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 watershed to another further downstream if you can dilute or flush nutrients with the water management.
Diversion is most often applicable to the rerouting of a point source effluent (e.g., sewage outfall) to a
detention pond, overland treatment, or larger reservoir.
Physicochemical treatments for in-lake water quality management (excluding mixers, aerators,
and oxygenation devices, etc.) include the addition of chemicals for algal and macrophyte control,
removal of macrophytes with mechanical harvesting, nutrient inactivation (e.g., phosphorus precipitation
with alum), nutrient addition (e.g., addition of nitrate to increase the N:P ratio), sediment covering,
sediment removal, and sediment oxidation. With the exception of chemical applications for macrophyte
and algal control, these techniques focus on control of internal sources of nutrients (even macrophyte
and algal control impacts internal nutrient cycling). The concept here is to limit production by limiting
essential nutrients (e.g., phosphorus). Nutrient inactivation and nutrient addition are designed to
balance the nitrogen to phosphorus ratio at a value that favors the growth of less obnoxious algal
species. Sediment covering, removal, and oxidation techniques attempt to limit available phosphorus
from the sediments and water column, thereby limiting algal production. These techniques may be
considered as a "bottom up" approach to nutrient management.
4.9.2 BIOLOGICAL MANIPULATION
Biological manipulation, a term first used by Shapiro et al. (1975), involves techniques to
manipulate the biological structure in the lake to improve water quality and centers around nutrient
availability and the trophic structure or food web of the lake. Usually, the approach is applied to lessen
or eliminate nuisance blue-green algal populations, change the fisheries to a more desirable population
(e.g., increase numbers or change species composition), or alter the macrophyte community for
fisheries or recreation. Biological manipulation utilizes techniques for manipulation of selected levels in
the food web (e.g., removal or control of fish that heavily graze on large zooplankton (which in turn
remove algae)) or a "top down" approach (Figure 4.9.1). Major factors to consider about the ability of
biomanipulation to provide water quality enhancement revolve around (1) the potential to reduce algal
biomass where loads cannot be controlled and (2) the potential to augment or accelerate the effects of
load reductions (National Research Council 1991). Additional information is available in Carpenter
(1988).
4.9.2
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