Plankton, including the microscopic "grass" of the sea (phytoplankton) and the tiny animals that feed on them (zooplankton), are composed of many different species. Many of these are benign, but some are harmful to marine life and humans, even though they are often only a small part of the total plankton community. In BC waters, Harmful Algae Blooms (HABs) regularly cause severe economic losses through finfish and shellfish mortality and shellfish harvest closures.

As part of the CitSci program, water samples are taken and examined under the microscope, to determine whether or not these harmful algae are present. The most important species of harmful algae in BC waters are:

• Alexandrium spp.: many species from this genus produce a neurotoxin (saxitoxin) which causes paralytic shellfish poisoning (PSP) in humans and other mammals. There are numerous shellfish harvest closures every year in BC due to saxitoxin in shellfish flesh exceeding regulatory limits.
• Chaetoceros convolutus and C. concavicorne: two species with long setae (spikes) that can be harmful to fish gills when present in large numbers.
• Dictyocha spp.: a silicoflagellate which is toxic to fish.
• Dinophysis spp.: a dinoflagellate that produce toxins causing diarrhetic shellfish poisoning (DSP).
• Heterosigma akashiwo: an ichthyotoxic raphidophyte, this species is an important source of salmon and herring mortality and probably the most prominent fish-killing algae in the world.
• Pseudo-nitzschia spp.: some species in certain conditions produce a neurotoxin (domoic acid) that causes amnesic shellfish poisoning (ASP). Although it is not generally considered a problem in the Strait of Georgia, the CitSci program started to enumerate this species in 2018.
• Rhizosolenia setigera: a centric diatom with long, needle-like spikes that can clog fish gills when present in large numbers.