may look like members of the Plant Kingdom, but few actually are.

The only common green plant on our outer shore is surf grass. Look closely and you’ll see the veins running length-wise down each leaf. Surf grass also has true roots, grass-type flowers and seedpods.

Although both groups make their own food, have rather tough cell walls, and generally don’t move, algae are distinguished from plants on several points. Although reproductive cells may develop in certain locations in some species, algae don’t have highly specialized reproductive structures, such as flowers, fruits, pods, or cones. Algae also don’t have highly specialized transportation systems, such as veins or roots, or protective structures, such as bark or waxy coatings. (By the way, “algae” is plural, “alga” is singular.)

Our common algae are divided into three very different groups: green algae, red algae, and brown algae. Although we tend to lump them together as “seaweeds,” the groups are not related: In addition to different basic pigmentation, each group has cell walls made of different material and each group stores different kinds of products in those cells.

The green algae seem most delicate — indeed, some are only one cell thick. Most often noticed by beach-walkers are the fine, delicate threads of Enteromorpha and the coin- to palm-sized bright, green, paper-thin sheets of sea lettuce.

Some of the red algae appear so tough they seem to be inorganic. The rounded splotches of bumpy, rose-pink coloring on the rocks and shells is truly algae, most often “pink-rock crust.” Though they look like masses of various small, branching pink corals, the coralline algae are several different red algae.

Most of what we’d call “kelp” are brown algae. Many of our brown algae have a mid-rib or stipe, a stem-shaped structure with attached blades, and a fingers-like holdfast that grips the rock. (The holdfast is not a root ball, since it doesn’t have vessels for transporting nutrients or water.) Our readily recognized brown algae include the stubby, branched rockweeds, the sea-palm that stands up to the heaviest surf, the iridescent kelp, and the familiar giant bull-whip kelp that washes ashore after storms.

A wide variety of brown algae may be piled on the lowest exposed rocks like so many buttered lasagna noodles. When the incoming tide returns the water, such piles turn into a dense underwater forest, lithely swaying and churning with each wave.

These roiling forests provide food and dwelling surfaces for diverse communities of animals and other algae, from one-celled diatoms to sea otters. Humans, too, utilize algae as food, both straight-away and highly processed as additives to gel, thicken, or smooth out other products. Processed algae also is used in medicines and cosmetics. Key to many human uses is the mucus in kelp — the stuff that makes kelp slippery.

“Slippery” may be the most memorable characteristic of intertidal kelps for people walking in tidepools. What’s the advantage of being slippery to the kelp?

Some say being slippery makes it harder for other organisms to attach. For intertidal kelps, being slippery helps keep them from drying out before the tide returns. And when the tide does return, being slippery will help the water “slip past” the kelp, giving it some protection against being torn up by the force of the waves.

There are rewards to poking about the seaweeds exposed on the rocks. In addition to investigation the algae, you can observe some of the other life on and among them as you peek under and between the blades. But take care to replace the algae to protect the life beneath.

Take good care of yourself in your seaweed exploration, too: the slick and supple algae can be the tidepool’s most hazardous residents!

For information on how you can arrange an exploration of our fascinating natural history, contact Marty at 541-267-4027,, or Questions and comments about local natural history are welcome.