Science of Water : Concepts and Applications

Water Ecology 165
Algae
FIGURE 6.8 Aquatic food chain.
Because of entropy, input of energy in any system is higher than the output or work done; thus, the
resultant, effi ciency, is less than 100%.
The interaction of energy and materials in the ecosystem is important. Earlier, we discussed
the biogeochemical nutrient cycles. It is important to remember that it is the fl ow of energy that
drives these cycles. Moreover, it should be noted that energy does not cycle as nutrients do in
biogeochemical cycles. For example, when food passes from one organism to another, energy
contained in the food is reduced systematically until all the energy in the system is dissipated as
heat. Price (1984) refers to this process as “a unidirectional fl ow of energy through the system, with
no possibility for recycling of energy.” When water or nutrients are recycled, energy is required. The
energy expended in this recycling is not recyclable.
As mentioned, the principal source of energy for any ecosystem is sunlight. Green plants, through
the process of photosynthesis, transform the sun’s energy into carbohydrates, which are consumed
by animals. This transfer of energy, again, is unidirectional—from producers to consumers. Often
this transfer of energy to different organisms is called a food chain. Figure 6.8 shows a simple
aquatic food chain.
All organisms, alive or dead, are potential sources of food for other organisms. All organisms
that share the same general type of food in a food chain are said to be at the same trophic level
(nourishment or feeding level). Since green plants use sunlight to produce food for animals, they are
called the producers, or the fi rst trophic level. The herbivores, which eat plants directly, are called
the primary consumers, or the second trophic level. The carnivores are fl esh-eating consumers;
they include several trophic levels from the third on up. At each transfer, a large amount of energy
(about 80–90%) is lost as heat and wastes. Thus, nature normally limits food chains to four or fi ve
links. In aquatic ecosystems, however, food chains are commonly longer than those on land. The
aquatic food chain is longer because several predatory fi sh may be feeding on the plant consumers.
Even so, the built-in ineffi ciency of the energy transfer process prevents development of extremely
long food chains.
Only a few simple food chains are found in nature. Most of them are interlocked. This interlocking
of food chains forms a food web. Most ecosystems support a complex food web. A food web
involves animals that do not feed on one trophic level. For example, humans feed on both plants
and animals. An organism in a food web may occupy one or more trophic levels. Trophic level is
determined by an organism’s role in its particular community, not by its species. Food chains and
webs help explain how energy moves through an ecosystem.
An important trophic level of the food web consists of the decomposers. The decomposers feed
on dead plants or animals and play an important role in recycling nutrients in the ecosystem. Simply
stated, there is no waste in ecosystems. All organisms, dead or alive, are potential sources of food
for other organisms. An example of an aquatic food web is shown in Figure 6.9.
FOOD CHAIN EFFICIENCY
Zooplankton
Perch
Earlier it was pointed out that energy from the sun is captured (via photosynthesis) by green plants
and used to make food. Most of this energy is used to carry on the plant’s life activities. The rest of
the energy is passed on as food to the next level of the food chain.
Nature limits the amount of energy that is accessible to organisms within each food chain. Not
all food energy is transferred from one trophic level to the next. Only about 10% (the 10%-rule)
of the amount of energy is actually transferred through a food chain. For example, if we apply
the 10%-rule to the diatoms–copepods–minnows–medium fi sh–large fi sh food chain shown in
Figure 6.10, we can predict that 1000 g of diatoms produce 100 g of copepods, which will produce
Bass