The phenomenon of a red tide event is enough to strike fear into the hearts of any avid shell-fish collector or shelf fish culinary enthusiast. The term “red tide” is however, somewhat of a misnomer, as the discolouration of the sea-water, caused by a bloom in microscopic plant species known as algae, can take on any hue ranging from colourless to green to brown. Extending the misnomer, there is no intimate link between these algal growth blooms and tidal movements, which are linked to the gravitational force of the moon and other celestial bodies.
Algal blooms form under favourable conditions, when there is a sufficient supply of nutrients such as nitrate and phosphate in the surface waters (think of the effects of fertilizer on terrestrial plants), coupled with sufficient sunlight to drive the algal growth. These algal blooms are not necessarily dangerous and in fact play important roles in restocking our atmosphere oxygen and taking out the harmful green-house gas carbon dioxide. However, some algae species produce natural toxins and although these species and their toxins are always present in ocean waters at small concentrations, under bloom conditions, their concentrations can increase to levels that become dangerous, mostly to other marine organisms but may also result in irritations to susceptible beach-goers and ocean lovers.
A more pronounced way in which these toxins can affect humans is through the ingestion of shell-fish, which, because of their filter-feeding habits, may accumulate high concentrations of these relatively persistent toxins (bioaccumulation). It is therefore advisable that, if a red-tide has moved through your area, one should first determine whether it was indeed a harmful algal bloom (consisting of toxin-producing algae (sometimes known as a “black-tide”)). If this was indeed the case, or if reliable information on the bloom cannot be attained, it is advisable to avoid the consumption of shellfish from the area for a few weeks therafter.
When the supply of nutrient has been exhausted and the algal bloom begins to die off, a secondary concern arises. Microbial decomposition of the sinking plant matter strips oxygen from the water column and can lead to low oxygen, or hypoxic conditions. If a water mass is isolated from ventilation with high oxygen waters, the situation can become dangerous for marine organisms and can result in large scale fish mortalities, detriment to fish egg development and of course, the infamous “lobster walk-outs”. These events are synonymous with the Southern Benguela upwelling region on the West Coast of South Africa. The earliest reported lobster walk-out event was as far back as 1869.
Although it is commonly thought that it is the low oxygen in the water that drives the lobster towards the oxygen surf zone, a recent study has shown that it is actually the flux of sulphide (another product of microbial decomposition) that chases the lobster from their depths. It is believed that sulphide ions precipitate at the breathing organs of the lobster as metal sulphides and organo-sulphate groups, thereby inhibiting the lobster’s ability to breath oxygen.
Whatever the cause, it is assuring to know that the Marine and Coastal Management now have systems (holding tanks, transport arrangements ect.) in place to collect beached lobster and thereby protect this important commercial fishery from collapse during these environmental disasters.
Bjorn von der Heyden