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 shells. Insufficient calcium may greatly affect distributions of clams and mussels, for example. Calcium
also affects alkalinity and hardness of water.
Magnesium is important for chlorophyll synthesis in photosynthesizing plants. Each chlorophyll
molecule contains one atom of magnesium. Magnesium also affects alkalinity and hardness and is (with
calcium) one of the most important divalent cations (positively charged ions with a charge of 2+, Ca2+
or Mg2+, for example).
Sodium (Na+) and potassium (K+) form monovalent cations and are also common chemicals in
water. Potassium is an important nutrient element for plants and animals. Sodium, however, does not
enter many reactions and can often be used as a conservative tracer for describing water movements.
During stratification, oxygen may be depleted in the hypolimnion forming an anaerobic zone. In
that zone, chemical reduction may allow iron or manganese (or methane and sulfides) to be produced in
or from the sediments. Such processes are common in reservoirs. Reservoir releases from penstocks
(openings in the dams that lead to the turbines) that withdraw from those depths will contain reduced
iron and manganese. If placed in an aerobic environment, the subsequent oxidation of iron is rapid. But
manganese oxidizes slowly and can remain in the reduced state long distances downstream even in
aerobic environments.
It is important that even if we can predict an outcome at equilibrium, the system rarely ever
actually attains such equilibrium and predictions must take kinetics into account. This is one reason that
chemical environmental models do not enjoy the degree of prediction accuracy that physical models
enjoy.
Oxygen depletion rates can be calculated using monitoring data collected over time. Such
calculations are useful as tools for predicting anoxia or decreased concentrations of oxygen in outflows.
Another way to measure an oxygen depletion rate is to use bottles or other enclosures to measure
oxygen loss directly.
If one reservoir releases to another reservoir, processes in one lake can affect processes or
distributions in the other lake. This is related to loading and is not confined to metals or nutrients. Such
interactions are illustrated by the historical distributions of September dissolved oxygen concentrations
in J. Strom Thurmond Lake. After the impoundment of Richard B. Russell Lake (RBR Lake) in 1984,
there was a trend of greater depletion of dissolved oxygen in J. Strom Thurmond Lake.
Variation over long time scales is a problem that only long-term monitoring programs can assess.
The long term effect of impoundments can sometimes be identified or predicted soon after
impoundment. For example, Richard B. Russell Dam altered the flow of the Savannah River and
decreased the nutrient loading to Clarks Hill Lake. The amount of water was undiminished but settling
of particles in inflows to RBR Lake decreased the amount of total phosphorus entering Clarks Hill
1.2-24
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