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roles in the economy of marine ecosystems: 
(a) phytoplankton includes the single-celled organisms, diatoms, 
dinoflagellates, and others of lesser abundance (Figure 2). These are the 
producers of organic matter in the sea (comparable to grass and trees on 
land) upon which all the animals ultimately depend. Since light is required 
to support photosynthesis, the phytoplankton is restricted to the depths of 
Penetration of sunlight. However, in temperate seas the major amount of 
photosynthesis occurs from just beneath the sea surface to depths no more 
than 15 meters, but this will depend upon turbidity of the surface waters. 
Phytoplankton is limited not only by light, but also by available nutrients, 
of which phosphorus and nitrogen are the two chief components. These ele- 
ments may become available in the surface waters through river runoff from 
land, through excretion and recycling of materials in the euphotic zone, 
through the rising of nutrient-laden waters from deeper layers, and the dis- 
posal of various human wastes. Both the upwelling of nutrient-rich waters 
and the disposal of sewage may cause immense phytoplankton blooms. 
The quantity of organic matter produced by the phytoplankton is referred to 
as primary production, and the rate of production is called primary produc- 
tivity. When the latter is high one will soon find a large pean pop- 
ulation feeding on the phytoplankton. 
(b) zooplankton of the sea is subdivided into the holoplankton that 
spend their entire lives as plankton, and the meroplankton in which only 
the larval stages are planktonic, the adult becoming an active fish or a 
bottom-dwelling animal. The meroplankton is much more abundant over the 
continental shelf than in deep oceanic waters. The ability to produce 
planktonic larvae permits many benthic species to achieve wide distribution 
throughout the ocean and to rapidly colonize new areas as appropriate envir- 
onmental conditions become available. 
The zooplankton is very diverse in its composition, and it includes repre- 
sentatives of nearly every major kind of marine animal (Figure 3). Small 
crustaceans generally predominate, and these include such forms as copepods, 
