7.2 ENERGY SOURCES, ENERGY FLOW & FOOD WEBS
Production and biomass / Food chains and food webs / The origins of organic material / Autotrophs and primary production / Particulate and dissolved organic matter / Functional feeding groups / Organic matter processing
The building blocks of life--
All living things need water, energy, carbon and nutrients if they are
to stay alive, grow and breed.
Aquatic organisms live in water and, under most conditions, it is not
in short supply. Almost all energy used by organisms is derived, directly
or indirectly, from the sun; the exception is some bacteria that derive
energy from chemical sources (e.g., iron bacteria: Section 6). Plants
use energy from sunlight to manufacture a range of sugars by the chemical
process of photosynthesis. Organisms that use the sun's energy in this
way are termed autotrophs. When animals eat plants, they make use of this
energy 'fixed' by the plant.
Carbon is the basis of the sugars and complex proteins
that are the major building blocks of any organism. A range of carbon-based
molecules provides the basis for structural materials (e.g., wood) and
energy stores (e.g., fat). The breakdown of these molecules (especially
sugars) provides the major source of energy for heterotrophs. They cannot
manufacture their own food using the sun's energy, and must consume other
organisms to obtain carbon and energy. Plants can use carbon, in the form
of carbon dioxide in the air. It is taken up, together with energy derived
from sunlight, and incorporated into sugar molecules during photosynthesis.
The sugars are stored in the plant body in the form of starch, but can
be combined with other chemicals to form different types of molecules
(such as protein). The most important of these essential chemicals or
nutrients are nitrogen, phosphorus and potassium. They are often in short
supply and, if so, can limit plant growth. Of secondary importance are
elements such as iron and sulphur and selenium. Nutrients are present
in the soil and the water, where they are derived from the erosion of
rocks, and they are taken up from solution by autotrophs. Heterotrophs
obtain nutrients that have already been incorporated into other organisms.
Increasingly, nutrients in water are derived from human sources, such
as sewage and agricultural fertilizers.
Production and biomass--
knowledge of the ways in which autotrophs and heterotrophs interact and
transfer energy and nutrients helps us understand ecosystem function,
and so provides a basis for management.
At its simplest level, two ecosystem processes are important. Primary
production is assimilation of organic matter and building of tissue by
autotrophs – mainly photosynthetic plants. It is expressed as a rate (e.g.,
the amount of wood produced each year).
Secondary production is the assimilation and of organic material and building
of tissue by heterotrophs. This may involve animals eating plants, animals
eating other animals, or microorganisms decomposing the dead bodies (or
parts) of other organisms (see Section 6). Again, this is expressed as
a rate (e.g., the amount of meat produced by grazing cattle each year).
In a productive environment, living plant or animal material will begin
to accumulate. The magnitude of this material is its biomass (B; also
known as standing stock or standing crop), and is an instantaneous measure
of the weight of organic material that is usually expressed on a per-unit-area
basis (e.g., kg/m²). This should not be confused with production (P),
which is the total weight (biomass) of organic material that is produced
over a specified time. Like biomass, the rate of production is expressed
on a per-unit-area basis (e.g., kg/m²/yr). It can be applied to all
organisms (i.e., production of all organisms in a habitat), a group of
organisms (e.g., all of the fishes), or one species (e.g., the Mekong
giant catfish, Pangasianodon gigas). Production is often difficult to
estimate, since it requires, among other things, repeated accurate measures
of biomass over a number of time intervals.
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High biomass does not necessary imply high production
(although it may). For example, the biomass of plankton in a reservoir
may not be high but, because it grows and reproduces quickly, plankton
may replace itself rather quickly after grazing by fishes. The relatively
large, long-lived fishes will represent a much larger biomass than the
plankton. Initially, it may seem puzzling that so much consumer biomass
is sustained by a small biomass of plankton. The key to understanding
this situation is knowledge of the rapidity with which living material
can replace itself measured by the production/biomass ratio (P/B). It
is high for plankton and relatively low for fishes, and provides a better
indication of the transfer of energy between trophic levels than instantaneous
measures of biomass.
A more complete understanding of energy flows can be attained if we know
the 'maintenance costs' of organisms. This cost is respiration (R), and
represents the energy the organism uses to keep its internal metabolism
functioning. Energy taken up by an organism is devoted to P (resulting
in growth or reproduction; i.e., greater B) or used up in R. The P/R ratio
indicates the efficiency with which an animal converts food into additional
animal tissue. A tree, for example, with roots, trunks and branches, has
a relatively low P/R ratio compared to a phytoplankton cell that does
not have the metabolic costs of roots and support structures.
