DESCRIPTION OF THE STUDY AREA AND DATA INVENTORY - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 92

The History of Lake Edko
Before going into the present status of the lake and its ecosystem, this section gives some historical
information on the lake area and its characteristics. The lake is a typical delta edge coastal lagoon lake
with a history of steady contraction from more than 200 km 2 before the 1940s to some 150 km 2 in the
1950s to the present 70 km 2 (Fisheries Department estimates). It is estimated that a reduction of some
30 percent of the lake area has occurred in the last 20 years by the development of drainage and
irrigation schemes in the eastern portion. In addition to the losses due to agricultural use, the changes
brought about by increasingly intensive agriculture, by lowered quantities of available water and by
greater economy in water use for irrigation, have lead to an acceleration in the growth of reeds (mostly
Phragmites ) and more recently the development of floating masses of water hyacinth ( Eichhornia ).
The lake also supports considerable stands of sub-aquatic plants ( Potamogeton ), with these occupying
a possible 50 percent of the apparently open water surface.
Typically this is a shallow, eutrophic lake at an advanced and accelerating stage of senescence, or
evolution to dry land. The average total of 100 km 2 can be assumed to be the productive area for the
assessment of possible available fishery resources. The lake occupies a very shallow basin with little
variation in a depth between 0.5 and 1.2 m. Encroachment of reeds and deposition of silt reduce the
margins to shallow marshes, which allow reclamation and encroachment by agriculture, particularly in
the south. To the north the lake boundary is fixed by the coast road and railway line which follow the
line of the coastal sand dunes. There is some encroachment of the lake margins on this northern edge.
In the east the lake edge is defined by the Edko Drain carrying irrigation drainage raised from the
neighbouring agriculture areas. Apart from the Edko Drain, the Barseek Drain enters from the middle
of the southern shore. The lake drains to the Mediterranean Sea through a single narrow channel at the
sea fishing port of Maadia. It is understood that occasionally this drainage reverses with the inflow of
sea water into the lake. There is evidence of this from the response of the vegetation in the vicinity of
Maadia and the area of the lake to the west. The drainage of agricultural, irrigated land into the lake
provides a nutrient-rich input with a salinity of about 2-3 ppt over much of the lake surface.
The water is not homogenous in appearance, varying from clear to at least the maximum 1.0 m depth,
to very cloudy with sediment/plankton and in some areas foul anoxic water smelling of hydrogen
sulphide. Such patchiness in the water mass would be expected due to the low flows and the localized
effect of water passing through, or being blocked by, stands of submerged plants or where reversal of
current flow, through tidal or wind effects, causes water to be flushed out from below consolidated
stands of water hyacinth. There is apparently no significant input of organic or industrial pollutants to
the lake except for untreated domestic waste water dumped into the drainage network, apart from the
enhanced nutrient status of the discharged drainage waters and the residual load of pesticides they
carry. There is however a considerable input of herbicides (currently Ametryn) in the October of each
year for the last 10 years as a part of a programme to eradicate or reduce the water hyacinth. It is not
known what effect these chemicals and their residuals will have on the fish or the edible quality of the
fish flesh. It appears that there is no monitoring programme for pesticide residues in the waters of the
area or edible products derived from them.
The lake is exposed to the prevailing winds with a reach over the sea or the flat agricultural lands of
the delta. With its shallow depth and relatively large area, it is unlikely that there will be any
significant stagnation or stratification. The exception to this will be those areas which are blocked by
floating and emergent weeds. These are localized and any deterioration in water quality would provide
little risk to the fish populations. Discrete water bodies are observable within the lake by colouring and
clarity, but their overall pattern could not be ascertained. The major significant heterogeneity is the
area to the west of the connection with the sea at Maadia. The lack of aquatic vegetation over a
considerable area suggests that this part of the water body is subject to considerable fluctuations in
salinity, with presumably the salinity changing with the reversal of flow through the lake-sea junction.