Image: Sean Bailey, Senior Scientific Technician, SAEON/SMC
Red tide has surfaced in Nelson Mandela Bay and scientists are due to collect samples in the bay on Thursday to assess what species it is and what danger it poses, if any.
SA Environmental Observation Network Elwandle coastal node acting manager Dr Shaun Deyzel confirmed on Wednesday the discoloured water associated with red tide spotted last week in St Francis and off the Gamtoos River had now appeared in Algoa Bay.
“We’re not sure at this stage which species it is, but I know there’s a lot of anxiety as to how dangerous it is, so my team will be going out into the bay tomorrow to sample from the red tide patches.”
He said the last serious outbreak of red tide in the region had been in 2013/ 2014, when vast blooms had stained Algoa Bay and waters further down the coast dark brown and had caused a considerable die-off of fish.
“We’ll know more in a few days as to which species it is and therefore how harmful it is once we have collected samples and then studied what we have.”
News of the arrival of red tide in the bay popped up early on Wednesday on ocean conservation and open water swimming WhatsApp groups.
Deyzel said there was no evidence yet that it was a danger to swimmers or even people eating fish.
“For now, as a precaution, it’s probably wise to avoid consuming shellfish, which can become contaminated by red tide toxins.”
He said red tide was an occasional phenomenon related to the prevailing easterly winds at this time of the year, the “upwelling” of nutrients to the surface of the sea and the accumulation of swathes of plankton.
“The plankton thrives on the nutrients and sunlight and occasionally within that process one species will proliferate and completely dominate the other plankton and then the water becomes discoloured various shades of pink or brown.”
He said the tiny carcasses of these short-lived organisms sometimes accumulated on the seabed in such numbers that they sucked oxygen from the surrounding waters.
“There is no evidence that we are facing that now, but in extreme cases, as in 2013/2014 areas can be deoxygenated, displacing marine animals and fish which can move away, and killing those which cannot.”
He said whatever the species the plankton would be broken up and washed out of the bay if there were several days of strong westerly winds.
“This process is hindered a bit here in the western section of the bay off Gqeberha because the water is more sheltered and stagnant and has pockets of nutrients from stormwater drains.”
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