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Golden algae confirmed as casue of fish mortalities at Zandvlei

posted Apr 20, 2012, 12:37 AM by Damian Gibbs   [ updated Apr 20, 2012, 4:28 AM by Damian Gibbs ]

It is confirmed that a extensive algal bloom comprising the species Prymnesium parvum, commonly referred to as “Golden algae”, is responsible for the unusual conditions and fish mortalities currently prevailing in Zandvlei. Significant numbers of this species have been identified in samples collected from Zandvlei on 16 April 2012. Of interest is the fact that it was also implicated in a fish kill at Zandvlei in 1973. The Golden algae is classified with the Haptophyceae and does NOT belong to the same family as the so called “Blue-green algae” or Cyanophyceae which are periodically problematic in other waterbodies around Cape Town.

When the population of Golden algae increases during a bloom cycle, the water may begin to turn cloudy yellowish, yellow-copper, or a copper-brown tea color. Another sign of P. parvum blooms can be foaming at the water surface if agitated or aerated, such as where there is wave action.

Prymnesium parvum is a microscopic (about 10 µm), flagellated alga that is capable of producing toxins that can cause extensive fish kills. This algal species is found worldwide and is most often associated with estuarine or marine waters, but it can also occur in inland waters that have a relatively high mineral content.This organism is especially notorious for seasonal harmful algal blooms wherein cell densities increase rapidly with the accompanying presence of potent toxins. The algal blooms do not always form toxins however and the onset of toxic conditions can be difficult to predict.

Golden algae produce a number of these toxins, collectively known as “prymnesins”. The toxin adversely affects gill-breathing organisms such as fish, bivalves, crayfish, gilled amphibians, and also some species of plankton. The toxin damages the permeability of gill cells, which lose their ability to exchange water and absorb oxygen and bleed internally, resulting in death of the organism by asphyxiation.

Although the algae can exist in waters without causing harm, under certain environmental conditions, P. parvum can gain a competitive edge over other algal species, and algal blooms can develop. The presence of blooms of P. parvum does not necessarily mean the algae will produce and secrete prymnesins into the water. The alga can remain dormant as cysts in the benthic sediments of water bodies for many years until favourable conditions for their proliferation exist.

Fish affected by P. parvum toxins behave erratically. They may swim slowly below the surface, accumulate in the shallows, or show no normal avoidance of human presence or other disturbance. They may try to leap from the water to avoid the toxins. If clean water flows into the affected body of water, fish will often accumulate around this freshwater source. Signs of intoxication include redness or bleeding in the gills, at the base of the fins, around the mouth area, and along the belly, and the fish may be covered with mucous. Young fish are often more sensitive to the toxins than adults. In the early stages of intoxication, the effects are reversible if the fish can move to uncontaminated water. Ecological impacts will vary depending on the length and severity of the toxic bloom. In larger water bodies with access to fresh water, toxic blooms may not kill all the fish present.

In addition to the toxicity of the algae, the fish at Zandvlei are also affected by diurnal depletion of dissolved oxygen levels due to night time respiration by the large algal bloom and all other aquatic organisms within the water body.

Prymnesium parvum blooms are not a public health threat. The toxins that golden algae produce appear to have no negative effect on other wildlife, livestock, or humans. However, as a common sense precaution, dead or dying fish should not be consumed.

P. parvum can thrive in a wide range of physical conditions, but nutrient availability has been shown to greatly influence bloom and toxin formation in this organism. Eutrophication of waters are commonly implicated in a growth surge in P. parvum populations. High nitrogen (in the form of nitrate or nitrite) and phosphorus (phosphate) concentrations support the rapid growth and formation of blooms that subsequently result in an imbalance of N and P sources. Lowered nutrient levels ultimately result in the algal population producing toxins.
Due to the predominance of blooms in low salinity and brackish water systems, many studies have examined the effects of salinity on prymnesin toxicity. In most cases, P. parvum has been recorded in highly-mineralized freshwaters between 3 and 8 ppt ; some assert that P. parvum can grow almost anywhere, in salinities ranging from 1 to 45 ppt. However, low salinity conditions have been shown to have a negative effect on growth rates of P. parvum.
The inactivation of prymnesins and eradication of P. parvum is of great interest to fish and wildlife biologists working to mitigate these blooms all around the world. The inactivation of prymnesin toxicity has been observed to occur under certain physicochemical conditions, but many methods are not feasible when balancing healthy fish and while retaining beneficial algae. Algaecides are commonly employed by fish farmers for the eradication of P. parvum in fish culture ponds but these chemicals have to be used at levels necessary to kill the alga without severely harming the fish. This treatment would not be appropriate in a natural ecosystem such as Zandvlei due to mortality of non-target organisms that would occur.
It is anticipated that fish mortalities at Zandvlei will continue until the bloom conditions subside. Reduction of salinity levels at Zandvlei if for example we experience heavy Winter rain may assist since the algae prefer saline conditions that typically prevail in estuaries.
City staff from the Biodiversity Management Branch are on site to assist with relocation of live fish and removal of dead fish.
Kind regards
Candice Haskins
Senior Professional Officer (Aquatic ecologist)
Catchment, Stormwater & River Management Branch
Roads & Stormwater Department
Transport, Roads and Stormwater Directorate
City of Cape Town
Tel: 021 4003088 / 710 8263
Fax: 021 4004554