Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 398

less than 1 mg/L, whilst between 6 and 10 m, the rapid reduction in DO concentration reflects the high
number of zooplankton which graze on the phytoplankton (Churchill and Nicholas, 1967). A similar
interrelationship was observed within the Boone Reservoir, Tennessee, although here the quantity of oxygen
used in respiration by the zooplankton exceeds that produced by phytoplanktonic photosynthesis.
Fig. 7.4 Vertical water quality profiles for summer in Cherokee and Boone Reservoirs, Holston River, U.S.A.: T =
temperature (ÛC); D.O. = dissolved oxygen (mg/L); PHY = phytoplankton (1000 cells/mL); and ZOO = zooplankton
(number/L) (after Churchill and Nicholas, 1967)
7.1.2.3 Eutrophication
Lakes and reservoirs act as nutrient sinks and considerable attention has been directed to the eutrophication
of natural and artificial lakes resulting from the accumulation of nutrients—particularly nitrogen and
phosphorus, although potassium, magnesium, trace elements (such as iron, manganese, and copper) and
organic materials also play a part. Figure 7.5 shows the blooming blue-green algae yano bacteria in the
Dianchi Lake in southwest China. The lake has received increasing amounts of sewage water from towns
and cities and suffers from increasing eutrophication in the past decades. The blooming blue-green algae
produce an oil-like green surface layer, which causes a serious DO depletion. Many fishes have been killed.
Fig. 7.5 An oil-like green surface layer due to blooming blue-green algae in Dianchi Lake in southwest China, which
causes serious DO depletion (See color figure at the end of this topic)