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Dredging in a lake may transform its micro-ecology, according to a new report.
Researchers from the National Institute of Oceanography and Fisheries in Cairo studied Lake Manzala, a coastal lake of Egypt, for seven years during 2015-2022.
They found that dredging increased salinity and reduced nutrients in the lake considerably in 2022 compared to 2015. This, in turn, changed the chemical parameters and zooplanktons in the lake's northern and the middle regions.
The findings were published in Evaluating the restoration of Lake Manzala after dredging using water quality parameters and zooplankton changes in the journal Nature in May 2025.
Manzala is situated in the northern part of Egypt, with the Mediterranean Sea to its north. It protects the coastal cities of the country from flooding during heavy downpours. It sustains biodiversity in the Mediterranean region and acts as a natural buffer zone between the saline water of the Mediterranean Sea and the agricultural fields of the coastal areas of northern Egypt.
In 2021, Manzala contributed to Egypt’s 16 per cent natural fish production and 3.6 per cent total fish production, according to the report.
The research indicated that the surface area of Lake Manzala, which was around 1,709 square kilometres until the 19th century, decreased by 67 per cent between 1907 and 2016, primarily due to land encroachment and aquaculture activities.
Additionally, eutrophication contributed to a further 45 per cent reduction. Currently, the lake covers an area of 572.41 square kilometres.
The lake receives water from two sources. In the north of the lake, saline water from Mediterranean Sea enters through the narrow channels of Al-Gamil, Ashtoum Al-Gamil and Al-Sofara outlets.
Another source is the drainage at the southern part of the lake which is supposedly a freshwater inlet which is through the Bahr Al-Baqar, Hadous, Al-Mataria, Faraskur and Al-Serw drains.
The latter drains also carry untreated wastewater from households, industries and agriculture — heavy metals, pesticides and organic nutrients enter the Manzala lake and pollute it to the brim. This wastewater contributes 98 per cent of the annual inflow to Lake Manzala.
The 190 kilometre Bahr al-Baqar drain passes through Qalyubia, Sharquya, Ismailia and Daqahlia and dumps around 16,000 million litres per day (MLD) of wastewater into the lake. The accumulation of pollutants and their concentrations had led to the vegetative growth in the lake, deteriorating the water quality and reducing fish production.
The fish production in Lake Manzala reduced by 20 per cent between 2012 and 2015, according to data published by the Central Agency for Public Mobilization and Statistics. Fishery is a source of livelihood for people residing around the lake for generations, and increased pollution in the waterbody across years has affected their sources of income.
There are around 304 islands in Manzala lake, constituting a total of 23 per cent of the lake area. The untreated domestic sewage from these islands also contribute to the pollution of the lake.
In 2017, the Government of Egypt started a massive project to clean Lake Manzala and construct a wastewater treatment plant in Bahr Al-Baqar. The dredging work started in 2017 when pollutant-rich sediments from the surface and bottom layers of the lake were removed.
This also included removing various islands and dense aquatic vegetation, which were home to dense pollution. The aim was to increase the capacity of the lake and remove the pollutant-rich sediment from this waterbody.
The world’s largest wastewater treatment plant was constructed near the lake in 2021, with a treatment capacity of 5,600 MLD.
The study done by researchers of the National Institute of Oceanography and Fisheries in Cairo has claimed that the lake has undergone significant changes due to dredging work. The capacity of the lake increased by 138 per cent during 2016-2022.
The researchers claimed that the storage capacity of the lake, after the dredging, was 903.64 million cubic metres (MCM) in 2022, which was 378.67 MCM in 2016.
The water quality and ecosystem of the lake have changed significantly after the dredging. The northern and central parts of the lake show prominent changes. The salinity and transparency of the lake water has increased, which has also led to changes in the zooplankton’s species.
There has been an overall reduction in the number of zooplankton but the saline species have increased their population in 2022. Additionally, the change in chemical properties of water has led to the growth of two new saline species of zooplankton.
The number of saline species has increased from seven in 2015 to 12 in 2022. Their population has increased in the northern and middle part of the lake, where salinity has increased. On the other hand, the freshwater species of zooplankton reduced in the lake during 2015-2022.
The aftermaths of dredging have been in debate for a long time. A 2023 research report in the International Journal of Geography and Environmental Management highlighted the environmental impacts of dredging in a waterbody. On one side, the process removes the pollutant-rich sediments from the waterbody and increases the overall capacity. But it also has a negative impact.
The release of pollutants from the sediment during dredging can lead to more hazardous impacts. The transported pollutant rich sediment can impact the new waterbody or place where the sediment is dumped. Most importantly, dredging can partially or completely change the micro-ecology of the lake.
Indian scientists consider dredging by itself to be a complicated and harmful method for revitalizing lakes, as it could disrupt the delicate lake ecosystem. Once the lake's ecology is disturbed, it may not recover naturally.
“Dredging is a very harsh and major destabilising operation for the health of a water body, especially when considered for a lake like Manzala which is situated in an extremely fragile ecologically balanced neighborhood,” said Ujjal Kumar Mukhopadhyay, retired chief scientist of West Bengal Pollution Control Board.
He added that India has diverse hydro-ecology and every lake has its own unique ecosystem. Processes like dredging should not be adopted blindly in any lake, as it will alter the basic structure of the waterbody.
The new study showed that following dredging, the lake's maximum salinity rose by 73 per cent from 2015 to 2022. In 2015, transparency ranged between 10 and 60 centimetres, which shifted to 20-65 centimetres in 2022.
The dissolved oxygen levels in the northern part of the lake increased from 12.22 to 12.7 milligrammes per litre during the same period. Despite this, the biological oxygen demand (BOD) and dissolved oxygen (DO) levels did not decrease significantly to safe limits, especially in the southern region where wastewater enters.
This discrepancy invites discussion on the research and suggests that reducing pollution is the most effective strategy to restore the lake's ecology, according to Mukhopadhyay.
However, the data showed that there has been an overall reduction in BOD and chemical oxygen demand (COD) in the lake.
This change in water quality and zooplankton communities has transformed the ecology of Lake Manzala.
Sanjeev Agrawal, National WASH Expert, Ministry of Jal Shakti, said, “Any lake can be saved from pollution only by controlling the pollution at sources. Dredging work may remove the pollutant-rich sediment for a shorter period, but it will disturb the natural ecosystem of the lake in the long term. The lake may take longer than the required period to recover to its natural condition, provided the pollution influx is controlled”.
Agarwal added, “Lakes are dynamic ecosystems, and they must be dealt to protect, sustainably manage and restore for human wellbeing, biodiversity benefits that address societal challenges effectively and adaptively to overcome problems of hysteresis (delayed recovery path of Lake Ecosystem) phenomenon in modified ecosystems in present days".
Indian scientists argued that lakes from India cannot be restored only through dredging. This process of removing nutrient-rich sediment may bring about irreversible damage to the health of the waterbody.
Currently, the government is revitalising lakes and ponds across the nation to effect changes in the ecosystem, groundwater replenishment and livelihoods through various programmes. It is important to recognise that dredging alone will not improve the quality of lake water and should not be considered a universal solution for countries with varied hydrogeological landscapes.