Hydrodynamic-ecological model analyses of the water quality of Lake Manzala (Nile Delta, Northern Egypt) |
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Authors: | E Kock Rasmussen O Svenstrup Petersen J R Thompson R J Flower and M H Ahmed |
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Institution: | (1) DHI-Water and Environment, Agern Alle’ 5, Horsholm, 2970, Denmark;(2) Wetland Research Unit, Environment Change Research Centre, UCL Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK;(3) Department of Marine Resources, National Authority for Remote Sensing and Space Sciences, 23 Josef Burrows Tito St. El Nozha, Al Gadida, P.O. Box 1654, Cairo, Egypt |
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Abstract: | Lake Manzala remains the largest of the Nile Delta lakes and has undergone substantial recent changes particularly over the
last 30 years due to regulation of freshwater inflows, land reclamation and increasing loads of BOD and nutrients. The latter
are particularly associated with polluted drains flowing into the lake. An increasing fraction of the N load may also come
from the expanding fish farming industry within and around the lake. Over the past 20–25 years, the N load from the drains
has increased three times whilst the P load has remained approximately constant. This is despite a 40% reduction in freshwater
inflows due to diversion of water towards the Sinai for land development. As a result of these changes, the concentration
of total N in the drains has increased 4.5 times while the total P concentration has increased by 50% over the same period.
Increases in nutrient concentrations can be attributed to increase in the use of fertilizer combined with an expanding population
served by sewers, improved living standards and increased industrial production. Nutrient enrichment has resulted in declining
water quality and eutrophication, especially in the southern parts of Lake Manzala. A combined hydrodynamic-ecological model
has been set up for the lake and is used to analyse the development of water quality as a function of the load of organic
material and nutrients. Model results show that 86% of the N load and 59% of the P load are retained within the lake. Despite
the efficient treatment of wastewater by the lake ecosystem, approximately 6,000 tonnes of N and 1,300 tonnes of P are exported
to the Mediterranean through the two main lake–sea connections. This nutrient export has important implications for coastal
fisheries production. The present investigation suggests that the current anthropogenic nutrient load to the coastal zone
exceeds that of pre-Aswan High Dam conditions despite large reductions in the annual pulse of water and nutrients brought
about by the dam. A series of model simulations with reduced nutrient load are investigated in order to identify the conditions
required to enable the propagation of vegetation throughout the lake. Submerged rooted vegetation is used as an indicator
for water quality as it needs light for growth and in polluted, eutrophicated waters turbidity increases hampering vegetation
growth. Simulation results indicate that the nutrient load has to be reduced to approximately 25% of the present loads if
submerged rooted vegetation is to become re-established in the more polluted south eastern parts of the lake.
Guest editors: J. R. Thompson & R. J. Flower
Hydro-ecological Monitoring and Modelling of North African Coastal Lagoons |
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Keywords: | North Africa Lagoons Lake Manzala Ecosystem model Mass-balance modelling Eutrophication |
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