Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 416

TABLE 15.1
Half-Saturation Constants for Nutrient
Limitation of Phytoplankton Growth
Nutrient K s

Phosphorus 1-5 μg P/L

Nitrogen 5-20 μg N/L

Silica (diatoms) 20-80 μg Si/L

Source: Chapra, S.C., Surface Water Quality Modeling ,
McGraw-Hill, New York, 1997.

concentrations would not be growth limiting regardless of the ratios between them. An analog
would be related to food, where if there is little food then the quantity limits growth, but where if
there were surpluses then growth would be limited by the rate at which an organism could eat. A
distinction must also be made between productivity or the rate of growth and biomass. For example,
under a variety of conditions (e.g., tropical forests), the biomass may be high but the rate of growth
may be slow or the opposite.

15.2.1.3 Shelford's Law of Tolerance
Shelford's law of tolerance describes the success or failure of organisms depending on how well
a complex set of conditions is satisied. That is, the absence or failure of an organism can be con-
trolled by the qualitative or quantitative deiciency or excess with respect to any one of several fac-
tors (limiting factors) that may approach the limits of tolerance for that organism.
For example, organisms may have a set of optimal conditions where they may be most success-
ful, while as conditions vary above or below those conditions, they may be less successful, or in the
extreme, they may die. For example, organisms may have a speciic range of temperatures within
which they can grow and be successful, while if the temperatures are outside of that range, their
growth or success rate would decline. The “range” of optimal temperatures may vary between
organisms. Eurythermal organisms, such as carp, can withstand a wide range of temperatures,
while stenothermal organisms may survive only under narrow ranges of temperature. Stenothermal
ish may, for example, be more common in the hypolimnion of lakes where temperature variations
are smaller, while carp and catish may inhabit the more shallow waters of southern lakes and res-
ervoirs with larger seasonal temperature variations.
The optimal conditions do not apply just to temperature, but to all environmental factors. For
example, phototrophs require light, so they are restricted to the photic zone. However, at certain
times of the day or year, and also as a function of depth, high levels of light may inhibit photosynthe-
sis. Substrate may also be limiting, as can be oxygen. Hydrodynamics can also impact organisms, so
that there may be an optimal range of velocities, above or below which the organism or populations
may decline.
An organism may be tolerant of one condition and not of another. The tolerances may also vary
with life stages. The U.S. Environmental Protection Agency (U.S. EPA) (USEPA 1974) identiied
the maximum weekly temperature for the survival of rainbow trout as 24°C. That condition is met
in much of the bottom water released from southern reservoirs, allowing a viable downstream trout
isheries. However, while trout may survive in these waters, often the tailwaters are not conducive
to natural reproduction due to their relatively high temperatures, the lack of suitable habitats, and
other factors. So, while they may survive, only a “put and take” isheries is sustainable. In addition,
in general the limits of tolerance for adult reproductive organisms are lower than those of nonrepro-
ductive organisms and are lower for young organism than for adults.