Estuary Processes and Managment - River Dynamics and Integrated River Management - page 496

and lakes, estuaries, coastal or marine waters, and has been recognized as an important and widespread
global environmental problem.
Over very large time scales, eutrophication can be viewed as a natural process particularly in lakes and
impounded waters. Young natural lakes are generally relatively barren bodies of waters. As ageing
proceeds the materials retained by the lake gradually increase in the bottom sediments and, as a result of
decomposition processes occurring there, nutrients are released to the lake water leading to an increase in
biological productivity. This is also affected in part by the change in shape of the water body brought
about by its filling; the increasing shallowness increases the re-cycling of available nutrients. As a result,
the lake undergoes major changes gradually from the oligotrophic phase to mesotrophic phase and
eutrophic phase (Table 8.4).

T able 8.4 Water quality impairment of lakes affected by nutrient enrichment
Limnological characteristics Oligotrophic Mesotrophic Eutrophic

Nutrient Low Medium High

Biomass Low Medium High

Transparency High Medium Low

Hypolimnetic oxygen content High Variable Low

Impairment of multi-purpose use Little Variable Great

The naturally slow eutrophication process can be greatly accelerated by human activities: intensive
agricultural development, growth of human population, urbanization, and industrialization—all of which
introduce excess nutrients into the receiving waters. For example, Fig. 8.32 shows a close relationship
between the human population and the number of “red tide” occurrences from 1976 to 1986 in Tolo
Harbor of Hong Kong (Lam and Ho, 1989). Human-induced eutrophication originally became the
environmental focus in European and North American lakes and reservoirs in the mid-20th century (Rodhe,
1969). However, it has become more widespread, both in inland waters and coastal waters. For example,
by the early 1990s, 54% of lakes in Asia were eutrophic; in Europe, 53%; in North America, 48%; in South
America, 41%; and in Africa, 28% (ILEC, 1988).

Fig. 8.32 Increase in human population and number of red tides occurrences in Tolo Harbor, Hong Kong

8.4.1 Symptoms and Environmental Impacts
Under favorable environmental conditions, excessive enrichment of nutrients in a eutrophic water body
can lead to extremely rapid growth of microscopic algae or phytoplankton. When this happens the algae
are said to have formed a bloom. High algal concentration gives the water a distinct greenish or brownish
color in lakes (“pea soup”), while “red tides” are often observed in many estuaries and coastal waters. An