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 or dissolved oxygen concentrations in releases and, more rarely, in the lakes. However, it is
important to remember that anything that alters the distribution of dissolved oxygen in the
hypolimnion can also alter the nutrient distributions in a reservoir. And any change that alters the
movement of water through a reservoir will also affect material distributions including nutrients
and organisms. Project modifications can alter the distributions and types of biota living in the
reservoir.
Nutrient Ratios: When a best management practice (BMP) is successful it may have a
perceived positive influence on water quality in downstream systems. This perception is
sometimes arguable. If a lake is negatively impacted by nutrient loading (eutrophication) the
most common nutrient that is implicated is phosphorus (P). Reduction of P loading is often a
major goal of BMP's for use or development in a watershed. Sometimes reservoirs can
accomplish this in complex river systems. For example, Richard B. Russell Lake (RBR) greatly
reduced the amount of P entering J. Strom Thurmond Lake via the Savannah River. In this
sense RBR acts as a nutrient trap.
If an ecosystem is adapted to certain available nutrient concentrations then changing one
of them will change the ratio of available nutrients. This can affect the limiting status of the
essential elements.
The simplest application of this idea applies logic developed around the "Redfield ratio"
(Redfield 1934 and Pearsall 1930, 1932). This ratio relates carbon, nitrogen, and phosphorus
and is approximately 120:16:1, respectively. In practice the ratio is often expressed simply as
nitrogen (N) to phosphorus (P) ratio because it is rare for carbon to be more limited than either
N or P in aquatic systems (although it does happen from time to time). Aquatic ecosystems can
be either N- or P-limited but P is the most common limiting nutrient. In extremely oligotrophic
lakes, both N and P can be limiting and in extremely eutrophic lakes, light (rather than nutrients)
can become limiting.
This is important to the algae because changing nutrient status may change the dominant
form of algae in the aquatic system (Kilham and Kilham 1984). For example, reduction of N-
loading without similar reductions in P-loading can result in N-limitation. This can give algae that
are capable of N-fixation a competitive advantage over those that can't and may shift the system
from green algae, for example, to cyanobacteria (Smith 1983). Although the biomass may not
change or it may even decrease, other qualitative aspects of this changed system may be judged
not to be improvements.
Hydraulics: One means of improving water quality that is being increasingly used in
reservoirs is in-lake aeration or oxygenation. This topic is presented in great detail during other
parts of this course but one aspect of it is gemaine to HABs. If an aeration or oxygenation
system is operated in such a manner as to mix a portion of the reservoir, then this water
movement (mixing or turbulence) can greatly alter the growth potential or the types of dominant
organisms in the lake.
1.2.49
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