Golden algae (Prymnesium parvum)

Common Names: Golden alga, toxic golden alga

Description:

Golden alga (Prymnesium parvum) is a microscopic, single-celled alga that can bloom in ponds, reservoirs, and rivers and release toxins that cause massive fish kills [1][2]. It lives worldwide, mostly in salty and brackish coastal water, but it has spread into inland fresh and slightly salty waters, where its first North American fish kill was recorded in Texas in 1985 [1][2]. MISIN tracks it because its blooms can wipe out fish and other gill-breathing life over long stretches of water [2][3].

Identification:

Golden alga is a single-celled alga far too small to see without a microscope ? a cell is only about 10 micrometers across, roughly a hundredth of a millimeter [1]. You will never spot the alga itself; instead you notice a bloom, when it multiplies until the water takes on a golden, coppery, or tea-brown tint, sometimes with foam [1]. The clearest sign of a harmful golden-alga bloom is a fish kill: fish gasp at the surface, swim slowly or erratically, or even try to jump onto the bank, and dying fish often have bleeding, mucus-covered gills [1][2].

Blooms and fish kills most often happen in cooler months ? late fall through spring ? in ponds, reservoirs, and slow rivers, especially where the water is somewhat salty or brackish [1][2]. Because golden alga can only be confirmed under a microscope, a suspected bloom needs lab testing to be sure ? and not every golden bloom is golden alga or toxic, since some strains are harmless [1].

At-a-glance cues:

1. Water turns golden, coppery, or tea-brown, sometimes with foam [1]
2. A fish kill ? fish gasping, swimming erratically, or leaping ashore, with bleeding, mucus-covered gills [1][2]
3. Other gill-breathers (mussels, clams, tadpoles) also die, while birds and mammals are unaffected [1]
4. Most common in cool months in ponds, reservoirs, and brackish or salty inland water [1][2]

Habitat:

Golden alga lives in salty, brackish, and slightly salty fresh water. It is found worldwide, most often in estuaries and coastal waters, but it also blooms in inland rivers, reservoirs, and ponds ? especially where the water is somewhat salty or mineral-rich [1][2]. It tolerates a wide range of salinity and temperature, which lets it survive in many kinds of water [1]. It is thought to be marine in origin; how it reached inland freshwater reservoirs is not fully understood [1].

Reproduction:

Golden alga is a single cell that multiplies by dividing ? under the right conditions, often cooler water with the right mix of salts and nutrients, it can multiply fast enough to turn the water golden within days [1]. It spreads through water and can be carried between water bodies in anything that holds even a little water: bait buckets, live wells, boat bilges, waders, and nets [2]. Once it is in a lake or river system, it can persist and bloom again in later years [1].

Because it moves in water and on gear, the best way to slow its spread is to clean, drain, and dry boats and equipment between waters and never move water, fish, or bait from one place to another [2].

Impact and Damage:

Golden alga is not the kind of invader you find in a yard, and its fish-killing toxins are not known to poison people, pets, birds, or livestock [1][2]. Even so, agencies advise caution during a bloom: don't swim in or drink discolored bloom water, and don't collect or eat fish that are dying or already dead [1][2].

The damage is to the water's animal life. When golden alga blooms, it releases toxins called prymnesins that attack the gills of gill-breathing animals, killing fish, mussels, clams, and tadpoles, while birds and mammals are unharmed [1][3]. These blooms cause massive fish kills: in Texas alone, golden-alga kills across dozens of reservoirs have destroyed tens of millions of fish and caused tens of millions of dollars in losses to fisheries and recreation [2]. Not every golden bloom is toxic ? some strains are harmless ? so a bloom has to be tested to know the risk [1].

Similar species:

Golden alga is microscopic, so it cannot be identified by eye ? a golden or discolored bloom is not always golden alga, and not every golden-alga bloom is toxic. Only lab testing can confirm it [1].

Blue-green algae (cyanobacteria) are the main source of confusion. Their blooms also discolor water and can kill fish, but they usually form blue-green or pea-green surface scums or paint-like streaks, and their toxins can also sicken people, pets, and livestock ? unlike golden alga's toxins, which mainly harm gill-breathing animals. Only lab testing can tell the two apart [1].

Monitoring and rapid response:

Signs to watch for:

1. Water turning golden, coppery, or tea-brown, sometimes with foam, in a pond, reservoir, or slow river [1]
2. A fish kill ? fish gasping, swimming erratically, or leaping ashore, with bleeding, mucus-covered gills ? most often in cool months [1][2]
3. Other gill-breathers such as mussels, clams, and tadpoles dying while birds and mammals are unaffected [1]

What you can do:

1. You cannot remove golden alga from open water, and there is no practical home treatment ? the important actions are reporting it and not spreading it [2]
2. Report a suspected golden-alga fish kill right away to your state fish and wildlife or environmental agency (and to MISIN, below), with the date, location, and photos [2]
3. During a bloom, as a precaution keep people and pets from swimming in or drinking the discolored water, and do not collect or eat dying or dead fish [1][2]
4. Don't move water, fish, or bait between water bodies, and clean, drain, and dry boats, live wells, waders, and gear before going to another lake, so you don't carry the alga to new water [2]
5. Pond and lake managers can reduce the nutrient runoff ? such as fertilizer ? flowing into a watershed, one of the few landscape-scale levers that influence blooms [2]

When to call a professional:

1. Any suspected golden-alga bloom or fish kill ? your state fish and wildlife or environmental-quality agency can test the water and confirm the alga [2]
2. Small private ponds where an owner wants to manage a bloom ? work with those agencies or an aquatic specialist, since open reservoirs and rivers cannot be practically treated [2]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Woods Hole Oceanographic Institution, Harmful Algae & Red Tides. Golden Algae, Prymnesium parvum. hab.whoi.edu
2. Texas Parks & Wildlife Department. Golden Alga (Prymnesium parvum). tpwd.texas.gov
3. Manning SR, La Claire JW (2010). "Prymnesins: Toxic Metabolites of the Golden Alga, Prymnesium parvum Carter (Haptophyta)." Marine Drugs 8(3):678?704. semanticscholar.org

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