The first red tide of the season has appeared in Algoa Bay, threatening fish life and sending a reminder of the perils of establishing an offshore aquaculture industry in the area.
SA Environmental Observation Network coastal node manager Prof Tommy Bornman said on Wednesday the average measure of plant plankton in the ocean was 0.5-5 micrograms per litre (ug/L) of water and anything over 10ug/L was considered a bloom.
“So, at 111ug/L — as the European Space Agency satellite data has calculated this week — is an exceptionally intense red tide.
“It is not toxic, but it causes harm by clogging the gills of fish and other plankton feeders.”
He said after the death of the plankton which made up the red tide, anoxic conditions could result, where there was low to zero oxygen.
This happened because bacteria used the oxygen in the water to break down the dead plankton.
“Anoxia can cause extensive fish kills, and creatures on the sea floor like octopus and crabs can die too.
“Algoa Bay is now a recognised hotspot for these harmful algal blooms, and the big risk is for the planned fish farm.
“A bloom like this will kill off all the fish stuck in a cage out at sea.
“It is also problematic for any terrestrial aquaculture or desalination activity that relies on the inflow of this water.”
The department of forestry, fisheries and the environment first proposed an aquaculture zone in Algoa Bay more than a decade ago.
After an outcry it was reconfigured and, despite continued objections, it was approved in 2020.
It now includes a shellfish farm off Summerstrand and an expansion of the existing oyster farm off North End, plus a fin fish farm off the Port of Ngqura and adjacent to the Addo marine protected area.
Nelson Mandela University marine biologist Prof Lorien Pichegru said at the time the fin fish farm was bad news for the critically endangered African penguin.
This was because the excess food and antibiotics from the floating fish pens would pollute the water, coat the reefs and chase away wild fish, including the pilchards and sardines the penguins fed on.
Bornman said the current red tide was the result of natural processes, but it was probably exacerbated by anthropogenic or man-made problems.
“East winds blew the warm surface water towards the shore and cold nutrient-rich bottom water rose to take its place, in a process called upwelling.
“Plant plankton called diatoms fed on the nutrients and, after the diatom died, they released ammonia into the water.
“Then the dinoflagellate plant plankton moved in, dominated for the first time in our experience by a species Tripos furca.”
He said the dinoflagellates could move up and down in the water column and were also armoured, making them less attractive to the animal or zooplankton waiting for their turn at the trough.
“The dinoflagellates would have consumed the ammonia and the diatom carcasses and then died themselves.
“The end of the bloom will come when the zooplankton consumes all the dinoflagellates, or the nutrients run out.
“So we think it is a natural bloom but record high water temperatures in Algoa Bay during the first three months of the year could have contributed to selecting specifically Tripos furca to bloom.
“The whole process would also have been exacerbated by the increasing nutrients flowing into the bay from sewage pipes and rivers.”
He said nothing would happen to open water swimmers or anyone else who inadvertently took a gulp of seawater, and it did not pose a problem for fishing.
“The red tide may kill the fish by suffocating it through clogged gills, but if the fish you catch is alive, then there is nothing wrong with it, and you can eat it.”
He said the red tide would probably not survive much longer than a week unless there was a fresh upwelling before then.
The Herald
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