Relationship of salinity, temperature, pH, and transparency to dissolved oxygen in the Bouregreg estuary (Morocco): First results

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.

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).

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.

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.