phosphorus is the limiting nutrient, which is the most common occurrence, this
correlation has been shown to be extremely satisfactory.
Information such as this is being used to indicate means of controlling productivity in lakes. One of the largest efforts in this direction is the study of controlling
nutrients in the Great Lakes of the USA and Canada. Knowing the phosphorus inputs
to a lake and the morphology of the lake, it can be determined what amount of
phosphorus control would be required to limit the productivity to a certain desired
level. This is presently being done by varied means, including the removal of
phosphorus from all wastewaters being discharged into the Great Lakes Basin, by
controlling urban runoff through the use of holding tanks and subsequent treatment,
and by placing buffer zones between farmland and streams into which fertilizers
would otherwise be leached. Based on these calculations, the most efficient and/or
economical system for control of phosphorus discharges into lakes can be
established.
In addition to the overall limitation to biological growth by the major nutrients,
special conditions are frequently controlled by minor nutrients. Two of these are iron
and silicon. Iron is essential to photosynthesis and can be limiting under certain
conditions. Silicon is essential to the growth of diatoms that produce a frustule that is
composed mainly of silicate.
Diatom growth is important in relation to control of blue-green algae growth.
Diatoms and green algae are considered the base of the food chain in that they can
convert inorganic matter to cell growth and oxygen in the presence of sunlight. In
addition, they are consumed by higher organisms, thereby controlling their population growth. The blue-green algae, although they also produce cell growth and
oxygen by photosynthesis, are much less desirable as a food source by higher
organisms. Therefore, they can grow uninhibited to massive numbers, or blooms.
Then when they die, their organic matter consumes the oxygen available in the
water, creating an undesirable condition.
Studies made on Saratoga Lake [22] indicated that when sufficient silicon is
available for diatom growth, they can out-compete the blue-green algae for the
available nutrients. However, after all the silicon is depleted by a large diatom
growth, their growth diminishes, and the blue-green algae can take over. Since the
blue-green algae are only slightly consumed, they proliferate until more soluble
silicate is made available, after which they decrease in numbers as the diatoms again
increase.
Rabalais et al. [23] studied iron and silicon concentrations along with nitrogen
and phosphorus in coastal plankton food webs. They particularly related their results
with the hypoxic conditions that exist in the Gulf of Mexico. Babbitt [24] reported
that as much as 7000 mi
2 of the Gulf are presently hypoxic. This has become a
concern since this restricts the production of shrimp, a major economic crop of the
Gulf. The hypoxic conditions correlate with the proliferation of blue-green algae in
the affected area. The depletion of silica in the Gulf correlates with the excess bluegreen productivity. Rabalais et al. [23] attributed the low levels of silicon reaching
the Gulf to the numerous dams along the Mississippi River that trap the silt and sand
which normally contribute to the silicate levels in the Gulf. The lack of this essential
282
D. B. Aulenbach et al.
correlation has been shown to be extremely satisfactory.
Information such as this is being used to indicate means of controlling productivity in lakes. One of the largest efforts in this direction is the study of controlling
nutrients in the Great Lakes of the USA and Canada. Knowing the phosphorus inputs
to a lake and the morphology of the lake, it can be determined what amount of
phosphorus control would be required to limit the productivity to a certain desired
level. This is presently being done by varied means, including the removal of
phosphorus from all wastewaters being discharged into the Great Lakes Basin, by
controlling urban runoff through the use of holding tanks and subsequent treatment,
and by placing buffer zones between farmland and streams into which fertilizers
would otherwise be leached. Based on these calculations, the most efficient and/or
economical system for control of phosphorus discharges into lakes can be
established.
In addition to the overall limitation to biological growth by the major nutrients,
special conditions are frequently controlled by minor nutrients. Two of these are iron
and silicon. Iron is essential to photosynthesis and can be limiting under certain
conditions. Silicon is essential to the growth of diatoms that produce a frustule that is
composed mainly of silicate.
Diatom growth is important in relation to control of blue-green algae growth.
Diatoms and green algae are considered the base of the food chain in that they can
convert inorganic matter to cell growth and oxygen in the presence of sunlight. In
addition, they are consumed by higher organisms, thereby controlling their population growth. The blue-green algae, although they also produce cell growth and
oxygen by photosynthesis, are much less desirable as a food source by higher
organisms. Therefore, they can grow uninhibited to massive numbers, or blooms.
Then when they die, their organic matter consumes the oxygen available in the
water, creating an undesirable condition.
Studies made on Saratoga Lake [22] indicated that when sufficient silicon is
available for diatom growth, they can out-compete the blue-green algae for the
available nutrients. However, after all the silicon is depleted by a large diatom
growth, their growth diminishes, and the blue-green algae can take over. Since the
blue-green algae are only slightly consumed, they proliferate until more soluble
silicate is made available, after which they decrease in numbers as the diatoms again
increase.
Rabalais et al. [23] studied iron and silicon concentrations along with nitrogen
and phosphorus in coastal plankton food webs. They particularly related their results
with the hypoxic conditions that exist in the Gulf of Mexico. Babbitt [24] reported
that as much as 7000 mi
2 of the Gulf are presently hypoxic. This has become a
concern since this restricts the production of shrimp, a major economic crop of the
Gulf. The hypoxic conditions correlate with the proliferation of blue-green algae in
the affected area. The depletion of silica in the Gulf correlates with the excess bluegreen productivity. Rabalais et al. [23] attributed the low levels of silicon reaching
the Gulf to the numerous dams along the Mississippi River that trap the silt and sand
which normally contribute to the silicate levels in the Gulf. The lack of this essential
282
D. B. Aulenbach et al.