Large-scale toxic red tides plague eastern and southern coasts of South Africa
The coastal areas between East London and Wilderness have been subjected to the largest, and most persistent, red tide in recorded history.
The red tide first made its appearance in mid-December at several coastal locations simultaneously and grew in size to cover more than 300 km of coastline (see satellite chl-a image below).
The red tide is caused by dinoflagellate species that rapidly increase in numbers to form a bloom. The high concentration of cells, together with the red to brown colouration of their photosynthetic pigments, creates the red colouration in the water (see image of sampling onboard R/V uKwabelana). The east and south coasts regularly experience red tides at a frequency of approximately one or two a year, but in the past these have always been caused by a non-toxic species, Noctiluca miliaris Suriray.
The latest blooms that extend from near East London all the way to Wilderness are different, even though these blooms were also characterised by spectacular bioluminescence displays (also known as phosphorescence). SAEON and the Nelson Mandela Metropolitan University (NMMU) monitored the red tide in the Algoa Bay area during the last three weeks of January. Samples were sent to USA and European colleagues and they positively identified the species as Lingulodinium polyedrum (F. Stein) J.D. Dodge (synonym Gonyaulax polyedra), which produces yessotoxins (YTXs) (see light micrograph on right).
Harmful algal bloom
This species is included in the IOC-UNESCO Taxonomic Reference List of Harmful Microalgae as the YTXs may accumulate in bivalves and are toxic to mice. Effect on humans is unknown (Paz et al., 2008; Gerssen et al., 2010). This is the first recorded occurrence of a harmful algal bloom along this part of the coastline.
A harmful algal bloom causes negative impacts to other organisms either via the production of natural toxins, mechanical damage to other organisms (cells stuck in the gills of fish), or by decreasing the oxygen levels in the water when the bloom decays.
Why only along this particular stretch of coastline and why have we not seen this species before?
Hypotheses abound and range from ballast water contamination to climate change. SAEON’s hypothesis is that this part of the coastline experienced typical summer wind driven upwelling that resulted in nutrients being brought to the surface. This event was followed by two to three weeks of relatively calm weather, during which the warm surface waters (up to 25oC) and lack of large oceanographic events (due to the Agulhas Current being far offshore), created the ideal environment for phytoplankton to bloom. Dinoflagellate cysts (resting cells) hatched and, because of the ideal conditions, rapidly divided to form a bloom.
It is still unclear where the cells came from and we would probably never know, but they could have been present in small numbers in the area for many years without being noticed. A more sinister explanation is that they could have been brought to our coast in the ballast water of ships from the Adriatic Sea, where the species is more common.
Whether the species is toxic or not to humans, it is having a deleterious effect on the marine life along this beautiful part of the coastline. The bloom started decaying at the end of January and the decomposition of the cells is using up the oxygen in the water. SAEON and NMMU measured oxygen concentrations as low as 1 mg/l in the bottom waters in Algoa Bay and these anoxic conditions have already resulted in several separate instances of marine organism mortality. At Sardinia Bay (a Marine Protected Area), more than 350 reef fish washed ashore, as well as several other invertebrates, macroalgae, etc.
Although the bloom appeared to be decaying along most parts of the coast, a recent east wind resulted in the upwelling of more nutrients and it would appear as if the blooms are developing again. Lingulodinium polyhedrum are known to produce temporary cysts at the end of a bloom and thereby remain in the plankton so that once the environmental conditions become favourable again, the cysts are able to re-seed the area and initiate another red tide event. The red tide may remain a feature of this coast for a few more weeks/months and the presence of the cysts in the sediment will mean that the area could be exposed to this kind of red tide more frequently in future.
SAEON and NMMU will continue to observe this unique phenomenon to gain a better understanding of the environmental drivers, ecology and ecosystem response to the red tide. This will enable SAEON to issue a timeous warning of a potential harmful algal bloom to government departments, coastal managers and mariculture operators.
Gerssen, A. et al., 2010. Marine Toxins: Chemistry, Toxicity, Occurrence and Detection, with special reference to the Dutch situation. Toxins, 2: 878 – 904
Paz, B. et al., 2008. Yessotoxins, a Group of Marine Polyether Toxins: an Overview. Marine Drugs, 6: 73 - 102