INTRODUCTION TO THE MARINE ENVIRONMENT AND POLLUTION - Marine Pollution: What Everyone Needs to Know - page 2

third-level consumers are carnivores that eat other carnivores;
and decomposers are microorganisms (such as bacteria and
fungi) that break down the organic materials from the plants
and animals (excretory products and dead bodies) into inor-
ganic materials, which are eventually reused by producers.
The decomposers are concentrated in the sand or mud on
the bottom, and play an essential role in recycling materials.
There are more producers than consumers, more primary con-
sumers than secondary consumers, and so on up the chain,
because at each step in the food chain a great deal of energy is
lost—it is not efficient. So top carnivores (for example sharks)
are the rarest animals.
The most important primary producers in the ocean are a
diverse group of microscopic floating single-celled photosyn-
thetic organisms called phytoplankton. They are the basis of
the food web that supports the rest of oceanic life. They are
widely distributed in huge numbers, but occur near the sur-
face of the water only down as far as light penetrates, since
light is essential for photosynthesis. Phytoplankton are eaten
by small floating animals called zooplankton. Zooplankton
consist of a wide variety of different types of generally small
animals, some of which spend their whole life as small plank-
ton, while others are larval stages of larger animals such as
clams or crabs that will subsequently go to the bottom to live
as adults. Zooplankton, in turn, are eaten by small fish, which
are eaten by larger fish, which may be eaten by very large fish
(or other large animals such as marine mammals). Animals
that live on the bottom are called benthos; some benthic ani-
mals obtain their food by filtering the plankton, while others
consume decaying plant or animal material (called detritus)
that sinks down to the bottom.
In shallow coastal areas or estuaries, additional kinds
of primary producers are found: larger algae (seaweeds) or
rooted plants like seagrasses that live attached on the bottom,
since the light can penetrate through the shallow water. These