Exotic algae spreads across Swartkops estuary

A swathe of exotic black algae is spreading across the middle reaches of the Swartkops and Sundays estuaries, suffocating fish and raising a red flag for human health.

By skewing oxygen levels and leaving a creeping mucus in the gills of fish,  Heterosigma akashiwo  nullifies the key ecological role of estuaries as fish nurseries which in turn support important fisheries needed for human nutrition and livelihoods.

But it could also be a sign of a much more direct threat to people in the form of disease, Nelson Mandela University (NMU) estuarine ecologist Prof Janine Adams said on Monday.

Adams said the algae, originally from Japan, had likely come to South African shores via ship ballast water or migratory birds, and had long been present in the Sundays in the form of mass blooms triggered by agricultural runoff.

“We’ve known about the same species in the Swartkops estuary but because that system is better flushed by the sea we had thought it would not get too dense.

“Now we are seeing a bloom of Heterosigma for the first time in the Swartkops.”

As in the Sundays, the bloom was clearly triggered by the injection of a heavy nutrient load, but in the Swartkops the source of the problem was sewage flowing in from the treatment works in Uitenhage and Despatch, exacerbated by pollution introduced by runoff from the recent rain, she said.

“We have had continual sewage spills through the years, apparently because the system is overloaded.

“But even when there is no spill and the effluent has been treated, it should not be channelled into the estuary because our water quality standard in SA in this regard is not high enough.

“Until we can improve that standard, we have to do something else with the water on land, like creating artificial wetlands. 

“We have to stop the source of the sewage, otherwise these harmful algal blooms will continue in the Swartkops.”

Harmful algal blooms radically changed the oxygen levels of the surrounding water, impacting on all estuarine life that could not take evasive action.

But even fish that could swim away could be impacted by Heterosigma akashiwo, she said.

“It generates a mucus that sticks in the gills of a fish and can kill it.”

The Sundays had already been lost as a functioning fish nursery where different marine species sheltered at different times in their life cycle and if the Swartkops was lost as well, the implications for fish conservation and the fishing industry would be severe, she said.

“We are at a tipping point.”

Adams said the issue should be of grave concern to the public even beyond the fish nursery issue.

“Besides the environmental problems this sewage causes, it also likely carries pathogens and viruses and we need more research in that regard.”

Zwartkops Conservancy spokesperson Jenny Rump said the algal bloom had been spotted by paddlers at the weekend and was on Monday clearly visible from the Wild Bridge at Swartkops village, extending up and down the river.

“We've had other kinds of algae before but not this. It has turned the water brownish-black and there is scum floating on top. It’s very bad news.”

NMU fish specialist and zoology department head Prof Nadine Strydom said Heterosigma's boom-bust life cycle caused severe disruption to indigenous estuarine life. 

“We know that when it blooms it supersaturates the water with oxygen, preventing the proper growth of, for instance, round herring, which is a major food fish for a variety of estuarine predators.

“Then when the algae runs out of nutrients and decays, it extracts almost all the oxygen from the water. The result is fish can move away but younger, less mobile fish and other estuarine animals can die.”