The murky green water that engulfed Balluta Bay over the weekend is a “red flag warning us that something unusual is happening”, according to marine biologist Alan Deidun, although the true cause of the phenomenon remains unclear.

Deidun was speaking to Times of Malta on Monday, a day after photos of the bay posted to social media caused widespread alarm.

Deidun said that the discolouration is caused by an algal bloom, a phenomenon in which algae in the water grows rapidly and excessively in a short span of time, becoming visible to the naked eye.

Studies suggest that the phenomenon is becoming increasingly common in the Mediterranean, with several reports of Alexandrium blooms, a type of phytoplankton that turns water into a green/yellow hue, recorded along Italy’s coast in recent years.

Alan Deidun. Photo: Chris Sant Fournier

Similar discolouration has happened before in Malta too, Deidun says, pointing to a 2017 incident in which the Tas-Sukkursu canal dramatically turned a deep shade of pink.

But, Deidun says, these algal blooms usually happen in late winter or early spring, not summer.

“It’s a little anomalous to have this in summer; it suggests that it could be driven by things that are not natural.”

Deidun says that algal blooms, such as the one that hit Balluta Bay, are often caused by specific environmental conditions, such as high temperatures causing low oxygen levels in the water.

But, Deidun says, other factors, such as sewage contamination, could also play a part in accelerating the growth of algae.

“Only extensive water testing can determine the cause of the algal bloom but it could well be a combination of factors,” he said.

The bay has been closed throughout the summer after two instances of E.coli contamination were detected. E.coli is commonly attributable to untreated sewage but the Water Services Corporation has insisted that the contamination is “not from the sewage system”.

Deidun says he recently noticed a similar, albeit less intense, case of an algal bloom at Golden Bay.

These sort of algal blooms are “usually very shortlived”, Deidun says, with the sea returning to its natural hue within a matter of days.

But, he is quick to add, “this really depends on the cause of the bloom”. If, for instance, a sewage runoff isn’t addressed, the same phenomenon could rear its head again.

The algal bloom itself, although unpleasant to bathers, is unlikely to be toxic, Deidun says. “These sort of blooms can be toxic in extreme cases but this isn’t usually the case in the Mediterranean,” he said.

In a brief statement on Sunday evening, the Environment and Resources Authority said that it is investigating the situation “in a number of bays around Malta” together with the Environmental Health Directorate.

The authority said that “algal blooms may occur due to high temperatures and reduced water circulation, conditions typical of heatwaves such as the one we are currently experiencing”.

Malta experienced above-average temperatures over the weekend, which prompted the Met. Office to issue a yellow weather warning.