Page 374 - Soil and water contamination, 2nd edition

Patterns in surface water 361

in total inorganic N. They hypothesised that the weak, delayed, or even absent response
might be attributed to long water transit time s in the soil water and groundwater, or to
mineralisation of organic nitrogen pools that had accumulated during several years before the
dramatic decline in agricultural inputs.
The total P concentrations in the Emajõgi river did not show a response to the decreased
fertiliser inputs, but this is probably due to the relatively high contribution of 40–60 percent
from point source emissions in this reach of the river, mainly from Tartu (Mourad et al.,
2006). The trends in other Estonian rivers could also be explained by changes in point source
emissions from wastewater treatment plants (Iital et al., 2005).

18.4 VARIATION IN LAKE WATER COMPOSITION

18.4.1 Role of lakes in catchment sediment and nutrient budgets
When a river discharges into a lake, its flow velocity diminishes rapidly and the suspended
matter starts to settle out from the water column to the bed sediments . Only suspended
particles that are too small to have settled out from the inflows or those that have been
introduced near the lake outflow are transferred to the lake outflow. Lakes are therefore very
efficient sediment traps and represent an important sink in catchment sediment budgets.
As well as being supplied by inflowing rivers, suspended particles may originate from
within the lake itself as a result of resuspension of bed sediments , erosion of shores, or the
production of autochthonous organic material. Bed sediments are eroded and resuspended
by current and wave action. In deep lakes this only occurs in the shallow parts along
the shoreline. When allochthonous sediment sources to lakes (river inputs, eroded or
resuspended material, and wind-borne dust) are limited, most of the suspended particulate
matter is autochthonous material, i.e. living organisms, organic detritus , calcite , and
diatomite. Organic detritus consists of the remains of dead organisms (e.g. algae , aquatic
macrophytes, zooplankton, fish) and faecal material. Photosynthesis causes an increase
2+
in pH , which may trigger the precipitation of calcite in hard-water lakes rich in Ca and
-
HCO . Diatomite consists of the debris of algal diatoms, which is rich in silica. The
3
processes of settling, resuspension, and river inflow usually bring about an inshore to offshore
decline in the concentrations and mean particle size of the suspended matter , whereas the
organic matter content and carbonate content of the suspended matter increase. These trends
of particle size and organic matter and carbonate content are also often found in the lake bed
sediments.
The settling of particulate matter and the sediment characteristics with respect to particle
size and organic matter content have important implications for the functioning of lakes in
nutrient retention in catchments (Hillbricht-Ilkowska, 1999). The sedimentation of particle-
3-
bound P (ortho-PO and organic forms of phosphorus ) causes lakes often to be a sink for
4
total P load , except if the lake is shallow and eutrophic : in the latter case, there is usually net
3-
P export during summer, when ortho-PO is released from sediments (see Section 18.4.4).
4
The anaerobic , organic bed sediments abundant in most lakes provide important sites for
effective denitrification , which reduces the total N load.
Marion and Brient (1998) quantified the retention of suspended particulate matter and
N and P load in Grand-Lieu, a shallow wetland lake discharging into the Loire estuary,
western France, for two hydrologically contrasting seasons. Grand-Lieu is 4000 ha in extent
during summer and 6300 ha during winter and is 1.2 to 1.7 m deep in its centre. The
retention of suspended particulate matter and nutrients were measured from October 1993
6
3
to May 1994, a wet season with a total lake inflow of 292 10 m , and from October 1995 to
3
6
May 1996, a dry season with a lake inflow of 76 10 m (Table 18.3). The retention rate (i.e.

10/1/2013 6:47:49 PM
Soil and Water.indd 373 10/1/2013 6:47:49 PM
Soil and Water.indd 373