The Bons Sinais Estuary is located in Central Mozambique, to the north of Zambezi Delta (17°54'S, 36°49'E; Fig. 1) adjacent to Sofala Bank, is the largest and the most productive of the eastern African coast. An updated and more complete description of the Bons Sinais Estuary is given by Hoguane et al. (2020). It’s a long (28 km) and shallow (10 m) estuary, with an extensive floodplain. It has a positive salinity gradient and is mostly well to partially mixed. The tides are semidiurnal, with amplitudes varying from 3.5 to 4.0 m, during the spring tide; therefore, it’s classified as meso to macrotidal (Mazzilli 2015).

Figure 1. 

Study area – Bons Sinais Estuary.

According to Köppen’s climatic classification (Köppen 1900), Mozambique’s climate is classified as tropical with two seasons: summer or rainy season and winter or dry season (Kottek et al. 2006). The average monthly wind speeds vary between 3 and 5 m s^-1, with higher values, southeast winds, observed during the summer, between September and January (Hoguane 1999; Rodrigues et al. 2000; Mazzilli 2015). The average annual precipitation is about 1,140 mm, of which approximately 80% falls during the wet season. Average annual evaporation is estimated at 1,650 mm; therefore, evaporation exceeds precipitation by about 500 mm per year.

The Bons Sinais Estuary is relatively shallow, funnel shape, highly hydrodynamic, with muddy strata and high mobility of sediments, being responsible for building and destroying continuously islands and sandy banks in the mouth. The channel depth varies from <2 to > 20 m. The greater depths are found near the harbour and its surroundings. The estuary is sinuous, braided with four large islands (Mazzilli 2015).

Mazzilli (2015) estimated, based on historical data and modeling, that the estuary receives an average of 500–840 m³s^-1 of freshwater from the tributaries per year. Bons Sinais Estuary used to be connected to Zambezi River in the past. Nowadays the Estuary is independent of the Zambezi, and the flow it receives depends on the precipitation in its hydrographic basin.

There were collected 15 bottom sediments samples in 15 stations (Table 1) along the estuary using a Van veen sampler. The sampling took place during the low tide of spring tides in October 2022. The samples were collected and stored in plastic bags. Each point was georeferenced with a GPS.

The samples were further analysed in the laboratory of civil engineering in Quelimane by conventional sieve and pipette methods. Data processing included the computation of statistical parameters (mean, sorting, skewness and kurtosis) and the plotting of Shepard and Pejrup diagrams in the software SYSGRAN 3.0 to infer about the facies distributions and sediment dynamics along the estuary.

Fig. 2, shows the relative abundance of the sediment classes in the Bons Sinais Estuary along the 15 sampling points. As seen in the figure, there are three textural classes in the Estuary, namely, Sand, Silt and Clay. Sand is the dominant class (51%), followed by Silt (25%) and clay (24%). Even though Sand is the dominant class, the abundance of thinner sediments is noteworthy.

Figure 2. 

Textural distribution of Bons Sinais Estuary.

The values of the mean grain size (Fig. 3), varied from values as low as 2.50 ϕ (fine sand) – in station B3 – to values as high as 7.30 ϕ (very fine silt) – in station C2. Generally, the sediments of the sampled area are coarse, as in most stations (7 stations) the sediments are classified as very fine sand.

The values of sediment sorting (Fig. 4) varied from 0.70 to 1.90 ϕ, in stations D2 and E1, respectively. Generally, the sediment sorting is classified as poorly sorted, since most of the stations (12), showed this classification. The remaining stations were classified as moderately sorted.

The values of skewness (Fig. 5), varied from -0.30 ϕ, in station C2 to 0.60 ϕ, in stations B3 and E2. The sediments were mostly classified as very fine skewed, as in most stations (9) were classified like this; three stations were classified as fine skewed, namely, E1, D2, and C3, the stations A1, C2, and D3 were classified as coarse skewed.

The sediment kurtosis (Fig. 6) varied from 0.60 ϕ, in stations A1, D2, and E1 to 3.70 ϕ, in station B2. Stations B1, B2, B3, and E2 were classified as extremely leptokurtic, stations A3, C1, C3, and D1, were classified as very leptokurtic, stations C2 and E3 were classified as mesokurtic, the stations A2 and D3 were classified as platkurtic and the stations A1, D2 and E1 were classified as very platkurtic.

Figure 3. 

Mean, showing the trends of all the samples.

Figure 4. 

Sediment sorting, showing the trends of all the samples.

Figure 5. 

Sediment skewness, showing the trends of all the samples.

Figure 6. 

Sediment kurtosis, showing the trends of all the samples.

The hydrodynamic of the Bons Sinais Estuary, as shown in the Pejrup Diagram (Fig. 7), varied from moderate to high. In the stations, A1, C2, and D3 are located in the band II-D, with a percentage of clay between 50 to 80%, and a percentage of sand between 0 to 10%. The Stations D2 and E1 were located in the band II-C with percentages of clay between 50 to 80% and percentages of sand between 10 to 50%. The stations A2, B2, B3, C1, E1, and E2, were located in the band II-B, with a percentage of clay between 50 to 80% and a percentage of sand between 50 to 90%. The stations A2 and E3 were located in band III-C, with a percentage of clay from 20 to 50% and a percentage of sand between 10 to 50%. The remaining stations were located in band III-B, with a percentage of clay from 20 to 50% and sand from 50 to 90%.

Figure 7. 

Sedimentary dynamics inferred by the Pejrup’s diagram.

The textural composition, shown in the Shepard diagram (Fig. 8), varied considerably. The Stations A1, C2, and D3 are located in position 3 at Shepard’s diagram, classified as Silty clay. The stations D2 and E1 are located in position 4, which is classified as Sandy silty clay. Station E3 is located in position 8 and is classified as Clayey silt. The stations B1, B2, B3, C1, C3, and E2 were located in position 9 and are classified as Sandy. The stations D2 and E1 are located in the position of Silty sand.

Figure 8. 

Bons Sinais Estuary textural composition.

The sediments of Bons Sinais Estuary in the sampled area are in majority sandy (about 50%) and the muddy fraction makes up the remaining 50% (Fig. 2). This result is not much different by the results found by Corrêa (2005) in the Estuary of Marajó Bay where the bottom sediments were composed of medium to very thin sand that graded to coarse to thin silt in direction of the continental shelf and Oliveira (2015) that found in the study of Lagoa the Patos that the bottom sediments are composed by thin to very thin sand with presence of muddy material (silt). However, in Bons Sinais Estuary, different of these other estuaries, the presence of clay is noteworthy (24%). Corrêa (2005) and Oliveira (2015) associated the presence of sandy sediments with places with relatively high hydrodynamics and shallow depth. Therefore, the amount of sand in the Bons Sinais Estuary. is probably related to the estuary’s high hydrodynamics as stated by many authors(e.g. Davis Jr. and Fitzgerald 2004; Corrêa 2005; Oliveira 2015). This aspect is reinforced by Hoguane et al. (2020) when they state that Bons Sinais Estuary has the highest tide amplitude (4 m) of the Western Indian Ocean, being therefore classified as meso to macro tide. According to Pinet (2009), most of the sediments in well-mixed estuaries are marine sediments. Therefore, probably most sediments found in this estuary are marine in origin, as this is well mixed, as argued by Hoguane et al. (2020). The concentrations of Silt and Clay in this estuary may be due to the local morphology or flocculation, as these sediments tend to be deposited at high depths or flocculate in the meeting of fresh and saltwater and due to the changes of pH, as Corrêa (2005) argues.

Although sandy sediments are dominant, muddy sediments concentrations are considerable, therefore the estuary may be considered susceptible to pollutants accumulation, as according to many authors (e.g. Karickhoff et al. 1979; Ellis and Revitt 1982; Horowitz and Elrick 1987; Singh et al. 1999; Krein and Schorer 2000; Ujević et al. 2000; Sutherland 2003; Ҫevik et al. 2008; Zhang et al. 2009; Zhao et al. 2010; Lima et al. 2019) pollutants tend to accumulate in thinner rather than coarser sediments.

In most stations the sediments were coarse (Fig. 3), with values that varied from 2.50 to 7.30 ϕ. These results don’t differ so much from those found by Oliveira (2015) that found sediment mean varying from thin sand to very thin silt. Haddout et al. (2022), in turn, found mean sediment values ranging from 3.07 to 9.80 ϕ and related his findings to the energy of the environment. Jesus and Andrade (2013) argue that environmental energy is closely related to the mean grain size, with coarser sediments revealing high energy conditions and thinner sediments revealing the opposite.

The sediments were generally poorly sorted (Fig. 4). Oliveira (2015) found, in his study that most of the samples collected varied from poorly to very poorly sorted and Haddout et al. (2022) found sorting varying form moderately poorly sorted to moderately well sorted. The bottom sediment sorting is related to the degree of reworking by waves and currents. Jesus and Andrade (2013) claim that with increased transport and hydrodynamics, the sediments tend to be sorted, on the contrary, occur a mixing of different sediment sizes, being therefore poorly sorted. According to Lima et al. (2019) sediments deposited near the source regions tend to be poorly sorted. Corrêa (2005) says that poor sorting is due to the existence of many depositional processes.

Generally, the sediments in the area of study were fine to very fine skewed (Fig. 5). Haddout et al. (2022) found, in their study, that the values of skewness have varied from coarse skewed to very fine skewed, and stated that the finer the skewness values, the thinner the sediments found in the environment. Therefore, even though the percentage of coarse sediments is high (Fig. 2) in Bons Sinais Estuary, skewness values reveal a relative abundance of thin sediments as many authors argue higher values of skewness are related to thin sediments (e.g. Jesus et al. 2014; Haddout et al. 2022). Hereupon, based on the values of skewness we can affirm that Bons Sinais Estuary is susceptible to pollutant retention.

The Kurtosis has varied from very platkurtic to extremely leptokurtic (Fig. 6). According to Jesus et al. (2014), variations in Kurtosis may be related to the degree of polymodality of the sediments. Thus, when there is only one source of sediments, the curve of sediments distribution tends to be unimodal, i.e., with kurtosis values closer to 1. When the curve of sediments has a second mode less expressive, the value of kurtosis will be higher than 1 and the curve will become more strongly leptokurtic. When there are different sources, each source will have a population that will represent a mode in the distribution curve. Thus, the value of kurtosis will decrease to values lower than 1 and the curve will tend to be bimodal or polymodal, i.e., platkurtic. It means that, although this estuary has both continental and marine sediments, one of these sources is dominant, which is probably the marine, due to its high tide amplitude as Hoguane et al. (2020) say. Haddout et al. (2022) associated mesokurtic to leptokurtic curves to the dominance of thin sediments. Jesus and Andrade (2013) state that very high and very low values of kurtosis mean that sediment may have been transported from its origin and deposited without losing its original characteristics.

The implications that thinner sediments have for the pollutions dynamics are the following: i) increasing pollutant retention, ii) reducing the mobility of pollutants, iii) interactions of the polluted sediments and the water column and iv) pollutants may affect living organisms through the trophic chain.

The findings of this study may contribute for understanding the Bons Sinais Estuarine Ecosystem since many estuarine processes; species distribution, pollutant transport and nutrient cycle are intrinsically linked to bottom sediment distributions influence.

From these findings, we propose to prevent dumping of pollutants direct in the estuary, and that pollution treatment plan be established in the city of Quelimane.

The hydrodynamics of the sampled region in Bons Sinais Estuary varied from moderate to high (Fig. 7) and most of the points are located in position 9 of the Shepard diagram (Fig. 8). The hydrodynamic regime showed in the Pejrup diagram, may be due to: i) the tide regime of about 4 m of amplitude, and ii) the shallowness of the estuary, with a mean depth of 10 m as mentioned by Hoguane et al. (2020). Corrêa (2005) in its turn associated the deposition of sandy sediments to shallow and high-energy locations in the Baia de Marajó Estuary. This observation reinforces our thesis that the Bons Sinais hydrodynamics and shallowness deeply control the sedimentation and consequently its susceptibility to pollutant accumulation. We can also assert that the factors above mentioned, also control the type of sediments found in the estuary as illustrated in the Shepard diagram (Fig. 8).

A feature that can be noticed through the detailed analysis of Fig. 7 is that the higher hydrodynamics, the lower the concentration of finer sediments. Therefore, we may affirm that the hydrodynamics plays a significant role in pollutants retention that is the higher the hydrodynamics, the lower the vulnerability to pollutants accumulation.

The authors have declared that no competing interests exist.

No ethical statement was reported.

No funding was reported.

Conceptualization: HAM. Data curation: JM, NLC, CAMAMH. Formal analysis: JM. Investigation: NLC, CAMAMH, HAM, JM. Methodology: JM, CAMAMH, NLC. Supervision: HAM, CAMAMH. Writing - original draft: HAM, JM, CAMAMH.

All of the data that support the findings of this study are available in the main text.