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reflected back into the atmosphere, and that which does penetrate the sea 
surface is rapidly absorbed by water molecules and by smaller suspended 
particles. For these reasons, sunlight is attenuated in seawater as a log- 
arithmic function of depth. Phytoplankton growth is limited by the quanti- 
ty of light available for photosynthesis, and phytoplankton generally can- 
not carry on photosynthesis when the light level is less than one percent 
of the surface value. Therefore, the depth (Figure 1) of the euphotic zone 
(i.e., the upper layer of the sea where light is sufficient to support 
photosynthesis) tends to be greter in the open ocean than in the more tur- 
bid coastal waters, greater in summer than in winter, and greater in tropi- 
cal latitudes than in temperate or polar latitudes. 
The floor of the sea is called the benthic area. For present purposes it 
is considered to include the water column for a few meters above the bot- 
tom, the bottom surface itself, and the bottom muds to a depth of about one 
meter. The benthic area is the ultimate resting place of all particulate 
material that sinks down through the water column. This includes the 
river-borne sediments, fecal pellets, the organic and inorganic remains of 
all plants and animals of the water column, and all non-floating debris of 
civilization that is dumped at sea. The bottom itself is a habitat for a 
very large variety of microbial and animal species. Through chemical ac- 
tion and through mechanical activities such as burrowing and pumping, these 
organisms stir and change the sediments and bring to the surface materials 
once buried to a meter's depth. 
Living Components 
The living components of the marine ecosystem include the plankton, nekton, 
neuston, and benthos. 
Plankton. The plankton of the sea includes the mostly microscopic 
plants and animals, which possess limited powers of locomotion and which 
are, thus, at the mercy of the water currents. Marine plankton is divided 
into two main groups: the phytoplankton, or plant plankton, and the zoo- 
plankton, or animal plankton. Each of these plays distinct functional 
