The basin is the subject of aggression multiples related to
human intervention which either reinforces already evoked natural degradations,
or new faces of pollution like developed eutrophication in Zio mouth and in
Lake (Lac-Togo). In fact, pollution in general is due to the bush fires and the
waste poured in the basin in particular the products hydrocarbons, toxic worn
water, from industrial and domestic use:
- Hydrocarbons, several standards, their
presence in the basin is due to the use of trucks and machines of any category
for extraction, the collecting, the washing of gravels and sediments transport.
Increasing man power of the machines, their maintenance involves the
hydrocarbon discharge which is spread and sometimes infiltrate in the ground.
The concentration of the high-carbon products is not without effects on the
environment; they take part in situ destruction or disturb the reproduction of
certain vegetable and animal species and could cause modifications on soil
physical property;
- Domestic sewage water, the thorough and
anarchic urbanization of Lome and its fringe or peripheral zones at ends,
tourist, residential involved the worn water increase which for is most of the
time drained or thrown in the basin without any specific treatment in order to
preserve the basin environment;
- Industrial sewage toxic water, it does not
know yet the specific impact of each type of industry (throwing metal, acid
ions amino, lipids, carbides...) which uses the basin as dump. This water
generates degradations which enter within the framework general of pollution by
sewage water and overloads surface water or in suspension;
- Eutrophication, according to Ryding and
Rast in «The Control of Eutrophication of Lakes and Reservoirs» (vol.
1, 1989, USA), it is an undesired over-fertilization of the water bodies with
inorganic nutrients, manly phosphorus, ?P? as phosphate and nitrogen ?N? as
nitrate and ammonium. Using sun light, air-born CO2 and these nutrients, if
they are available in excess, phytoplankton (micro algae, cyan bacteria) or
macrophytes (water hyacinth) can produce through photosynthesis enormous
amounts of organic matter (biomass):
+P, +N, + light
6 CO2 + 6 H2O -* C6H12O6 + 6 O2
Phytoplankton biomass will sink down later into the lower
dark layer where it cannot survive. Their dead are finally decomposed by
heterotrophic bacteria in the water deep layers causing strong oxygen depletion
through this biochemical process:
Darkness
C6H12O6 + 6 O2 -* 6 CO2 + 6 H2O
Main control strategies can be derived from the phytoplankton
balance equation:
dX/dt = D . Xo + u . X - D . X - B . X - G . Z .
X
import growth export sedimentation grazing
* Where/ X = lake phytoplankton biomass; Xo = inflowing water
biomass; t = time; D = dilution (or flushing) rate ; u = phytoplankton growth
rate; B = sedimentation rate ; G = zooplankton grazing rate; Z = zooplankton
biomass. Rates are related with ecosystem variables in following way: D = Q/V
or water residence time t = 1/D (Q = discharge, V = lake volume) ; Z = f
(fishbiomass) ; u = f (light intensity, temperature, nutrients) ; G = f
(temperature, biomass phytoplnkton) ; B = Vs/Zmix (Vs = phytoplankton sinking
velocity, Zmix = water depth)
* When dX/dt is/
- Positive (+dX/dt), means that phytoplankton
biomass increases if gains (import and growth)
- Negative (-dX/dt), means that phytoplankton
biomass exceed losses (export, sedimentation and grazing)
