Science of Water : Concepts and Applications

Water Ecology 167
2. Likewise, biomass pyramids (in which biomass is used as an indicator of production) are
usually pyramid shaped. This is particularly true of terrestrial systems and aquatic ones
dominated by large plants (marshes), in which consumption by heterotrophs is low and
organic matter accumulates with time.
However, biomass pyramids can sometimes be inverted. This is common in aquatic
ecosystems, in which the primary producers are microscopic planktonic organisms that
multiply very rapidly, have very short life spans, and are heavily grazed by herbivores. At
any single point in time, the amount of biomass in primary producers is less than that in
larger, long-lived animals that consume primary producers.
3. Numbers pyramids can have various shapes (and not be pyramids at all) depending on the
sizes of the organisms that make up the trophic levels. In forests, the primary producers are
large trees and the herbivore level usually consists of insects, so the base of the pyramid is
smaller than the herbivore level above it. In grasslands, the number of primary producers
(grasses) is much larger than that of the herbivores above (large grazing animals) (Ecosystems
Topics, 2000).
PRODUCTIVITY
As mentioned, the fl ow of energy through an ecosystem starts with the fi xation of sunlight by
plants through photosynthesis. In evaluating an ecosystem, the measurement of photosynthesis is
important. Ecosystems may be classifi ed into highly productive or less productive. Therefore, the
study of ecosystems must involve some measure of the productivity of that ecosystem.
Primary production is the rate at which the ecosystem’s primary producers capture and store a
given amount of energy in a specifi ed time interval. In simpler terms, primary productivity is a measure
of the rate at which photosynthesis occurs. Four successive steps in the production process are:
1. Gross primary productivity—The total rate of photosynthesis in an ecosystem during a
specifi ed interval
2. Net primary productivity—The rate of energy storage in plant tissues in excess of the rate
of aerobic respiration by primary producers
3. Net community productivity—The rate of storage of organic matter not used
4. Secondary productivity—The rate of energy storage at consumer levels
When attempting to comprehend the signifi cance of the term productivity as it relates to
ecosystems, it is wise to consider an example. Consider the productivity of an agricultural ecosystem
such as a wheat fi eld. Often its productivity is expressed as the number of bushels produced per acre.
This is an example of the harvest method for measuring productivity. For a natural ecosystem,
several one-square-meter plots are marked off, and the entire area is harvested and weighed to give
an estimate of productivity as grams of biomass per square meter per given time interval. From this
method, a measure of net primary production (net yield) can be measured.
Productivity, both in the natural and cultured ecosystem, may vary considerably, not only
between types of ecosystems, but also within the same ecosystem. Several factors infl uence
year-to-year productivity within an ecosystem. Such factors as temperature, availability of nutrients,
fi re, animal grazing, and human cultivation activities are directly or indirectly related to the
productivity of a particular ecosystem.
Productivity can be measured in several different ways in the aquatic ecosystem. For example,
the production of oxygen may be used to determine productivity. Oxygen content may be measured
in several ways. One way is to measure it in the water every few hours for a period of 24 h. During
daylight, when photosynthesis is occurring, the oxygen concentration should rise. At night the
oxygen level should drop. The oxygen level can be measured by using a simple x–y graph. The
oxygen level can be plotted on the y-axis with time plotted on the x-axis, as shown in Figure 6.11.