Many species of phytoplankton can cause Harmful Algal Blooms (HABs). HABs are any bloom that causes harm to the ecosystem, other marine animals, or humans. HABs can be harmful in a variety of ways; some species produce toxins, some form such dense blooms that they deplete oxygen, some species clog or lacerate fish gills, and others strip seabird feathers of their
waterproof coating.

At the Santa Cruz and Monterey wharves, a phytoplankton called Pseudo-nitzschia is a common HAB former. Pseudo-nitzschia can produce a potent neurotoxin called domoic acid. Domoic acid can build up in the tissues of filter feeders like mussels, and be dangerous to marine animals or humans who consume them.

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Pseudo-nitzschia