Seaweeds and Seagrasses

General Biology


World wide there are 7000 estimated seaweeds, 4000 microalgae and 50 seagrasses.  Here in the Pacific Northwest we have an estimated 700 of these species making our area one of the richest in seaweed biodiversity in the world.  (Harbo, 1999)

Sites visited:

Fox Island - South Puget Sound

Willapa Bay - WA Outer Coast

Trip aboard the Centennial - San Juan Island

Argyle Lagoon - San Juan Island

False Bay - San Juan Island

Cattle Point - San Juan Island

Salt Creek - Olympic Peninsula

Sokal Point - Olympic Peninsula



Both seaweeds and seagrasses are primary producers, this means they are at the base of the food chain and support much of the life that lives in our oceans. Primary producers make their own food from the sun by photosynthesis.


Photosynthesis: Sun + CO2 + H2O = Nutrients for the Plant or Seaweed + Oxygen for us

Both seaweeds and seagrasses use photosynthesis to support themselves, but they absorb the nutrients in very different ways. Plants (seagrasses) have roots, stems and leaves making up a complex system that transports the nutrients from their roots to other parts of the plant. Algae (seaweeds) absorb what they need on a cellular level through any part of their structure.

The structure of seaweeds is a bit different then that of a plant, but it still has some of the same elements. Seaweeds have a holdfast, stipe and blades, which look similar to roots, stems and leaves on a plant (see picture)


The holdfast anchors the seaweed to a rock or other sturdy object, but does not serve as the primary sorce of absorption for water and nutrients like in a plant. The stipe acts as a middleman between the holdfast and the blade (many seaweeds don’t have a pronounced stipe and appear to go from the holdfast to the main bladed portion of the plant.) The blade is usually the largest and most noticeable part of the seaweed. It floats on or near the surface and absorbs as much sun light as possible.

Because seaweeds and seagrasses rely on sunlight to support themselves they only inhabit waters that the sun can penetrate. In the Pacific Northwest, seaweeds and seagrasses are found from high intertidal zones to about 20m deep, depending on plankton levels and the clarity of the water.


green   red   brown


sea grass

purple sea star