weathering of evaporite minerals (NaCl and CaSO 4 ) in the catchment, and the
concentration of dissolved salts by evaporation, leads to much increased dissolved
chloride and sulphate compared to all the other mature rivers, explaining its near
central position on the diagram.
5.5 Biological processes
In streams and small rivers, biological activity in the water has little influence on
water chemistry because any effects are diluted by the rapid flow. Conversely, in
large slow-flowing rivers and in lakes, biological activity can cause major changes
in water chemistry.
All photosynthetic plants absorb light and convert this to chemical energy
within a chlorophyll molecule. The liberated energy is then used to convert CO 2
(or HCO 3 - ) and water into organic matter. This complex biochemical process is
crudely represented by the familiar equation:
eqn. 5.19
CH 2 O represents organic matter in a simplified way as carbohydrate. The
reaction depicted by equation 5.19 requires the input of energy (DG°=
+475 kJ mol-^1 ) (see Box 4.8) to proceed and this is provided by light. In shallow
freshwater, large plants and drifting microscopic algae (phytoplankton) are
responsible for photosynthesis, while in deep lakes (and in the oceans) phyto-
plankton account for almost all photosynthesis. The reverse of equation 5.19 is
organic matter decomposition, i.e. oxidation or respiration, which liberates the
energy that sustains most life:
eqn. 5.20
Since photosynthesis requires light, it is confined to the surface layers of waters—
the euphotic zone (the region receiving>1% of the irradiance arriving at the
water surface). The depth of the euphotic zone varies with the angle of the sun,
the amount of light absorbed by suspended matter (including phytoplankton) and
the presence of dissolved coloured compounds in the water.
The decomposition of organic matter, which is almost always bacterially
mediated, can occur at any depth in the water column. Decomposition consumes
oxygen (eqn. 5.20), which is supplied to the water largely by gas exchange at
the water/air interface and partly as a byproduct of photosynthesis. Temperature
influences the amount of oxygen that can dissolve in water. Oxygen-saturated
freshwater contains about 450mmol l-^1 oxygen at 1°C and 280mmol l-^1 at 20°C.
In summer, the surface layers of many lakes are warmed by insolation. The
warmer surface water is less dense than the cold deep water, causing a stable
density stratification. Stratification limits exchange of oxygenated surface water
with the deeper waters. Organic matter produced in surface waters sinks into the
deeper waters where it is oxidized, consuming and depleting dissolved oxygen.
In some cases, oxygen levels fall below those needed to support animal life. This
CH O22 22()ss+Æ +O() CO()gH O()lDG∞=- 475 kJ mol-^1
light
CO 22 ()gl+H O()––ÆCH O 2 ()sg+O 2 ()
The Chemistry of Continental Waters 161
