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plants of the sea take up carbon, hydrogen, oxygen, nitrogen, and the vari- 
ous salts and metals required by the living plants. Once these materials 
become incorporated as organic compounds (carbohydrates, proteins, fats) 
into phytoplankton cells, they become available to the animals of the sea 
that graze upon phytoplankton. 
Within the lighted euphotic zone the phytoplankton organisms are consumed 
By smaller zooplankton organisms, and through a series of eat-and-be-eaten 
steps the chemical materials are eventually transferred to the largest 
predatory carnivores (Figure 6). Taken together, these steps form the 
links of the food chain of the euphotic zone. Some of the phytoplankton 
sink to lower levels, but very little of it reaches depths below 200 meters 
before being eaten. This is one very great difference between land and sea 
ecosystems. Whereas most of the primary production of the sea is eaten 
directly, on land only a small percentage is eaten by herbivores. If this 
were not the case, we would not have trees, lawns and gardens, much of the 
production of which is recycled by fungi and bacteria rather than by her- 
bivores. 
In order for the chemical elements to move through the food webs of the 
sea, there must be power, and this power is derived from the sun. In the 
process of photosynthesis, phytoplankton cells convert certain wavelengths 
of solar radiation to chemical energy stored in the cell body as carbohy- 
drates, fats, and proteins. These materials are passed around the living 
system through the food chains and webs. When plants or animals require 
energy to support their metabolic activities, growth, and reproduction, 
they oxidize some of the stored chemicals and release the energy for bio- 
logical work. According to the second law of thermodynamics, no energy 
transformation is 100 percent efficient. Some of the energy becomes avail- 
able for useful work, but the remainder is lost, primarily as heat. The 
ratio of work output to fuel consumed is a measure of efficiency. Most 
biological systems are approximately 10 percent efficient, which means that 
90 percent of the energy is lost at each step in the food chain. This high 
rate of energy loss effectively limits the marine food chains to no more 
(usually Tess) than six links. 
