Marine algae are organisms living in the sea that utilize sunlight, via a process called photosynthesis, to sustain themselves. An arbitrary (not scientific) division of this huge group into microscopic and macroscopic components can be made.
The vast majority of microscopic, unicellular marine algae collectively comprise the vital community of life known as phytoplankton. Phytoplankton form the base of most marine food chains and, as such, have an impact on all other life in many ways.
A significant oceanic phenomenon is the phytoplankton bloom, essentially a gigantic reproductive occurrence that happens primarily in response to increases in nutrients and sunlight. Such blooms usually occur in spring and summer. In the Pacific Northwest, they generally cause seawater to become cloudy, which affects the marine naturalist’s underwater visibility.
Two types of phytoplankton bloom are of particular interest to commercial and recreational users of the Pacific Northwest’s marine environment. One, popularly called red tide (photograph A), involves several species of single-celled algae called dinoflagellates. One of these algae, Alexandrium catenella, contains a toxin that can become concentrated in the tissues of clams and mussels—bivalve mollusc predators of the alga—and cause severe illness or even death for those who consume them, including humans. This is known as paralytic shellfish poisoning (PSP). If planning to harvest these shellfish, you are well advised to consult local authorities or postings before consuming them. A tingling sensation in the lips is an indication of PSP.
At night, a second, more benign type of phytoplankton bloom produces spectacular bioluminescence. The single-celled organism involved in this bloom has the capacity to chemically generate a flash of light. In an intense bloom, any moving object—such as a diver—is illuminated in the surrounding darkness.
Macroscopic marine algae, typically referred to as seaweeds, are readily visible to the naked eye and are almost always attached to solid substrata. Unlike the complex flowering plants (Phylum Anthophyta), seaweeds have no flowers, stems or leaves. A superficially root-like holdfast merely attaches the alga to a solid substrate and derives no nutrients.
In the following section, seaweeds are presented in three major groupings (photograph B, by Michael Hawkes): Phylum Chlorophyta—green algae, Phylum Ochrophyta—brown algae, Phylum Rhodophyta—red algae. A few miscellaneous entries appear at the end.
In general, the colour distinction for recognizing the three main seaweed groups works well with the green and brown algae. The brown algae are typically a characteristic olive-brown colour, whereas some shade of green is seen in the green algae. Red algae are more problematic: those living in the intertidal zone are often black, brown-red, or even yellowish. Subtidal species are easier to recognize as red algae, being bright red or pinkish—most of the time. The colour in these plants varies because of the dominance of different photosynthetic and accessory pigments within them.
In this book, the marine flowering plants and seaweeds are arranged according to the vertical tidal zones they inhabit: high intertidal, mid-intertidal, low intertidal and subtidal. Some species live within very narrow bands, whereas others frequent a wider vertical distribution. This listing is somewhat arbitrary, as wave exposure, surge channels, shading, tidepools and other factors affect vertical distribution from site to site.
More than 600 species of macroscopic seaweeds and seagrasses live in the Pacific Northwest.

Further Reading
Conner, Judith, and Charles Baxter, 1989, Kelp Forests, Monterey Bay Aquarium, Monterey, CA, 64 pp.
Druehl, Louis, 2000, Pacific Seaweeds: A Guide to Common Marine Seaweeds of the West Coast, Harbour Publishing, Madiera Park, BC, 190 pp.
Graham, L.E., and L.W. Wilcox, 2002, Algae, Prentice-Hall, Upper Saddle River, NJ, 640 pp.
Mondragon, Jennifer, and Jeff Mondragon, 2003, Seaweeds of the Pacific Coast: Common Marine Algae from Alaska to Baja California, Sea Challengers, Monterey, CA, 97 pp.
O’Clair, Rita, and Sandra Lindstrom, 2000, North Pacific Seaweeds, The Plant Press, Auke Bay, AK, 161 pp.
Technical References
Gabrielson, P.W., T.B. Widdowson, S.C. Lindstrom, M.W. Hawkes and R.F. Scagel, 2000, Keys to the Benthic Marine Algae and Seagrasses of British Columbia, Southeast Alaska, Washington and Oregon, Phycological Contribution #5, UBC, Department of Botany, Vancouver, BC, 189 pp.
Gabrielson, P.W., T.B.Widdowson and S.C. Lindstrom, 2004, Keys to the Seaweeds and Seagrasses of Oregon and California: North of Point Conception, Phycological Contribution #6, PhycoID, Hillsborough, NC, 181 pp.
Scagel, R.F., P.W. Gabrielson, D.J. Garbary, L. Golden, M.W. Hawkes, S.C. Lindstrom, J.C. Oliviera and T.B. Widdowson, 1993, A Synopsis of the Benthic Marine Algae of British Columbia, Southeast Alaska, Washington and Oregon, Phycological Contribution #3, UBC, Department of Botany, Vancouver, BC, 535 pp.