Of the non-cyanobacterial inland HAB taxa, Prymnesium parvum is likely the most problematic in U.S. waters.  This unicellular haptophyte is tolerant of a wide range of salinities and temperatures, making it suited to environments ranging from brackish coastal waters to inland rivers and streams. Blooms of P. parvum can stretch for hundreds of kilometers, and are often monospecific because this alga consumes other competitors in the ecosystem. At high cell densities the water takes on a golden color, giving these blooms their common name, “golden alga.” It is important to note that there are also non-toxic strains of this species, therefore the appearance of a golden bloom does not always mean that the water is toxic. Conversely, low densities of highly potent P. parvum can impact ecosystems even if a bloom is not visible.

While P. parvum is best known for causing massive fish kills, toxins from these species can also kill larval amphibians and bivalves. Prymnesium parvum produces multiple kinds of toxins, some of which become more potent once they are released into the environment after the cell dies. Some toxin forms are light-sensitive and will break down in sunlight, others are rendered less harmful at lower pH levels. Affected fish display bleeding gills and mucus, and exhibit behavioral changes including slowed swimming or attempts to leap out of the water and onto shore. Species impacted by P. parvum include catfish, carp, gar and bass. Recovery of fish populations is dependent on the duration and frequency of P. parvum blooms, as well as the fecundity and mobility of the fish species.

Prymnesium parvum blooms occur worldwide, causing mortality events in Europe, Asia, and even Australia. In the United States, Texas has been particularly affected by P. parvum blooms, which were first documented in 1985. The majority of major kills have occurred since 2000 as this toxic alga has been found in an increasing number of river basins in the state; annual events have occurred in the Colorado River drainage since 2001. Prymnesium parvum has also been confirmed in New Mexico, Colorado, Wyoming, North Carolina, South Carolina, Georgia, Arkansas, and Alabama and has been suspected in Oklahoma and Nebraska.  In New Mexico, P. parvum has entered into stream environments where it threatens the survival of the Pecos bluntnose shiner, which is listed as a threatened fish species.