TROPICAL 



JAN. ' FEB. 'mCH. ' APL. ' MAY 'JUNE 'JULY 'aUG. ' SE P T ' OC T. 'N OV ' DEC . 

 PLANTS ANIMALS 



Fig. 29. The seasonal variations of plant and animal plankton in polar, temperate and tropical 



zones. Note that the increase in plants takes place first, when food is available the animals increase 



and graze down the plants. The dotted hne in the temperate zone represents the abundance of 



dinoflagellates(see pp. 37-38). 



shallower the water the earlier the growth will be because the light reaches 

 all the way to the bottom and water movement cannot carry the plants 

 away from it. We find the onset as early as February or March in coastal 

 estuaries etc., March or April in the North Sea and as late as June in the 

 open ocean. 



The spring plant growth in shelf water continues until the winter 

 accumulation of nutrients is diminished and the plant numbers are then much 

 reduced because they arc grazed by the herbivorous zooplankton. There is 

 always some replacement of nutrients from the metabolic products of the 

 animals feeding on the plants and from the disintegration of dead plants and 

 animals, enough to maintain a reduced summer population. In the autunm 

 while there is sufficient light left, and the thermocline is less intense because 

 of surface cooling and wind turbulence, a secondary outburst will occur as 

 the surface water is replenished with nutrient rich water from below. The 



III 



