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(a) phytobenthos includes plants associated with the bottom such as 
seagrasses and attached marine algae (kelps and seaweeds). These are most 
abundant in the shallow marginal areas of the sea and its estuaries. In 
estuaries the phytobenthos provides nursery areas for shrimp, crabs, and 
many finfish that as adults live on the continental shelf. 
(b) zoobenthos includes representatives of nearly every major group 
in the animal kingdom (Figure 5). Especially important are the protozoans, 
various groups of marine worms, mollusks, crustaceans, and starfish and 
their relatives. Demersal fishes (bottom-feeding fishes) that live upon or 
just above the bottom are very important components of the benthos. Com 
mercially important demersal fishes and benthic shellfishes are 
1. Cod 9. Halibut 
2. Haddock 10. Sablefish 
3. Hake 11. King crab 
4. Butterfish 12. Tanner crab 
5. Pollock 13. Shriinp 
6. Sole 14. Prawns 
7. Flounder 15. Red crab 
8. Plaice 16. Blue crab 
Bacteria are also common and a very important part of the benthos. They 
break down various organic compounds, some of which are toxic to higher 
organisms, and recycle metals and nutrients. 
Studies have revealed that benthic organisms are far more abundant in shal- 
low waters than they are in deeper areas. Standing crops in the nearshore 
waters may reach a kilogram or more per square meter of bottom surface 
area, whereas in the abyssal zone the living matter may constitute only a 
gram or less per square meter of bottom. 
Matter and Energy Transfers in Marine Ecosystems 
Ecosystems exist by repeated cycling and recycling of chemical elements 
from the environment through various steps of the living systems and back 
to the environment. By means of photosynthesis and other mechanisms the 
