NATURE ADRIFT 



good fertility of the water, through the plankton, can yield a good fishery. 

 It does not always work out quite that way, especially if the change is too 

 sudden. An example is the 'red tide' mentioned at the end of Chapter 3 ; 

 this name is given to a sudden bloom of a toxic dinoflagellate associated with 

 an exceptionally rich nutrient supply. One occurred off the west Florida 

 beaches in 1959, the fourth in ten years. Red tides can have a serious adverse 

 effect on the local fisheries and amenities. The nutrients are so rich in the area 

 affected that the dinoflagellates can multiply extremely rapidly and reach 

 such enormous numbers that the toxin given off can poison many of the 

 fish. Other fish die through lack of oxygen in the water because it has been 

 absorbed by the bacteria during the process of decay of the fish and other 

 creatures. 



The spray blown ashore by the wind may contain enough poison to 

 cause an irritation to those ashore and this can be serious to those with chest 

 or heart troubles. This, and the smell of decaying fish, makes these otherwise 

 popular seaside resorts thoroughly unpleasant, with the resulting loss of 

 tourist trade. As the dinoflagellates multiply they use more and more of the 

 nutrient until it is exhausted and there follows the sudden death of multi- 

 millions of them through sheer starvation. 



Red tides occur in places other than Florida; in India, Japan, Australia 

 and Chile, for example. They are also known from fresh or brackish waters in 

 the Mediterranean area and as near home as Belgium and Denmark, but not 

 to anything like such a severe degree as at Florida. What can sometimes 

 happen is a case of indirect poisoning. Shellfish such as mussels feed on bac- 

 teria and other small organisms including dinoflagellates — mostly of course 

 the non-poisonous ones. If they happen to include too many poisonous ones 

 in their diet, and are then eaten, they can cause poisoning even after cooking. 

 The chance of this is rare, and in Britain it is particularly remote as the 

 Shellfish Purification Schemes, designed to get rid of pathogenic bacteria, 

 would naturally free them from poisonous dinoflagellates too. 



13S 



