Red Tide at Noetzie in Knysna - photo by Wendy Dewberry

Red Tide at Noetzie in Knysna – photo by Wendy Dewberry

On Friday, the Noetzie Conservancy in Knysna reported a red tide which is an algal bloom (large concentrations of aquatic microorganisms). Yesterday, on our Love Knysna Facebook, Mark Taylor, a surfer, stated that the “red tide was blooming at Buffalo Bay” as far back as New Year’s day. “Water has been strangely warm considering all the easterly winds… we surfed in a weird, brown type of sea… tasted like iron or money.”

You may know not to eat shellfish during and after a red tide but maybe you don’t know how it occurs and exactly what the dangers are.

The “red” in “red tide” is not definitive as it can also be yellow, brown, orange or purple. The oceans colour change is determined by the angle of the sun and the species of phytoplankton (small sea organisms) whose production is suddenly boosted when cold currents bring them to the surface to encounter appropriate salinity, carbon dioxide, warmth and light. states that “most red tides along the South African coast are caused by a group of phytoplankton known as dinoflagellates. These single-celled organisms are able to swim short distances by means of two whip-like appendages called flagella.”

anatomy of dinoflagellate - taken from

anatomy of dinoflagellate – taken from

In most cases, a red tide is a natural process and, as in Knysna, an indication of our nutrient rich waters. On rare occasions, when the concentration of dinoflagellates becomes too dense (more than a million cells per millilitre), they can be harmful. Some species toxicity can cause death in humans but be assured that this is not the norm in South Africa and, even then, mostly restricted to the West Coast). It’s more likely to cause mild skin irritation, stinging eyes, respiratory problems, nausea and vomiting. However, indirectly, through shellfish (mussels, clams and oysters), which filter sea water, and consequently the red tide within which phytoflagellates have possibly accumulated toxins, there can be fatality via PSP (Paralytic Shellfish Poisoning).

Please not that this is meant to be an informative and not a scare article. The chances of catching PSP in Knysna from the latest red tide is low (just play safe). But for national and international readers, note that PSP can cause death in 2-24 hours. Wikipedia says that “symptoms can appear ten to 30 minutes after ingestion and include nausea, vomiting, diarrhea, abdominal pain, tingling or burning lips, gums, tongue, face, neck, arms, legs, and toes. Shortness of breath, dry mouth, a choking feeling, confused or slurred speech, and loss of coordination are also possible.”

phytoplankton cycle

Phytoplankton Cycle

Marine life is more in danger as severe concentrations can block fish gills or remove oxygen from the water. Irma van der Vyver and Dr Grant Pitcher state that, “In March 1994, South Africa experienced its worst recorded marine mortality in the west coast area of St. Helena Bay. The event was caused by the entrapment and subsequent decay of an extensive red tide dominated by the non-toxic Prorocentrum micans and Ceratium furca, with the toxic species Alexandrium catanella and Dinophysis acuminata present in lower concentrations. Marine life died because of suffocation or hydrogen sulphide poisoning. The low oxygen conditions allowed anaerobic, sulphate-reducing bacteria to convert sulphates in the water column to hydrogen sulphide gas, which corroded metal objects and caused respiratory problems amongst some residents of the area. These chemical reactions also caused the sea to turn black, and the event was soon dubbed a ‘black tide’ by the media. Approximately 60 tons of crayfish and 1500 tons of fish, comprising about 50 species, washed ashore.”

Locally, SANParks failed to respond to 2 queries of mine and the national Red Tide Emergency Line rang unanswered. However, in contrast, on the ground, Wendy Dewberry, of the Noetzie Conservancy, disagreed, saying that, “Shamley Titus of SANParks reacted immediately yesterday to our call and arrived at Noetzie, and then at my door, to confirm the Red Tide. SANParks officials are out in full force and Shamely would not even accept a cooldrink, explaining that he was extremely busy and had a long list of call-outs to attend to. I have to just put that right… SANParks in Knysna are hero’s on the ground and in every department I have experienced only support, information and assistance, day or night.”

In contrast, the Knysna Municipality and Knysna Tourism failed to put up any information on their websites.

Red tides are on the increase worldwide with global warming and human activity such as phosphates (used in fertilisers) the likely suspects. However, it is important to note the earlier point which is that phytoplankton blooms are a necessary part of reproduction, part of a natural chain essential to life in water.

After a red tide, best not to eat locally acquired shellfish for 2 months. Be safe, not paranoid – simply ask your chefs where they get their stocks from. No need to give up on a succulent seafood platter as many restaurants are supplied by wholesalers from stocks out of town.

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