City blames nutrient loading, heat, for fish die-offs
A CITY probe has found that low oxygen levels with high daily temperatures and high nutrient loads possibly caused the recent fish dieoff at the mouth of the Milnerton Lagoon.
About 500 dead fish, consisting of juvenile mullet species, primarily southern mullet, were found earlier this month in the water.
This follows widespread complaints from residents about a worsening stench at the site, and a suspected sewage spill.
Mayco member for Water and Sanitation Zahid Badroodien said investigations into the incident indicated that fish die-offs had been observed to occur in late summer when dissolved oxygen levels are lower, with high daily temperatures.
“High nutrient loads further increase the likelihood of fish die-offs. Fish die-offs relating to nutrient loads are often linked to toxic ammonia levels. Nutrient inputs can drastically reduce oxygen levels in the lagoon. In low-oxygen conditions, ammonia predominantly presents in the un-ionised form. This is extremely toxic to fish with an acute toxicity level of 0.1mg/l.
“The high summer temperatures and low oxygen levels combined with high ammonia levels may have led to high un-ionised ammonia levels,” said Badroodien.
According to him, there were multiple sources of pollution entering the lagoon, which all contributed to the nutrient loading in the water course over time.
“In addition to the recent repairs at the pump station, the City continues to implement special measures to limit the amount of illicit greywater which flows to the lagoon via the stormwater system, by over-pumping from the stormwater system into the sewer system at strategic points, as and when required.”
He said final effluent from the Potsdam Wastewater Treatment Works had an overall average effluent compliance of 80% since May 2021.
The Milnerton Central Residents Association said to claim the fish dieoffs were caused by anything other than the pollution was either disingenuous or a failure to understand the situation.
Spokesperson, Caroline Marx said the incident was not normal or natural, but the consequence of the increased nutrients, including ammonia, in the lagoon.
“This has been caused by noncompliant effluent discharges, repeated pump station spills and highly sewagecontaminated water entering the river at the Erica Road outlet.
“All of these causes were identified in the directive from the WC government.
“The over-pumping at Erica Road is appreciated but insufficient; E Coli levels here measured 70 million on February 21.”
Marx said while the Potsdam performance had improved, there had also been serious malfunctions particularly at the end of December last year, leading to enormous quantities of effluent – allegedly not meeting safety standards – being discharged.