Water Quality Management - River Dynamics and Integrated River Management - page 586

waters contain small counts of organisms but many different species of aquatic plants and animals, whereas
eutrophic conditions create large numbers of plants and animals of fewer species. Eutrophication diminishes
water quality because dissolved solids increase, transparency decreases, blue-green algae are more
prevalent, rooted aquatic plants are more abundant, and finer fish are replaced by more tolerant species.
In particular, the presence of blue-green algae (also known as cyanobacteria) results in nauseous odors
and tastes, interference with stock watering, poisoned water fowl and cattle, poisoned mussels, shorter
filter runs in water-purification plants, and interference with industrial water uses.
Fig. 9.17 Algal bloom in Rosenow Creek near Oconomowoc, Wis., U.S. (photo provided by Mike Hahn)
Masters (1991) notes the quickest way to control eutrophication is to identify the limiting nutrient and
reduce its concentration. As previously noted, in eutrophic lakes the dominant species of algae are often
blue-green (cyanobacteria), which are able to obtain nitrogen directly from the atmosphere (known as
nitrogen fixation). Thus, to control blue-green algae phosphorus must be controlled. Fair et al. (1971, p. 664)
note that from the mean stoichiometric relation for every milligram of phosphorus, 7 mg of nitrogen, and
41 mg of carbon enter into algal protoplasm. They further note that lake waters can usually obtain enough
carbon dioxide from the atmosphere, decaying organic matter, and bicarbonate alkalinity to supply required
amounts of carbon (Fair et al., 1971). The phosphorus source is orthophosphate and the nitrogen source
is ammonia (typically preferred) or nitrate. Sawyer (1947) suggested that phosphorus concentrations in
excess of 0.015 mg/L and nitrogen concentrations above 0.3 mg/L are sufficient to cause blooms of algae.
Further, Thomann and Mueller (1987) suggest that nitrogen to phosphorus (N/P) ratios in a body of water
over 20 generally indicate that phosphorus is the limiting nutrient, whereas N/P ratios of 5 or less reflect
nitrogen limited systems. These recommendations suggest that phosphorus is typically is the limiting
nutrient and that reductions to 0.015 mg/L are difficult to achieve considering that most current phosphorus
effluent limitations based on the “Best Available Technology Economically Achievable” are around
1 mg/L. Further, complicating the control of phosphorus concentrations are the abundant natural sources.
Phosphorus released from rocks can enter the water directly, but more commonly it enters the water in
the form of dead plant matter (Davis and Masten, 2004). It is extremely difficult to reduce the natural
inputs of phosphorus.
In addition to nutrients, the major physical factors that influence aquatic-plant production are temperature,
light, residence time of the water, and thermal stratification (Clark et al., 1977, p. 279). A very short
residence time in a watercourse or impoundment seems to reduce the effects of eutrophication. Because
of short residence times and blocking of light by turbidity due to high sediment loads, many major rivers
are not subject to high algal production even though their nitrogen and phosphorus concentrations exceed