A detailed description is given in Field Document 1. The present report stresses features salient to aquatic weeds.
Lake Edku is situated approximately 30 km east of Alexandria on latitude 30°N and longitude 31°E. It is a shallow eutrophic lake rarely exceeding 1.5 m in depth, with occasional tidal inflows through a permanent opening to the sea at Maadi. The area of the lake was approximately 20 000 ha before 1950 and has reduced to approximately 10 000 (of which 3 000 ha are overgrown) in 1985 primarily through reclamation for farming and drying of the littoral. In areas where the water is less than 1 m deep, the reed Phragmites australis is, and was as far as human memory goes, the dominant macrophyte. More recently, commencing in the 1960s, water hyacinth Eichhornia crassipes has covered large areas, particularly away from the tidal influence.
Submersed plants, principally Ceratophyllum demersum and Potamogeton pectinatus, densely cover large areas of the less saline sections of the lake.
Inflow from the sea occurs sporadically, sometimes daily and sometimes weekly, depending on the tides, wind direction and volume of drainage inflow. The tidal influence, judged by the macrophytes, is limited at present to about 2 000 ha adjacent to the opening to the sea.
Total drainage inflow from the Edku, Boseilly and Barsik drainage canals is approximately 2 062 million m3 (2 062 000 megalitres). Pumps are situated on each of these canals.
The water movement/water balance of Lake Edku could be postulated as:
Freshwater drainage inflow - 2 062 000 megalitres
Loss from evapotranspiration (at 1.5 m/year) = 150 000 megalitres
Net water remaining (after evapotranspiration loss) = 1 812 000
megalitres.
An unknown amount of water is lost through seepage and the balance flows into the sea.
The total dissolved salts concentration varies from around 1 500 parts per million in the drainage inflow water, to more than 5 000 parts per million adjacent to the opening to the sea at Maadi. Evapotranspiration calculated to be about 1.5 m/year slightly increases the Lake Edku salinity. Concentrations of nitrogen and phosphorus are sufficiently high to cause explosive growths of aquatic plants - particularly water hyacinth.
The drainage water also contains unspecified quantities of urban, industrial and agricultural chemicals from the Beheira Governorate and beyond. Based on the diffuse sources of pollution many chemicals from human activities would be present in the drainage water. Most of these chemicals would be at trace (less than ppb (thousand million) levels). Nevertheless nothing is known of the synergistic effects on non-targets of water containing trace levels of many chemicals.
Barsik Fish Farm and Lake Edku use this water for fish production and it is considered essential that on-going research examines the level of chemicals in the sediments and in vertebrates and invertebrates, particularly in Lake Edku.
Fishing in Lake Edku is an unhealthy activity. About seventy-five percent of the 5 000 full-time and part-time fishermen in the lake suffer from schistosomiasis and other parasites. The problem is exacerbated by the dense growths of emergent and submersed plants that prevent water movement and provide a habitat for parasite hosts such as snails.
Control of the submersed vegetation with grass carp (Ctenopharyngodon idella) will probably cut down on health problems in Lake Edku. However, unless stringent health regulations are enforced, e.g., against defaecating and urinating in the lake it will always present a major health problem to fishermen.
A preliminary checklist of 23 aquatic macrophytes in Lake Edku is presented as Appendix 1. It is based on plants collected during this investigation. The most important species are the floating water hyacinth Eichhornia crassipes, the emergent reed Phragmites australis, and the submersed Potamogeton pectinatus and Ceratophyllum demersum.
Lake Edku is a shallow, occasionally tidal, eutrophic lake about 10 000 ha in area. Approximately half of the lake is less than 0.75 m deep, and if current water levels are maintained it will slowly turn into marsh or dry land. The speed with which this will happen depends on a number of factors including the level of plant nutrients in the inflowing water and/or the direct resumption of land for agriculture including fish farming. Of these, the most important factor is water inflow. Any reduction in total yearly inflows will result in the shallow sections rapidly turning to marsh or dry land.
Changing demands for water in Egypt which will result in less drainage water reaching Lake Edku is always a possibility, and if this happens it will, as the lake water level drops, increase the amount of Phragmites australis. Less freshwater in the lake may result in greater salinity levels, through increased tidal influence.
Many variables are likely to affect Lake Edku in the future and the changes in the macrophytes cannot be predicted with certainty. However, if existing water inflow and water quality conditions remain static, a steady increase in the amount of reeds is likely as the organic matter builds up.
The water hyacinth in Lake Edku, growing near the three major drains supplying the lake, is characterized by very luxuriant growth, i.e., long, dark green stems and leaves, few flowers and short roots. The water hyacinth found in the saline sections is exhibiting leaf necrosis and a stunted appearance. Water hyacinth found in the zone between the strong tidal influence and the eutophic water of the drains has noticeably longer roots, is a lighter shade of green and is profusely flowering; all these characteristics are a sure sign of mesotrophic conditions and indicate that the nitrogen and phosphorus are being stripped out of the water by the dense submersed and emergent macrophytes flourishing at the mouth of the drains.
The reed Phragmites australis dominant in the lake varies from sparse, thin plants carrying unhealthy leaves to strong growth up to 5 m tall. The unhealthy reed is found near the tidal entrance to the lake where salinity levels are high enough to prevent optimum growth. Phragmites is killed by sea strength salinity (typically 35 000 ppm total dissolved salts) but will survive in tidal situations if there is a daily wash of fresh or brackish water over the roots.
Phragmites is one of the early colonizers in salt marsh reclamation and its use for this purpose in countries like the Netherlands is well documented. It is therefore likely to continue as a monospecific stand in the tidal area.
The significant submersed plants are Potamogeton pectinatus and Ceratophyllum demersum.
P. pectinatus is one of the world's more cosmopolitan submersed plants, growing in water from 1 to 4 m deep and thriving in water containing at least 5 000 ppm total dissolved salts. It is a perennial which reproduces mainly by specialized tubers that grow in large numbers in the root system. A 24-m2 culture of P. pectinatus produced 36 000 tubers in one growing season demonstrating its considerable capacity for regeneration. This species will grow in water temperature varying from 10° to 25°C and higher. When water temperatures fall below 10°C, as probably happens in Lake Edku in mid-winter, considerable dieback of stems and leaves would occur. However, this suggestion was not verified by Lake Edku fishermen.
Ceratophyllum demersum is another cosmopolitan submersed aquatic plant. This plant is rootless and the stems, up to 2 m long, float around with current or catch on other vegetation. The leaves assume neutral buoyancy, and dense growth (as seen in Lake Edku) occupies most of the water column. No fruits were observed in Lake Edku, but this is not important as the main form of propagation is from pieces. The plants over-winter by thickened lateral tips which sink to the mud and grow again under favourable conditions. These dormant pieces contain increased quantities of starch. Numerous snails representing intermediate hosts of Schistosoma were evident on the Ceratophyllum demersum leaves and on Potamogeton pectinatus.
A detailed account of this farm is given in the Field Document 1. This farm, which was constructed in the southeast sector of Lake Edku and covers approximately 685 ha of water, consists of 104 production ponds, each about 6 ha in size, and 50 smaller (about 1 ha each) ponds for breeding and raising broodstock. The rate of aquatic weed infestation in these ponds differs from pond to pond, with some ponds entirely clear of weeds and others completely overgrown by predominantly Phragmites. The rate of overgrowth is largely directly related to the intensity of pond management, those ponds without any management usually having the most dense weed cover. Weed control in ponds has been carried out on a limited scale and with varying success by using a floating aquatic weed cutter and by stocking grass carp.
