I wish to respond to the letter in Wednesday’s (August 20), edition of the Seychelles Nation by Mr Yakub Chaka on this theory concerning the recent occurrence of algae bloom.  I found Mr Chaka’s explanation to be rather amusing, let alone very far-fetched.

Algae bloom is a natural occurrence that, although very rare in Seychelles waters, is a natural phenomenon that commonly occurs in certain areas of the world, such as western Australia and the Gulf coast of Florida, which is caused by phytoplankton.  As explained by Mr Jude Florentine in Tuesday’s NATION (August 19), this phenomenon was most likely caused by the unusual period of calm weather (and high water temperature) during last week (although he did not elaborate on the scientific explanation).

This sudden rise in sea-level temperature coupled with an abundance of nutrients (in the coastal waters) as a result of a run-off from the uncharacteristic high rainfalls of recent weeks may have resulted in ideal conditions for a population explosion of these microscopic organisms.  Localised upwelling brought about by changes in water current patterns, further promoted the growth of these organisms.  The appearance of these prolific organisms in the coastal waters is directly dependent on the availability of light, temperature, and minerals.  Therefore, the absence of any of these three factors will lead to a spectacular demise of these organisms.  The rapid decaying process of such a mass of organisms would lead to fish dying of oxygen starvation.  The fact that winds were blowing towards the shallow waters of Mahe could only have exacerbated this situation.

I hope the above explanation will help to elucidate your readers on this phenomenon that, although not fully understood in its entirety, is short of being magical.

Finally, may I remind Mr Chaka that we have qualified Seychellois scientists who can give plausible explanation to such natural occurrence and with all due respect to French and British scientists, I do not believe that we need to carry out a survey to obtain further information on this natural occurrence

Thanking you for publishing my response.

Joel Nageon de Lestang

Fisheries Biologist   

*****

Regarding the article and comments on the recent topic of the algal blooms in Seychelles, I should like to comment that the mechanisms leading to algal blooms have been fairly well studied over the years and contrary to what many believe they are not necessarily associated with polluted waters being a contributary cause. Algal blooms have been seen and documented since recorded history began when general global pollution was nowhere on the scale that we are currently experiencing.
It is probably safe to assume that the calm sea conditions did help to produce the right marine conditions for the formation of the bloom, and that once the sea state became rougher there is every likelyhood of the blooms dispersing quickly.
The trigger mechanisms for the formation of this particular bloom will need to be investigated further before coming to any conclusions on whether human activity is a factor and if the phenomenon is likely to repeat itself in the future.

John Collie

****

Over the weekend the seas around the inner granitic islands, including Praslin, La Digue and Mahe turned a green colour. This was caused by a tremendous increase in naturally occurring phytoplankton in the sea. The population explosion of the phytoplankton was most likely triggered by an increase in localised Sea Surface Temperatures around the granite islands. This in turn was induced by either the drop in wind over a period of some days thus preventing mixing of waters and leading to rapid surface heating in enclosed bays or to a pool of warm water in the North East to us. A combination of these and other factors may have led to the event witnessed over the last few days.

Phytoplankton are microscopic, single-celled organisms that float in the sea. They are able to photosynthesise and use nutrients in the sea and they form the basis of food webs in the oceans. In the South East monsoon, prevailing winds lead to cold waters, which are rich in nutrients, to rise from the depths. This phenomenon known as upwelling is normal.

An unusual combination of nutrient rich water, sunlight, warm water, and calm conditions led to ideal conditions for rapid growth by certain phytoplankton. This resulted in millions of living cells per litre of water and giving the sea the pea-soup, green colour. This is known as a phytoplankton or algal bloom. The colour of blooms typically ranges from red to green depending on species composition.  Since the winds died down and conditions were calm for a few days, the phytoplankton were caught in bays such as Beau Vallon, where they used up all the nutrients in the water and died. 

Phytoplankton blooms can sometimes lead to highly toxic conditions, such as those caused by the infamous red tides that happen in several countries. Over the last few days hundreds of fish and other marine organisms have indeed died. However, in this particular case, death is most likely the result of lack of oxygen than toxins. Death by oxygen depletion is caused when gills of fish and other sea creatures become clogged or when the oxygen in the water is used when the phytoplankton die and rot. Anaerobic bacteria use up oxygen as part of the normal decay process.

Lobsters, sea cucumbers, octopuses and other bottom-dwelling animals are more vulnerable to oxygen depletion because they cannot swim up to the surface where there is more oxygen. They die or perhaps try to crawl to the surf zone where the waves generate oxygen in the waters but become stranded when the tide retreats.

The itchiness and sore eyes reported by some bathers may well have been caused by  stinging zooplankton associated with blooms. These zooplankton eat phytoplankton and their numbers may have increased with the multiplication of their food source. Some of the phytoplankton may also have produced itchiness among bathers because some toxic organisms such as dinoflagellates may have been present in some quantities. We therefore have to be on the lookout for real cases of poisoning in both human and marine populations.

Fortunately, the wind speeds started to increase, which mixed the waters, cooled the sea surface and moved phytoplankton offshore. If that had not occurred, then the waters may have turned toxic as poisonous hydrogen sulphide gas (smelling of rotten eggs) was produced as part of the normal decay process. This would have caused more deaths possibly affecting corals and other larger organisms in bays and lagoons.

Nirmal Jivan Shah

LINKS