Environmental Engineering Reference
In-Depth Information
The psammolittoral is the beach zone, and is home to a variety of specialized copepods, rotifers,
and nematodes. The littoral zone is the shallow water zone where light may penetrate to the bottom,
where macrophytes may be found. The limnetic or pelagic zone is the deep-water zone where phy-
toplankton dominate primary productivity. The benthic zone is the zone on or in the bottom. The
epifauna, such as crayish and dragonly larvae, live and move about on the lake bottom, while the
infauna, such as nematodes and some insect larvae, live beneath the mud surface.
The relative proportion or abundance of each of these zones directly affects their biological
characteristics and is also often taken as an indication of the successional stage. If one considers
that as lakes age, they ill in, then the rate of succession would be greater for lakes dominated by a
littoral zone than for lakes dominated by a pelagic zone. Or, as lakes age and ill, the littoral zone
will become increasingly dominant.
15.2
FACTORS AFFECTING DISTRIBUTION
15.2.1 G eneraL p rIncIpLeS
The distribution of organisms is affected by factors such as light, temperature, substrate, and nutri-
ents. For example, phototrophs will be restricted to the photic zone of lakes, while heterotrophs may
not be so restricted. Some of the factors affecting the distribution of organisms are discussed in the
following sections.
15.2.1.1 Stoichiometry and Redield Ratios
Autotrophic production takes inorganic materials and energy and produces organic materials from
them. The building blocks for this organic material consist of carbon, nitrogen, phosphorus, and
other materials. The speciic ratios of the materials in organic materials are commonly based on
some assumed stoichiometry, such as in the following approximation (Redield et al. 1963, Chapra
et al. 2007):
CHONP
106
263
110
16 1
which can be used to determine mass ratios, such as
C:N:P
Molar basis
106:16:1
Mass basis
106 × 12:
16 × 14:
1 × 31 = 1272: 224: 31
So, the organic matter of, say, algae is composed of mass of about 40% carbon, 7.2% nitrogen,
and 1% phosphorus, or on a molar basis, of 16 times more nitrogen than phosphorus.
These ratios may be used, for example, to determine if phosphorus or nitrogen is in limited sup-
ply for plant growth. This equation suggests that organisms require (molar basis) 16 times as much
nitrogen as they do phosphorus. So, for example, if the molar ratio of N:P is greater that 16:1 (or the
mass concentration ratios are greater than 7.2:1), then there is an excess of nitrogen in relation to
phosphorus, and phosphorus may limit growth.
15.2.1.2 Liebig's “Law of the Minimum”
All organisms have requirements for growth, and that growth may be limited by those require-
ments not being met. A limiting factor to biological activity is that material available in an amount
most closely approaching the critical minimum required to sustain that activity (Odum 1971). But,
if you have a variety of requirements for growth, how do you determine which is limiting? One
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