|While having a stunning kayak trip organized by Paddlers last Wednesday evening we were all struck by the rusty brown colouration of the sea water, especially close to Fisherman's Beach near Murdock Valley. We also marveled at the phosphorescence. The sages in the group all pontificated on this phenomena known as red tide and much was proposed on its origins and characteristics. Being a group of kayakers of a most noble breed much of what was spoken was profound and true but, as the evening drew to a close in a beery haze, some remarks appeared to be closer to myth than fact. This short compilation of information attempts to substantiate some of the truths and debunk some the myths and maybe to add to our knowledge of the briny environment in which we chose to relax and have fun on our kayaks and boats.
What Is Red Tide? The term RED tide is misleading as the sea water may turn brown, orange, purple, yellow, or even black. The phrase is also a misnomer since it has nothing to do with tidal fluctuations. The colour in the water is caused by the dense concentration of very tiny microscopic plants called phytoplankton. On the South African coast the most common type of microscopic plants are known as dinoflagellates. The latter are so called because they have two whip like appendages called flagella which assist them in moving through the water. When reading about this relationship I was struck by the parallels between a kayaker with his two appendages ploughing through the water and his tiny cohabitant with his similar dual propulsion system.
How And When Do Red Tides Occur? They are most common along the west and south coasts of the Cape in late summer and autumn. During this period the prevailing southerly winds cause the cold nutrient-rich water from the deeper parts of the ocean to rise up by a process known as upwelling. When the dormant phytoplankton reside in the sediments in the deep ocean, in an environment which is dark and oxygen poor, they are known as dinoflagellate cysts. When these cysts are brought to the surface, a complex combination of environmental factors can contribute firstly to their germination and secondly to their rapid growth. Ideal conditions result from optimum sea temperatures (warm), calm conditions, salinity, the duration and intensity of sunlight and the presence of nutrients. During this bloom period each single cell may replicate itself one million times in two to three weeks. When all these conditions combine the result is a bloom known as red tide.
Can Red Tides Be Harmful? While kayaking last week in False Bay many kayakers were nodding sagely and agreeing that Red Tides are harmful and poisonous and that you should never swim in them. After all the Red Tides can produce crayfish and mussels which are poisonous. The following summary may allow you to better understand these very broad generalizations.
In general a Red Tide results from a normal natural event in which phytoplankton is produced. Phytoplankton is a key component in the food chain within the marine environment. However under abnormal conditions the plankton can congregate into such unusually high concentrations that the following unnatural aberrations result:
- In 1962 the shear density of phytoplankton within the waters of False Bay resulted in the mortality of some 100 tons of fish. According to reports the reason for death was that the phytoplankton caused irritation to their gills which resulted in them not being able to extract oxygen from the sea water. A more recent example points to the fact that the dinoflagellate species feed on fish tissue which results in fish kills.
- Oxygen depletion may result in the indirect killing of organisms in sea water. Essentially what happens is that as phytoplankton grow in the water they use up oxygen and nutrients. Once these have been used up the plankton die and produce large quantities of bacteria in the sea. The decomposition process further depletes the water of oxygen which leads to the death of other marine animals. In exceptional circumstances such low oxygen levels are believed to be the reason why crayfish vacate their environment to die on the rocks and beaches.
- The trauma for life forms in these oxygen depleted environments can be further exacerbated if a Red Tide event is trapped within a closed environment such as a bay or a density layer in quiet conditions. For instance if large volumes of red tide organic material is entrapped and decays it will result is massive oxygen depletion which provides an environment conducive for the production of anaerobic sulphate-reducing bacteria. Such bacteria convert sulphates in the water column to hydrogen sulphide gas. Such a situation which occurred at St Helena Bay in 1994 caused the sea to turn black and resulted in the death of hundreds of tons of fish, crayfish, mussels, limpets and sea urchins.
- Toxins produced by some dinoflagellates are some of the most potent poisons known to man. In 1980 mussels were killed by a neurotoxin at Elands Bay by direct poisoning. When such shellfish are eaten by man numbness and paralysis can result in body extremities (lips, tongue and fingertips) and death can occur through respiratory failure.
- Animals such as mussels, clams and oysters are particularly vulnerable to red tides because they feed by filtering particles including phytoplanktons from the water. Toxic phytoplankton can accumulate in these filter feeders and cause illness or possible death to consumers such as birds, mammals and man. The worst type of poisoning is called paralytic shellfish poisoning ("PSP") which occurs via a nerve poison. Several hundred deaths have been reported world-wide. It is possible that eating a single mussel could be fatal so don't take any chances. The less dangerous type of poisoning is diarrheic shellfish poisoning ("DSP) which results in well known symptoms. The third form, which has been noted in False Bay is neurotoxic shellfish poisoning ("NSP") where olive green discolouration of the seawater commonly occurs during the month of June. The symptoms are sensory abnormalities and include dizziness, tingling sensations, dilated pupils and hot-cold reversals. It is reported that such poisoning can be transmitted by contact with sea spray.
- With regard to aerosol (air-born) toxins. In 1995-6, people living close to the sea in False Bay, Hermanus and Walker Bay areas were overcome by the discomfort of coughing burning of the nasal passages, difficulty in breathing, stinging eyes and irritation of the skin. This phenomina was linked to the discovery of bloom of a toxic dinoflagellate species Gymmodinium first recorded in False Bay in 1998. Although aerosol toxins can lead to considerable discomfort during the event no long-term effects were noted.
Where To Find Out More About Red Tide: In South Africa the responsibility for monitoring red tide rests with the Chief Directorate Sea Fisheries of the Department of Environmental Affairs and Tourism. Researchers from the Sea Fisheries Research Institute regularly monitor the waters around the coasts. Currently (16th March 2005) Sea Fisheries is informing the public of a Red Tide warning along the Cape West Coast. If you need further and more detailed information you can contact the department's RED TIDE Information line at 021 4344457. Members of the public can call this number to get more information and current Red Tide status from the Sea Fisheries Research Institute at 021 439 4380.
Discussion Points: Well after all the above I guess we are allowed to speculate a little about Red Tide. Maybe when we paddle again we can talk about the following:
Man Made Pollution and Red Tide: Most of the articles perused indicate that Red Tides are occurring with increasing intensity and frequency with a wider global distribution. I don't know too much about increasing intensity of pollution along the West Cape but we certainly are getting increasingly worrying information on pollution around the coast of False Bay. Although I am tempted to fall for the easy pollution answer I don't get the feeling that the blooms we observed can be attributed to local pollution because the blooms would be focused around areas of easily monitored pollution such as river estuaries and sewerage disposal sites. Pollution and the associated Red Tides should be easy to identify by means of satellite imagery. The South African Red Tide is clearly defined by very low radiances from the sea along the West Cape coast from a SeaWiFS satellite image but I can't see any correlation with pollution! What do the kayakers think.
The Greenhouse Effect: The more subtle effects from global warming are a possible but very difficult to prove without major research.
Ballast Pollution: Big transport ships carry dormant cysts within the dark bilges of their ballast tanks. When these are washed out during routine bilge cleaning operations they enter a new environment. If by chance these cysts are dumped into an environment where optimal breeding conditions are present they can proliferate very efficiently. If you just think about how infectious diseases are spread under optimal conditions I quite like this hypothesis! What do you think?
Paddling and Swimming Through Red Tide: Under most circumstance I believe that we are safe kayaking and swimming through Red Tide. It goes without saying that we should not eat shellfish and crayfish during high alert periods. Maybe we should think about cooling our intake of such food especially if one considers possible pollution problems that occur close the nodes of human population. The section on airborne toxins makes interesting reading but I am not going to leave the beach quite yet.
16th April 2005