Environmental Engineering Reference
In-Depth Information
example of typical seasonal variation of environmental factors and algal bloom patterns in temperate
climates is shown in Fig. 8.33 (Welch, 1980).
Fig. 8.33 Algal bloom patterns in temperate waters: general seasonal cycle of environmental forcing factors and
phytoplankton biomass (modified from Welch, 1980)
Municipal effects —The excessive growth of phytoplankton can give rise to a variety of environmental
problems: unsightly algal blooms, decreasing water transparency, and emission of noxious odors due to
decomposition of algal scum; all these inevitably lead to deterioration of water quality and destroy the
aesthetic and recreational uses of water bodies (e.g., swimming and boating activities). Some types of
algae, e.g., cyanobacteria and filamentous species of chlorophytes may clog filtering systems in
waterworks, making it difficult to treat drinking water for human consumption. Also, the treated water
may have unacceptable taste or odor due to the secretion of organic compounds by microbes. Further,
when water with high concentrations of dissolved organic carbon is disinfected by chlorination in the
waterworks, several potentially carcinogenic and mutagenic byproducts may be formed.
Toxins —Harmful algal blooms (HABs) are caused not only by microalgae but also by cyanobacteria
and protozoan agents that secrete toxins. Algal toxins can pose a grave health risk by damaging the liver,
intestines, and nervous system. In inland waters, about half of all bloom species are toxic; in coastal waters,
about 40 or 50 of the phytoplankton species may produce toxins. Drinking toxin-tainted water can cause
vomiting, gastrointestinal disorders, headache, muscle pain, paralysis, respiratory failure, and even death.
On the other hand, toxins from algae blooms may enter the food chain and become highly concentrated
in various aquatic products which may become very dangerous for human consumption; for example,
consumption of sea food contaminated by algal toxins can lead to Paralytic Shellfish Poisoning (PSP).
Hypoxia —Algal blooms and red tides can result in severe rapid oxygen depletion, especially in waters
with poor hydrodynamic conditions—e.g., isolated lakes or semi-enclosed bays with poor flushing rates.
Although phytoplankton can release oxygen to the water as a byproduct of photosynthesis during the day,
water has a limited ability to store dissolved oxygen (DO). On the other hand, the respiration of phytoplankton
and zooplankton, and the bacterial decomposition of organic matter (e.g., settled algal detritus) can
consume large amounts of DO. The oxygen in the water can be rapidly depleted if the renewal processes
of DO (at the free surface or with the open ocean) are very slow due to poor hydrodynamic conditions.
Figure 8.34 illustrates the photosynthetic production of algae and bacterial consumption of oxygen in a
stratified lake.
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