Producers 475 



flowering" or "bloom"— an amusing misnomer since these algae are far 

 removed taxonomically from flowering plants. In the Gulf of Maine, 

 for example, the number of diatoms per unit volume in the spring 

 averages 1000 times greater than the winter population and may be 

 over 60,000 times greater in local regions. Hardly has the diatom 

 population reached its maximum size in March or April than numbers 

 begin to drop off and characteristically continue to fall to a low level. 



An intriguing interplay of ecological forces accounts for these 

 enormous changes in the numbers of plant producers. The thorough 

 stirring of the water in the upper layers of the ocean during the 

 winter brings about a replenishment of the nutrients to the euphotic 

 zone from the deeper levels, but the same vertical movement of water 

 also carries the diatoms down to levels below the compensation 

 depth. As the spring comes on, two things happen: the compensation 

 depth extends to deeper levels because of the greater intensity of sun- 

 light, and stirring becomes less because of the progressive stratifica- 

 tion of the water. As a result the diatoms remain for longer and 

 longer periods above the compensation depth, until after a time con- 

 structive growth of the population becomes possible (Sverdrup, 

 1953). Under favorable conditions diatoms can divide at a rate 

 greater than once in 24 hours. Exponential growth of the population 

 ensues and the sea may become green with diatoms within a week. 



Certain definite ecological reactions bring the spring increase in 

 phytoplankton to an end. As diatom growth goes forward, nutrients 

 are progressively used up until their concentration drops to a point 

 where all the plant cells are starved and either die or form cysts. 

 Furthermore, the great abundance of diatoms has tended to reduce 

 the transparency of the water and at the same time has provided a food 

 supply for a new generation of zooplankton. Copepods and other 

 herbivores "graze" on the diatoms, and, as the zooplankton increases 

 in abundance, the diatom population is consumed at an accelerating 

 rate. 



The diatoms of the open sea characteristically remain at a low 

 ebb throughout the summer months because the majority of dead 

 cells decompose and release nutrients again at levels below the com- 

 pensation depth. Because thermal stratification prevents deep stir- 

 ring, the euphotic zone remains depleted of its nutrients during the 

 summer. In the autumn when stronger winds and lower surface tem- 

 peratures allow effective stirring to take place once again, nutrients 

 are restored to the upper layers of the ocean. A rather sudden in- 

 crease in the diatom population may then take place; this is known 

 as the autumn "flowering" or "bloom." As the diatoms again grow 



