Tide Pools

Intertidal Marine Plants

Many marine plants, especially seaweeds, thrive in the harsh environment of the intertidal zones.

Flowering Plants

Seagrasses, like surf grasses (Phyllospadix spp.) and eelgrasses (Zostera marina), are the only submerged angiosperms (flowering plants) found in the ocean. Seagrasses are often abundant in intertidal areas.

Marine Algae (Seaweeds)

Marine algae characteristics:

  • Unlike land plants, marine algae lack true roots, stems, leaves, and flowers.
  • Most possess a holdfast (a rootlike structure), a blade (leaflike structure), and many have a stipe or stemlike structure that connects the holdfast to the blade. The weight of algae is supported by water.
  • Some marine algae also have pneumatocysts (gas-filled bladders) that float the blades close to the water's surface to maximize access to sunlight for photosynthesis.
  • Marine algae can range in size from microscopic phytoplankton (free-floating, single-celled algae) to 45.7 m (150 ft.) tall for giant kelp (Macrocystis pyrifera), which grows in coastal, underwater forests.

The three phyla of marine algae are mainly characterized by the dominant pigment coloring the plants.

Phylum Chlorophyta (green algae) - This phylum contains more than 7,000 species of green algae. The color of green algae stems from the photosynthetic pigments Chlorophyll a and b. Green algae possess these pigments in the same proportions as green land plants. The intertidal zone contains many kinds of green algae including:

  • sea lettuce (Ulva spp.) - This bright green algae is extremely thin (only two cell layers thick) and translucent.
  • gutweed/hollow green weeds (Enteromorpha spp.) - These species of green algae have flattened green tubes and commonly inhabit the high intertidal zone of coasts and estuaries.
  • dead man's fingers (Codium fragile) - This marine alga is dark green with fingerlike blades. It is found from the middle intertidal to the subtidal zones.


Phylum Phaeophyta (brown algae) - The color of brown algae results from the dominance of the xanthophyll pigment, fucoxanthin, over the Chlorophyll a and c and other pigments. The largest brown algae species are called kelp. 

Brown algae are commonly found in the intertidal zone and many contain alginic acid within their cell walls; this gel-like acid absorbs waters and keeps the algae from drying out during low tides. Algin is commercially harvested and used in pharmaceuticals, cosmetics, waterproofing and fireproofing fabrics, paper and textile production, dentistry (preparing molds), and to thicken and emulsify many different food products (ice cream, drinks, soups, jellies, salad dressings).

Some brown algae species commonly found in the littoral zone include:

  • sea potato (Leathesia spp.) - This light brown algae forms small, hollow sacs about 5 to 6 cm (2–2.4 in.) in diameter. It is found in the rocky intertidal on both the Pacific and Atlantic coasts of North America.
  • common rockweed (Pelvetiopsis limitata) and (Fucus spp.) - The branching tips of these brown algae are swollen on mature plants. While both plants range along the Pacific coast of North America, Pelvetiopsis occurs on more exposed coastlines and is smaller than Fucus. In addition, the branches of Fucus are flattened and have a midrib.
  • gulfweed (Sargassum muticum) - This Sargassum species is golden to dark brown with abundant tiny air bladders along the long, narrow blades. Introduced to many regions, gulfweed is invasive and can outcompete native algae species.
  • sea palm (Postelsia palmaeformis) - This large greenish brown alga resembles a miniature palm tree with upright stipes that each support up to a hundred straplike blades. It grows to about 61 cm (2 ft.) tall. A strong holdfast anchors the sea palm enabling it to withstand the pounding surf of the middle intertidal zone. It competes with mussels for anchorage in areas of heavy surf exposure—colonizing bare rock patches where mussels have been dislodged. The sea palm ranges from British Columbia to Central California. In northern California the blades are harvested and consumed raw or dried for use in soups and salads. In California, harvesters of edible marine algae, including sea palms, must possess a California Fish and Game license.
  • oar weed (Laminaria spp.) - This golden brown algae is characterized by a single, large, unribbed blade that is sometimes split into longitudinal strips. This kelp may reach 5 m (16.4 ft.) in height. Smaller species grow in the low intertidal and subtidal zones. In the past, humans burned down oar weed to make soda ash used for making soap and glass. Oar weed is currently harvested, and even cultivated, in some regions for algin, fertilizer, and food.
  • feather boa kelp (Egregia menziesii) - The dark to golden brown fronds of feather boa kelp can grow up to 10 m (33 ft.) long. The stipe branches and bears numerous small blades and floats along the edges. In the upper portion of the plant, the stipes are flattened and edged with small blades and no floats.
  • giant kelp (Macrocystis pyrifera) - As the largest species of kelp, giant kelp attains lengths up to 45.7 m (150 ft.)  Giant kelp grows in the cold, coastal waters of western North America, South America, South Africa, Southern Australia and New Zealand. It forms dense, underwater forests just offshore in water temperatures ranging from about 50° to 60° F (10°–15.5°C) and thrives at up to 30.5 m (100 ft.) depths. In ideal conditions, giant kelp fronds can grow as much as 0.6 m (2 ft.) per day. Although primarily subtidal, giant kelp frequently washes up into the intertidal—often still harboring invertebrate inhabitants. In Southern California, giant kelp is harvested for its algin and also used as food for livestock and farmed abalone. This commercial harvest is managed by the California Department of Fish and Game.

Phylum Rhodophyta (red algae)  - The dominant pigments phycoerythrin and phycocyanin produce the red color of the more than 5,200 species of algae in this group.

  • coralline algae (Order Corallinales, Family Corallinaceae) - Coralline algae contain calcium carbonate deposits within the plants' cell walls giving these algae a crusty texture. Coralline algae growth may be branching or encrusting depending on the species. Many kinds of encrusting coralline algae contribute to the structure of some coral reefs.
  • nori (Porphyra spp.) - The purple to blackish blades of nori are just a single cell-layer thick. Humans cultivate and harvest nori for consumption due to the abundance of vitamins and beneficial proteins it contains. Nori is also commonly used to wrap sushi.