Evaluating the restoration of Lake Manzala after dredging using water quality parameters and zooplankton changes

The northern coastline of Egypt features five lakes connected to the Mediterranean Sea. These lakes serve as a significant source of fish production, contributing over 50% of Egypt’s total fisheries output in the 1980s. However, between 2015 and 2020, this contribution declined to less than 11%¹. Lakes are particularly vulnerable to rapid environmental changes resulting from various human activities, which lead to decreased water quality and increased eutrophication. This situation is evident in the Mediterranean lakes of Egypt, which receive substantial amounts of untreated industrial, domestic, and agricultural wastewater^2,3,4,5,6. As a result of these human activities, these coastal lakes have suffered severe ecological damage, leading to a decline in fish production. Lake Manzala is the largest lake among the northern coastal lakes of Egypt, covering a surface area of approximately 572.41 km^27,8. It is recognized as one of the most significant wintering and nesting sites for various species of migratory birds⁹. Additionally, Lake Manzala is expected to play a vital role in mitigating the impacts of climate change. It is also essential for protecting coastal cities from storm surges and flooding. Furthermore, Lake Manzala is invaluable to the biodiversity of the Mediterranean region¹⁰. Moreover, it functions as a naturally occurring oxidation basin, acting as a buffer zone to prevent saline coastal water from infiltrating groundwater and agricultural fields, while also serving as a natural barrier between the drainage systems of the Mediterranean Sea and the Nile Delta¹¹.

There are two main sources of water for Lake Manzala: (1) Saline water enters the lake from the Mediterranean Sea to the north through narrow channels, specifically the Al-Gamil, Ashtoum al-Gamil, and al-Sofara outlets¹². Seawater flows into the lake through the al-Sofara outlets, located in the northwest corner, via several openings, with El-Boghdady being the most significant. (2) Freshwater sources enter the lake through numerous drains and pumping stations to the south, particularly Bahr al-Baqar, Hadous, Al-Mataria, Faraskur, and Al-Serw⁶. Untreated wastewater, laden with various pollutants—including heavy metals, pesticides, PCBs, high concentrations of nutrients, and organic matter—constitutes approximately 98% of the annual inflow to Lake Manzala. These pollutants have led to an increase in vegetation area while simultaneously decreasing water quality and fish productivity¹³. Additionally, the lake contains a substantial number of islands, which cover about 23% of its total area³. Since surface water is a complex mixture of soluble and insoluble chemicals, water quality encompasses all characteristics of a waterbody influenced by surrounding environmental factors. Waterbodies deteriorate to varying degrees due to the direct impact of dissolved and insoluble contaminants on surface water characteristics¹⁴. Overall, water pollution is a global issue that affects most waterways¹⁵ as a result of human activities and the ongoing, steady increase in pollution¹⁶.

The Egyptian government embarked on a massive project to restore the lake to its natural habitat. This initiative includes a large wastewater treatment project in Bahr El Baqar. Additionally, the lake restoration and treatment plan encompass an extensive dredging operation, which began in 2017 and is scheduled for completion in late 2022. The dredging process aims to increase lake depths in addition to removing islands and dense aquatic vegetation. Dredging is a lake restoration technique that involves the removal of surface-bottom layers containing pollutants, thereby controlling their release and nutrient bioavailability^17,18,19. Sediment treatment is often necessary to enhance water quality and other ecosystem components²⁰. Currently, dredging is the most widely used method for addressing pollutant-rich sediments. However, the positive and negative effects of dredging remain subjects of ongoing debate, particularly regarding its role in the ecosystem debate^{22,23,24,25,26,27}. In several studies, dredging activities have been shown to cause significant changes in various components of lake ecosystems. Numerous investigations have documented the negative environmental impacts of dredging^{26,27,28,29,30,31,32,33}. Among these negative impacts are increases in nitrate, phosphorus, ammonia, nitrogen, alkalinity, and conductivity following dredging^19,34. Furthermore, the adverse consequences of dredging can lead to alterations in the structure and distribution of biota within a lake. In this context, Rehman et al.²⁵ concluded that dredging had modified the water quality in Dal Lake, Kashmir, India, resulting in a dramatic shift in the zooplankton community structure. Factors such as predation and competition became influential in shaping the zooplankton community. Conversely, dredging may also serve as an effective strategy for improving lake environments by reducing internal nutrient loads, which can promote the dominance of less eutrophic zooplankton species and decrease the presence of indicator species associated with high eutrophication. Consequently, changes also occur in the composition of zooplankton communities²⁴. Therefore, the current study aims to conduct a preliminary assessment of the environmental state of Lake Manzala by monitoring chemical changes and the community composition of zooplankton with the dredging process in its final stages. This study is characterized by the integration of chemical and biological assessments of water quality at different time intervals to evaluate the impact of dredging on the physicochemical parameters and zooplankton community structure of Lake Manzala, Egypt, by comparing data collected before (2015) and after (2022) the dredging process.

Site description

Lake Manzala is a rectangular, brackish, shallow, and turbid water basin. Lake Manzala is a rectangular, brackish, shallow, and turbid water basin. As of 2022, the lake measures approximately 43.1 km in length and has a mean width of 13.1 km, with depths ranging from less than 0.5 m to more than 2 m. The shallowest depths are found in the southern zone, while the deepest areas are located near the entrance to the Mediterranean Sea. The lake is situated between latitudes31° 07′ 03.2ʺʹ N and 31° 23′ 53.7ʺ N and longitudes 31° 47′ 45.4ʺ E and 32° 14′ 35.0ʺ E³⁵. As previously mentioned, the lake receives saline water from the Mediterranean Sea through various outlets and fresh water (brackish water) from several drains that carry a significant amount of wastewater. Additionally, Lake Manzala is connected to the Suez Canal via a bit of exploring channel known as Al-Qabouti. Accoeding to Elshemy³, Khedr et al.⁸, and Abd Ellah⁹ this canal, referred to as the El-Raswa Canal, links the Suez Canal to an isolated pond in the northeastern part of the lake. This isolated pond serves as a treatment reservoir for the sewage from Port Said City. Lake Manzala is of great importance both nationally and globally; it contributed approximately 16.8% of Egypt’s natural fish production and 3.6% of the total fish production in 2021^8,36. Over the past few decades, the surface area of Manzala Lake has steadily decreased from 1709 km² in 1907 to 565.91 km² in 2016, primarily due to illegal land reclamation and aquaculture practices. Furthermore, the overgrowth of aquatic vegetation and the proliferation of islets have reduced the open water area, which shrank from over 70% to approximately 45% of the total lake area between 1986 and 2016. However, due to extensive government efforts, the lake area increased to 572.41 km² in 2020, and the open water area rose to about 75% of the total lake area^9,10. According to Abd Ellah³⁵, the depth of Lake Manzala increased significantly between 2016 and 2022 as a result of dredging operations, where the volume of water increased from 378.67 MCM in 2016 to 903.64 MCM in 2022 with about an increase of 524.94 MCM in the water volume. For example, the area with a depth of more than 2 m increased from 9.41 km² in 2016 to 166.49 km² in 2022.

Sampling sites

Samples were collected during the winter and summer seasons of 2015 and 2022 from 12 sites, dividing the lake into three sections: north, middle, and south/southeast. The northern section includes sites 1, 2, 3, 11, and 12, and extends along the Mediterranean coast and is connected to it by several canals. The middle section is represented by sites 4 and 5, located at the center of the lake. The southern and southeast sections comprise stations 6, 7, 8, 9, and 10, which are characterized by receiving significant volumes of water discharged from various domestic, agricultural, and industrial sources (Fig. 1 and Table 1).