The Cameroon coastal rivers that are located in the south of the Nyong river are Lokoundjé, Kienké, Lobé, and Ntem. They drain the entire Atlantic watershed in the south of the Nyong river while the eastern section is drained by tributaries rivers of the Congo river and few one by Ogoué river [15]. The watershed of Kienké is characterized by an equatorial climate with four seasons the major dry season (from November to March), the small rainy season (from April to Jun), the small dry season in July, and the large rainy season from August to October [16] [17] [18] [19] [20]. The Kienké estuary belongs to the downstream part of the Kienké watershed. It is located in the Ocean Division between 02˚934' - 02˚943' North and 09˚901' - 09˚909' East where it crosses a forest reserve. The lower course drains its waters through the city of Kribi and empties into the Atlantic Ocean through and estuary system that shelters a small port. Its watershed covers an area of 1435 km² dominated by a dense equatorial forest while the average flow is estimated at 49.2 m³/s [15]. The tidal regime is semi-diurnal with two ebb and two flood phases (of approximately equal intensities each) per day (Figure 1). The vegetation found in the Kienké estuary indicates an area of traditional crops characterized by the presence of the Ceibapentadra, Munsangasmibii, and palm trees [21]. The Kienké estuary is a shallow estuary due to its geological structure and measures approximately 1000 m whereas many anthropogenic activities are taking place; the Rapid Intervention Brigade (BIR) and the naval navy are situated on the right bank whereas the fisheries port and many hotels are located on the left bank.

The survey was carried out during the large rainy season i.e., August 2019. It started at 7 am and ended at 7 pm on the 4^th. We conducted our 13-h time series observation on a river transect (stations K9, K8 and K7), a midstream transect (stations K4, K5, and K6), a downstream transect (stations K1, K2, and K3) (Figure 2). The downstream transect was fixed in the estuary mouth whereas the midstream and the upstream transects were located at 0.5 km and 0.8 km from

the mouth. In addition to this spatial sampling, vertical profiles were installed at 0 m, 1 m, 1.5 m, 2.5 m and 3 m depth of each transect. Temperature, salinity, conductivity, pH, dissolved oxygen, and Total Dissolved Solids (TDS) were measured using a HANNA multiparameter (HI 9829). These measurements were carried out with a sampling time step of one in every hour.

The tidal effect on temporal variation of physicochemical parameters was highlighted by the hovmuller analysis. The latter is generally used to plot the temporal evolution of vertical profiles of scalar quantities of ocean constituents as a function of time. The longitudinal section of each physicochemical parameter was made to show the spatial distribution of physicochemical parameters in the estuary. Both analyses have been performed with Ocean Data View software. The stratification index η_s was calculated in each profile using [11] formula:

η s = δ S / S ′ m

with: δ S = S bott − S surf ;   S ′ m = 1 / 2 ( S bott + S surf ) .

If η_s < 0.1, estuary is well mixed.

If 0.1 < η_s < 1, estuary is partially mixed.

If η_s > 1, estuary is stratified.

The Kruskal-Wallis test was applied to the dataset in order to examine the eventual difference between the physical and the chemical variables during high and low tide.

The temporal variation of temperature, salinity, conductivity, TDS, pH, and dissolved oxygen in the Kienké river estuary are shown below. The tidal variation was made possible by observing the behaviour of the investigated parameter under ebb and flood conditions. In this study, the flood tides occur around 07:30 (first flood) and 19:00 (second flood) while the main ebb phase having an interest in this work occurs at 13:30 (Figure 2).

The average values of vertical distribution of temperature, salinity, conductivity, total dissolved solids, pH and dissolved oxygen during high tide and low tide are showed in Figures 6-11 respectively. During high tide, surface water temperature (0 - 1 m) is lower (24˚C - 25.5˚C) than bottom water (1 - 3 m) where temperature varies between 25.5˚C to 26.5˚C. A warm water mass is observed in the bottom closed the estuary mouth and it is expended around 200 m. During low tide water temperature seem homogenous for the whole estuary (Figure 6). The Kruskal-Wallis test (p > 0.05) revealed that water temperature does not vary

significantly. This result can be explained by the fact in shallow estuary, surface water temperature in shallow estuary is controlled by local condition of the atmospheric temperature [20] [27]. The vertical stratification observed during high tide could be explained by the penetration of marine water (low temperature at surface and high temperature in the bottom) into the estuary. Furthermore, the limit of this vertical stratification before 0.5 km could be explained by the limit of the sea water intrusion.

Like temperature, the vertical stratification of the estuarine salinity revealed highest values at the bottom (Figure 7). A minimum of 0.01 PSU was measured in the upper river estuary during both tides. The highest salinity was recorded close to the estuary mouth between 2 m to 3 m depth indicating the limit of high influence of marine water. The Kruskal-Wallis test (p < 0.01) revealed that the longitudinal variation of salinity varies significantly from high to low tide. Salinity increased from the surface to the bottom and the horizontal gradient showed that salinity increased upstream to downstream. The halocline (1.51 PSU) observed respectively during high tide and low tide marked the barrier between freshwater from the Kienké river and salt water from the Atlantic Ocean. These results also showed that, salt water penetrates further upstream during flood tide (0.5 km) compare to ebb tide (0.2 km). A similar trend has been noticed by [27] in the Wouri-Nkam estuary indicating the positive nature of the horizontal salinity gradient of these estuaries.

The stratification index calculated based on the salinity profile are shown in Table 1. During high tide the stratification index varied from 0.04 to 0.42 obtained respectively at the stations K1 and K9. While during low tide, the stratification index is comprised between −0.25 to 0.49 (Table 1). Except for the stations K1, K5 where the stratification parameter is less than 0.1 during high tide and for stationS K7, K8 where it was greater than 0.1, the stratification parameter was in general greather than 0.1 in the Kienké river estuary during the flood and less than 0.1 during the ebb phase. The result showed that, throughout the tidal variation, the Kienké estuary fluctuated from a partially mixed estuary during high tide to well mixed estuary during low tide. When the vertical stratification is important, the estuary behaves approximately as two layered systems, comprised of a low salinity surface layer and high salinity lower layer. When water column is completely mixed, salinity increases toward the sea but varies little with depth and the two layers can no longer be differentiated [28]. This situation commonly occurs when the dynamics of the estuary is mainly controlled by one of the two mains hydrodynamics forces (freshwater flow and tidal exchange). Thus the Kienké river estuary hydrodynamic is mainly controlled by river discharge during the ebb phase but during the flood phase estuary dynamic is govern by river and tidal forces.

Conductivity and total dissolved solids (TDS) are higher in the Kienké estuary during high tide than the low tide. For conductivity, its average ranged between 2000 - 13,000 μS/cm and 2000 - 5000 μS/cm (Figure 8) during high and low tide respectively. Total Dissolved Solids ranged between 1000 - 7000 mg/L and between 1000 - 3500 mg/L during high and low tide respectively (Figure 9). The Kruskal-Wallis test with p < 0.05 has revealed that conductivity varies significantly from high to low tide.

The vertical gradient of both variables increased from surface to bottom and the longitudinal variation, revealed a decreased from the estuary mouth to upstream riverine end. The vertical and axial trend in TDS and conductivity could be due by the salt gradient produce during the intrusion of marine water [27]. Depending on the ionic properties of water, excessive total dissolved solids can produce toxic effect on fish eggs. When total dissolved solids range above 2200 - 3600 mg/L salmonids, perch and pike all showed reduced hatching and egg survival rates. Dissolved solids are also important to aquatic life by keeping cell density balanced. In water with a very high total dissolved solids concentration, cells will shrink while in distilled or deionized water cell will swell. These changes can affect an organism’s ability to move in water column, causing it to float or sink beyond its normal range.

pH is not a conservative parameter as salinity. Many redox reactions such as biodegradation, nitrification, denitrification, sulfide oxidation, sulfate reduction, etc., involve H+ as a reactant or product [29]. Figure 10 illustrated the vertical distribution of the average pH in the Kienké estuary during high and low tide. pH concentration ranged from 7 to 7.7 during high tide while during low tide it ranged from 7 to 7.7. Despite the fact that the higher values of pH were obtained during high tide in the estuary mouth (distance lower than 0.2 km upstream) due to the seawater influence, the two sample t_test with a p_value greater than 0.05 revealed that pH variation is not significant between both tide. However, the axial pH gradient observed during flood tide disappeared during ebb phase. Despite the non-conservative nature of pH, its axial gradient of flood period highlighted a domination of a basic character (marine water) near the estuary’s mouth and a les basic or neutral property at the riverine upstream. The disappearance of the axial trend (flood phase) lead to the establishment of a vertical stratification with higher pH values at the surface. The high pH values observed in the surface layer during ebb tide could be explained from surface photosynthesis that may have led to the uptake of huge amount of CO₂ thereby reducing the hydrogen potential of surface water.

Dissolved oxygen is vital for microorganism activities and impacts numerous reactions. The dissolved oxygen content ranged between 7 mg/l to 8.25 mg/l during the high tide and between 7.7 mg/l to 8.3 mg/l during the low tide (Figure 11). The maximum DO was recorded at the surface during low tide and

the minimum of was observed at the bottom. As it recognized that DO concentration pattern can be explained by air-sea interaction and photosynthetic process, Figure 11 also revealed that tidal action has an influence on oxygen distribution indeed, during low tide the distribution of dissolved oxygen concentration was almost homogeneous but during high tide a gradient is observed with an oxycline of 7.75 mg/L. The Kruskal-Wallis test revealed that dissolved oxygen concentration variation was not significant (p > 0.05) from high tide to low tide indicating that they are no significant variations of the external factors influencing oxygen concentration such as atmospheric pressure, air temperature, wind speed, circulation and tides. However, the permanent vertical stratification with higher DO at the surface could be attributed to the domination of photosynthesis process in the surface and respiration at the bottom (oxygen consumption).