Many reservoirs experience intense and prolonged "blooms" of algae dur-
ing summer months, particularly in the transition and lacustrine zones of the
The occurrence and degree
reservoir (see Part II) where the water is clearer.
blooms can often be directly linked to a high income and a high
in-reservoir concentration of nutrients, especially phosphorus (Rast and Lee
A substantial reduction in nutrient loading to the reservoir, as would
occur in the case of sewage diversion, will usually lead to a predictable
decline in concentration in the reservoir.
If the decline has been signifi-
cant, then algal blooms may decrease in frequency and extent, and the degree
of eutrophication or trophic state of the reservoir may shift to a far less
The dramatic improvement of Lake Washington following
sewage diversion is one illustration of this type of response (Edmondson
Phosphorus release to the water column from enriched sediments, espe-
and temperature, is
cially under conditions of low dissolved oxygen or high
This "internal loading," as
well known (Bostrom, Jansson, and Forsberg 1982).
described in Part II, can be a very significant source of nutrients, espe-
cially phosphorus, to the water column.
Internal nutrient loading may prolong
the eutrophic state long after nutrient diversion.
Shallow reservoirs, espe-
cially those with a prolonged history of nutrient and organic matter loading,
are especially susceptible to the impact of internal nutrient release. In
these reservoirs the regenerative zone or recycling zone (the sediments) is
very close to the lighted, productive zone (the surface waters), and algal
blooms may therefore persist after a reduction in nutrient income.
Phosphorus inactivation is a procedure to accelerate the recovery of a
reservoir, following a reduction in nutrient income, in those cases where
The target of the treatment is
internal phosphorus release is extensive.
phosphorus in reservoir sediments, and the procedure is to add an aluminum
salt to the sediments to bind the phosphorus to aluminum hydroxide.
of aluminum hydroxide will persist, even under conditions of anoxia, and has
produced a significant decrease in phosphorus concentration in the water
column of natural lakes and maintained an improved trophic state in them for