Dissolved oxygen (DO) concentration is an essential indicator for assessment of estuary ecosystems. According to our knowledge, there is no research on the depth profiles of the DO and associated water quality variables of the entire Bouregreg estuary (Morocco). Water samples were collected from three stations at 0.5-m depth intervals during high and low tides (10/11/2021, 20/03/2022, and 02/07/2022). Our study shows that the DO concentrations ranged between 5.5 and 11.8 mg/L and demonstrated an inconsistent stratification pattern. When the tide was high and low in most of the study area, the bottom layer had a higher concentration of DO than the surface layer. There was a relatively high concentration of DO in the bottom layer of the estuary and a relatively low concentration in the surface layer of the estuary due to a high rate of freshwater flow on the surface (the release of Sidi Mohammed Ben Abdellah dam and the existence of estuarine gravitational circulation). Salinity, pH, water temperature, and water transparency ranged from 19.4 to 35.8 ppt, 7.0 to 8.7, 17 to 23 °C, and 16 to 21 cm, respectively, considering the depth profiles in the study area. The study provides scientific support for ecology operation and considerable advances in understanding the ecosystem dynamics of the Bouregreg estuary.

  • Depth profiles of the DO and associated water quality variables are studied.

  • The result shows that the DO concentrations ranged between 5.5 and 11.8 mg/L and demonstrated an inconsistent stratification pattern.

  • The study provides scientific support for ecology operation and considerable advances in understanding the ecosystem dynamics of the Bouregreg estuary.

Many definitions and classifications have been proposed for estuaries, which mostly depend on their application (Pritchard 1967; Hume & Herdendorf 1988; Davidson & Council 1991; Savenije 2006). In most cases, estuaries were defined based on the relative influence of tides, waves, rivers, sediment types, sediment supply, vegetation, geology, or time (Savenije 2006, 2005). Additionally, estuaries play an essential role in the human–earth system, affecting freshwater resources, the mixing between the ocean and river water, and the health of aquatic ecosystems (Savenije 2015; Wei et al. 2017). This makes the functioning of estuarine systems an important field of research. The dissolved oxygen (DO) concentration in a river is an essential indicator of ecosystem assessment. DO decrease in rivers impacts biological activities and results in reductions in benthic animal populations (Bu et al. 2021). A low DO concentration may slow the degradation of pollution, weaken the capacity of water's self-purification, and even decrease the quality of the aquatic ecosystem. In recent decades, hypoxia (DO < 2 mg/L) has been frequently reported, which is an urgent warning for river ecosystem health (Ji et al. 2017). DO is required for respiration by most aquatic biota. Apart from this, DO combines with other important elements such as carbon, sulphur, nitrogen and phosphorus that could have been toxicants in the absence of oxygen in the water bodies to form carbonate, sulphate, nitrate, and phosphate, respectively, contributing, in this way, to the required compounds for the survival of aquatic organisms (Rouf et al. 2022). Therefore, natural stream purification processes require adequate oxygen levels in order to provide aerobic life forms. As DO levels in water drop below 5.0 mg/L, aquatic life is put under physiological stress (Rouf et al. 2022). Oxygen levels that remain below 1–2 mg/L for a few hours can adversely affect the growth and survivability of aquatic organisms. DO is, therefore, an indispensable element to all forms of aquatic life (Sarker et al. 2021). DO generally varies horizontally along the course of the waterway in shallow rivers but is most likely to vary vertically in the water column in deep rivers. A theoretical deduction shows that vertical stratification of DO concentration can be explained by the extended Hansen and Rattray's central region theory, which suggest that vertical DO profiles are mainly controlled by biological factors such as photosynthesis, biochemical oxygen demand (BOD), sediment oxygen demand (SOD), and physical factors such as surface re-aeration, river flow, and estuarine gravitational circulation (Lin et al. 2006). Freshwater from inland and saline water from the sea can generate a stratified layer and limit the DO in the epilimnion and the hypolimnion region (Rouf et al. 2022). Alternately, a deep river can also generate an unstratified water column due to the mixing of water. The stratification pattern of DO in estuaries is considered a highly informative variable for obtaining the ecosystem's functionality and behaviors. Stratification of DO has been reported from various estuaries of the world in recent years (e.g., Lin et al. 2006; Liblik et al. 2020; Rouf et al. 2022). It appears that vertical distributions of DO concentrations in estuaries are affected by different parameters in different systems. The dominant parameters in a system should be unknown a priori. Most of the studies regarding the Bouregreg estuary are focused on pollution assessment (Cherkaoui et al. 2005; El Harim et al. 2019), salt-intrusion (Haddout et al. 2020, 2021), distribution of water quality variables (El Amraoui et al. 2015), sediment/ heavy metal studies (Priya et al. 2018, 2021, 2022). All these studies were based on surface layer observation instead of considering the vertical profile of the river. Thus, an analysis of the vertical profile of DO and its stratification, in the Bouregreg estuary is needed that helps in improving. The understanding of the river ecosystem, which is beneficial for estuarine ecologists and coastal managers.

Study area

The Bouregreg River is located in northwestern part of Morocco, stretching about 240 km from its source in the Middle Atlas Mountains to the Atlantic Ocean. It belongs to a watershed covering an area of approximately 9,800 km2 with elevation ranging from 0 to 1,627 m (Zellou & Rahali 2017). The climate is semi-arid with an average annual rainfall of 440 mm and an average annual temperature of 18 °C. The Bouregreg estuary is located on the Atlantic coast between the two cities Rabat and Sale 34°N and 6°50′W (Figure 1). It has a length of 23 km, limited by the Sidi Mohammed Ben Abdellah dam and an average width of 150 m. Its average freshwater flow varied from 3 to 84 m3s−1. Additionally, the tide near the estuary mouth is mainly semi-diurnal with a 12.25-h tidal cycle and is a meso-/micro-tidal estuary with an average tidal range of only 2.3 m. The absence of a real fluvial flow, following the disappearance of flooding events, clearly influences the estuary's functioning, where the hydrodynamics are governed by the tides (their height varies from 0.5 to 3.3 m) (Cheggour et al. 2005). Tidal excursions, i.e., the distance that a water particle travels between low tide and high tide ranges between 3.5 and 7 km. The channel width is about 150–140 m to 57 m and the average depth ∼3.5–7 m, approximately. This estuary is dominated by seawater, with a minimal influx of freshwater for most of the year. Near the mouth, there are agricultural and industrial areas. Some urban sewage from Rabat and Sale is directly discharged into the estuary.
Figure 1

Map showing the Bouregreg estuary and location of measuring stations.

Figure 1

Map showing the Bouregreg estuary and location of measuring stations.

Close modal
On the other hand, the physical–chemical structure of the Bouregreg estuary is characterized by two gradients: one is the increment of particulate organic matter and nutrients and another is the enrichment of salts, the latter playing therein a more important role. The estuarine waters, due to oceanic influence, show a double thermal gradient horizontally with the season. In fact, their temperature decreases of the upstream towards the downstream during the dry period than the wet period. The saltwater intrusion of the estuary was found to increase with time since the construction of the Sidi Mohammed Ben Abdellah dam. The waters, in general, are well oxygenated and rich in suspended particulate matters. High concentrations of nutrients occur at midstream and upstream parts. In addition, Figure 2 gives the longitudinal evolution of measured river width, cross-sectional area, and depth elaborated with an intensive field survey.
Figure 2

River width, cross-sectional area, and depth characteristics of the Bouregreg estuary.

Figure 2

River width, cross-sectional area, and depth characteristics of the Bouregreg estuary.

Close modal

Data observation and preparation

Water samples were collected on the following days, months, and years, 10/11/2021; 20/03/2022; and 02/07/2022 at the stations indicated in Figure 1, for in situ measurements and laboratory analysis (study desk). Sampling was conducted during the slack period of both high tide and low tide. Water sampling date, tidal conditions, weather, and atmospheric temperature are given in Table 1. Water samples were collected by using a bottom water sampler in 0.5 m of depth intervals. During in situ measurements, salinity, water temperature, and pH were measured by YSI Pro30 conductivity meter attached to a 10-m cable. Also, a Global Positioning System (GPS) was used to record the positions of every measurement. The equipments used are adequate to carry out various measurements in cost-effective way. The measurements started from the mouth moving upstream, keeping pace with the tidal wave. The processed water samples were taken to the study desk at Casablanca (Morocco) for analysis using the Winklers method (APHA, AWWA & WEF 1995) was applied for the determination of DO. An accuracy of 0.1% was observed while comparing with the standard-based dissolving known quantities of O2 in O2-free water. Mann–Whitney test was conducted at a 5% significance level in order to determine the statistical difference between the surface and bottom layers of the DO of each depth profile. To calculate the percentage saturation of DO, a monogram was used to get a quick approximate oxygen saturation value (Behar et al. 1996).

Table 1

Station-wise water sampling date with their respective, tidal level, weather, and atmospheric temperature

StationSampling dateTide conditionsWeather and atmospheric temperature
Station 1 at 1km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 
Station 2 at 4km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 
Station 3 at 7km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 
StationSampling dateTide conditionsWeather and atmospheric temperature
Station 1 at 1km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 
Station 2 at 4km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 
Station 3 at 7km from the mouth 10/11/2021 HW Cloudy 
LW Cloudy 
20/03/2022 HW Cloudy 
LW Sunny 
02/07/2022 HW Sunny 
LW Sunny 

HW = high water; LW = low water.

DO stratification

The depth profiles of DO in three stations during high and low tides during 10/11/2021; 20/03/2022; and 02/07/2022 are shown in Figure 3. DO concentrations ranged from 5.5 to11.8 mg/L at both tides during the sampling months throughout the depth profiles. The range of DO in the stations indicates a healthy water mass for estuarine biota throughout the profiles. EL Harim et al. 2021 recorded the DO as 3.37 mg/L at 7 km to 9.57 mg/L at the mouth (during the autumn period) and from 2.93 mg/L at 7 km to 9 mg/L at the mouth (during the summer period) at around 0.2 m depth for the samples obtained from the lower stream of the Bouregreg estuary. During the cold period, heavy rainfall increases the exchange of oxygen with the atmosphere and facilitates the entry of air into the water bodies of this estuary and therefore a dilution of the polluting load. Furthermore, lower temperatures favour the dissolution of oxygen thereby enhancing the DO concentrations. The lowest concentrations of DO are recorded in station 3 where strong anthropogenic pressure is experienced by solid waste, air pollution, the action of microorganisms (depleting the oxygen medium by degrading the abundant organic matter in this station), but especially by the warming of the water of this station by discharges of raw sewage rich in fermentable organic matter, which are liable to be oxidized causing a decrease in the dissolution of oxygen (Brahimi & Chafi 2014). The DO results in the present work are still higher than those recorded in this estuary before the development works of the Bouregreg valley (from 1.5 to 5.5 mg/L) (Cherkaoui 2006). According to the Moroccan classification of surface waters, the waters of this estuary are classified from average to good quality. On the other hand, higher DO values were found in the low tide than that of the high tide in most of the profiles due to strong tidal mixing in the estuary, with cold incoming seawater and warm outgoing freshwater. During the low tide, the salinity values are lower than that during the high tide. DO values are relatively lower in high saline waters as the mass transfer of atmospheric oxygen is reduced in ionic water thereby causing a decrease in the dissolution of oxygen in saline water. It was noticed during the sampling that the water current was relatively strong during the low tide than that of the high tide in stations 1 and 2. Higher turbulence enhances the surface area and increases the dissolution of oxygen; so, it is suspected that this phenomenon also influences the DO variation in the observation sites. Supporting this, Pereira et al. 2013 also reported that intense water–atmosphere interactions and photosynthetic activity dominated the concentration of DO in the Amazon Coast. Small depth may also be responsible for mixing or little stratification in the study area. DO was found relatively higher in November and March than July due to the autumn and spring seasons. Maximum three units (mg/L) of variation in DO were noticed in the depth profiles of the stations. It is noticeable that the DO concentration was relatively high in the bottom layer and low in the surface layer during both the high and low tides in most of the months. This may be associated with the temperature variations; bottom cool layers exhibiting higher DO than warm top layers. The obtained results from Mann–Whitney test suggest that both significant difference (P < 0.05) and no difference (P > 0.05) between the surface and bottom layer DO profiles were evident in the study area. Therefore, it can be said that the stratification of DO was inconsistent in the depth profiles of study area. Lin et al. 2006 found similar stratification pattern in the Cape Fear River Estuary of North Carolina due to higher estuarine gravitational circulation; and same results are investigated by Rouf et al. 2022 in the Pasur-Rupsha estuary of Bangladesh. In contrast, Liblik et al. 2020 reported strong stratification of DO due to wind forcing circulation patterns, upwelling and downwelling in the Changjiang (Yangtze) Estuary, China.
Figure 3

DO profiles during high and low tides in three months in Stations 1–3.

Figure 3

DO profiles during high and low tides in three months in Stations 1–3.

Close modal

Relationship of DO profiles with salinity, temperature, pH, and water transparency

Depth profiles of DO, salinity, temperature, and pH during the high and low tides in 3 sampling months in the study area are shown in Figure 4 (DO from Figure 3). The vertical profiles of DO, temperature, salinity, and pH showed an inconsistent stratification pattern throughout the study period.
Figure 4

Salinity, pH, and temperature with depth profiles during high and low tides in 3 sampling months in Stations 1–3.

Figure 4

Salinity, pH, and temperature with depth profiles during high and low tides in 3 sampling months in Stations 1–3.

Close modal

Temperature controls the lives of aquatic organisms (Hauer & Hill 2007). The temperature of a particular water system usually affects the solubility of substances on it. Certain substances dissolved significantly in water at high temperatures, others do so at low temperature (Averill & Eldredge 2012; Rouf et al. 2022). Temperature has direct relation with sedimentation and filtration rate as well as with the metabolism rate of aquatic organisms. The water temperature of the Bouregreg estuary varies according to the seasons and the nature of the waters that are salted downstream and freshwater upstream. The average water temperature in the surface of this estuary oscillates between 19.98 and 21.80 °C (EL Harim et al. 2021). The temperature rise in the upstream sector can be explained by the direct influence of the air temperature, given the shallow depth of the water layer upstream of the16-km stretch (EL Harim et al. 2021). During the study period, water temperature varied from 17 to 23 °C in the vertical profiles of the stations. One or less than one unit (°C) variation in temperature was noticed during study period. Less variation (<2 units) of temperature within the profiles indicates a weak stratification, which addresses a level of water mixing. This mixing is thought to be regulated mostly by the water movement following ebb and flood tides in the estuary. The relation between the temperature and DO was found inversely correlated (−0.252) with high significance level (P < 0.000). Some temperature profiles showed inverse relation with DO profiles while others showed opposite or no relation. Seasonal variation of temperature within the temperature profile was found responsible factor for that minimum level of inverse correlation coefficient.

Water salinity in the stations ranged from 19.4 to 35.8 parts per thousand (ppt) during the study period. Maximum one unit (ppt) variations in salinity were noticed in all profiles. Relatively low variations observed in those stations are located downstream. The salinity profiles, however, indicate a weak stratified profile in the stations. Variation in salinity profiles from the high tide to low tide, shown in Figure 4, addressed that the water movement is regulated by ebb and flood tides along with the differences in the density of water. The movement of water due to the density differences in turn expresses the existence of estuarine gravitational circulation. These regulatory mechanisms may be responsible for the weak stratification in the estuary. These results are comparable to those recorded in the same estuary by Haddout et al. (2020, 2021). The relation between salinity and DO in this study was not found significant (P > 0.3) in the profiles. Some salinity profiles showed inverse relation with DO profiles while others showed opposite or no relation.

pH determines the acidity or alkalinity of a water system. It regulates the chemical and biological states of nature water. The changes in pH bring the dissociation of weak acids and bases that affects the toxicity of most compounds in water. Hence, the ability of water to support life is a function of pH (Simon & Beevers 1952). Water pH was found between 7.5 and 8.7 in the profiles during the study period, in three stations. During the study period, this stability of values at the mouth of the estuary can be explained by the alkalizing effect of marine waters, and upstream by the marly and calcareous nature of the geological formations of the Bouregreg catchment area, thus promoting the release of carbonates and bicarbonates, which increases the pH (Benmessaoud 2007). The pH profiles rarely showed any major stratification in the stations. The pH is defined as negative log to H+ ion concentration. As temperature increases, H+ concentration increases due to increased activity and there is a drop in pH. For example, pH of water at 100 °C is 7.47 and that at 1000 °C is 6.14. Also, the variation of pH with temperature is just one scale for a temperature difference of 1,000 °C. Thus, the relationship between pH and temperature and hence DO in the estuary was found not significant (P > 0.05) in the profiles. DO and pH would increase when photosynthesis by aquatic plants removes carbon dioxide (CO2) from the medium and decline during respiration by aquatic organisms and decomposition by decomposers release CO2 into the medium (Mistry et al. 2019). The observed pH range (7.5–8.7) in the surface layer of the stations suggests a reasonable extent of photosynthesis by aquatic plants in the surface of the estuary. While a nearly similar pH range (7.0–8.0) in the bottom layer suggests insufficient respiration and decomposition at the bottom of the estuary. In addition, water mixing is also responsible for similar pH in the profiles (Muduli & Pattnaik 2020; Rouf et al. 2022).

Transparency of the Bouregreg estuary was also observed during the study period and the records are shown in Table 2. The values ranged from 16 to 21 cm. The range of transparency observed in the estuary was noticeably low and represents (in general) extensive turbid water.

Table 2

Transparency of water in the head of the Bouregreg estuary

StationTideTransparency (cm)
10/11/202120/03/202202/07/2022
Station 1 HW 20 18 21 
LW 18 17 19 
Station 2 HW 19 18 – 
LW 17 17 18 
Station 3 HW 16 16 17 
LW 17 18 17 
StationTideTransparency (cm)
10/11/202120/03/202202/07/2022
Station 1 HW 20 18 21 
LW 18 17 19 
Station 2 HW 19 18 – 
LW 17 17 18 
Station 3 HW 16 16 17 
LW 17 18 17 

HW = high water; LW = low water.

The Bouregreg estuary has significant ecological importance in Morocco. Despite its ecological importance, the estuary is seldom studied, and consequently, the ecosystem's structure and function are seldom known. Following a comprehensive field investigation with vertical profiles of the selected water quality parameters (10/11/2021, 20/03/2022, and 02/07/2022), the purpose of this study is to explore the vertical stratification of DO and its association with salinity, temperature, pH, and transparency. DO concentration ranged from 5.5 to 11.8 mg/L with maximum three units (mg/L) of variation in the depth profiles and demonstrated an inconsistent stratification pattern. Relatively low DO concentration near the surface layers and high near the bottom layers were remarkably noticed in most of the sampling periods and is associated with cooler bottom waters complimented by gravitational circulation and mixing. Strong freshwater flow in the surface layer, existence of estuarine gravitational circulation, and lack of oxygen consumption in the bottom layer due to high sediment particles ameliorated the pattern of higher DO concentration in bottom and lower in the surface layer of the estuary. The DO variations are significant over the tidal cycle with higher DO values during low tides than high tides that are attributed to the higher saline water during high tides. Variations of DO are more influenced by the temperature and salinity than pH. By analyzing the DO profile in the Bouregreg estuary of Morocco, this study will provide considerable insights into ecosystem dynamics relating to DO profile. In perspective of this research, actual work of the authors focuses on 1-D/2-D modeling of DO along an estuary axis, which may give novelty in results and information for scientific community (Preliminary results: see Appendix, Supplementary material).

The authors gratefully acknowledge two anonymous reviewers for their scientific suggestions and constructive comments which led to significant improvements of the manuscript. Also, the authors would like to express their gratitude to colleagues (Dr Amine ES-Saidi and Dr Otman ELmountassir); and National Agency of Ports-Morocco, for their availability and help. Additionally, the authors specifically acknowledge Dr Chen at University of British Columbia, Canada for English improvement.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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