South coast red tide (blog post)

Image credit: NOAA

The red tide that occurred along the Cape south coast in January-February was caused by the dinoflagellate Lingulodinium polyedrum (formerly Gonyaulax polyedra), which before now had never been seen in any numbers in our waters, although the species had been recorded here. It’s well known from southern California, where it frequently forms spectacular blooms – given that it’s one of a number of bioluminescent phytoplankton species - and put on a fine display in the Maldives in December, as captured by a honeymooning tour guide from Taiwan. Phycologists of the German Botanical Society are so taken with it that they named it Alga of the Year in 2013!

The bloom made its presence known all along the coast from Port Alfred to Hermanus at various stages in January and February. Such an extensive event was unprecedented for the south coast – red tides are more common on the West Coast, where the prevailing south-easterly winds in spring and summer cause regular upwelling of cold, nutrient-rich water, as well as dinoflagellate cysts, from the dark depths to the surface. It’s the conditions between upwelling events - when the upper water column is calmer and warmer - that stimulate germination and growth of dinoflagellates though. By beating their whip-like flagella, they can ‘swim’ and maintain their position in water layers with optimal light and nutrients, unlike diatoms that need turbulent water to tumble them about. Winds and currents concentrate their numbers, particularly in bays where they may get trapped inshore. Depending on the species and its dominant pigments, the bloom can colour the water various shades of red, green, brown or purple.

Since blooms may not be red and don’t necessarily come and go with the tides, the more ‘pc’ term is a harmful algal bloom, and the Lingulodinium bloom was certainly harmful to marine life. It resulted in fish and invertebrate mortalities, possibly due to its depletion of oxygen in the water column or gill-clogging. Although its yessotoxins are known to accumulate in filter-feeding shellfish, laboratory experiments with mice have revealed low toxicity with oral ingestion, and there are no documented cases of human poisoning.

Upwelling does occur along the south and east coasts too, although on the east coast and Agulhas Bank the mechanism is different, related to the Agulhas Current and continental shelf. From Cape Padrone (Algoa Bay’s eastern boundary) along the south coast, strong and persistent easterly winds result in intermittent upwelling at prominent headlands.

Marine scientists will be looking at recent weather patterns and satellite imagery (both photographs and data products such as sea surface temperature and algal indicators) to try to figure out what environmental conditions fuelled this huge HAB event. Were there calm seas and light onshore winds following wind-induced coastal upwelling? Was the Agulhas Current meandering unusually, or had breakaway eddies swept warm, nutrient-rich water close inshore? Did flooding rivers flush large quantities of nutrients into inshore waters and intensify stratification of the water column?

Climatic conditions in early January were certainly unusual – an unseasonal upper-level low brought heavy rains to the south-western parts of the country, with the result that the month's rainfall stats were somewhere between 200 and 1000 per cent above the long-term average. In contrast, the south-eastern parts were hot and humid, with warm inshore waters.

bloom-JeffSchmaltz-NASAThe first official warnings about the red tide came from Knysna on 8 January, after a bloom was seen at Knoetzie and low numbers of the dinoflagellate were found in water samples collected from Knysna Lagoon. By then sightings had also been reported in Port Elizabeth’s Algoa Bay, further east. This satellite image, taken at the end of December, suggests that the bloom may actually have originated in Algoa Bay, although a large bloom is already visible off Mossel Bay. This could have been transported inshore and westward by winds and currents, and formed a red tide where it was concentrated in bays along the coast.

Image credit: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC

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