Distribution of Foraminiferal Assemblages in Contemporary Bottom Sediments of Qua-Iboe River Estuary, Southeast Nigeria

Estuaries are transitional environments that form an interface between fresh water and seawater (Nash et al., 2010; Schröder-Adams, 2006). Contemporary estuaries have been extensively studied based on their physical, biological and sedimentological characteristics (Dalrymple et al., 1992; Wang, 1992; Schröder-Adams, 2006). They are dynamic coastal ecosystems that often adapt to natural changes, such as variations in temperature, salinity, pH and other parameters, resulting from freshwater inputs and penetration of marine waters (Crossland et al., 2005; Nash et al., 2010). Estuaries with high sediment preservation potentials are important archives for geologic history (Dalrymple et al.,1992; Knudsen et al., 2012).

Foraminifera are among the most diverse shelled organisms present in brackish and marine environments (Kumar & Manivannan, 2001; Wang & Chappell, 2001). The distributional pattern of foraminifera in sediments of modern estuaries plays an integral role in the identification of estuarine sub-environments (Wang & Murray, 1983; Schröder-Adams, 2006). The ecological record they leave in sediments is important for the interpretation of present and past environmental conditions (Reymond et al., 2012; Itam et al., 2019). Understanding the present distribution pattern of foraminifera is important for the assessment of future changes within the environment due to anthropogenic influences (Dublin-Green, 1990; Mendes et al., 2004; Ukpong et al., 2015; Fajemila, et al., 2020; Ye et al., 2021).

Foraminiferal distribution in modern brackish water environments has been widely studied. Chen (1989) used quantitative analysis to study foraminiferal distribution from the surface sediments of Yamen, Pearl River estuary. The author observed a gradual seaward increase of foraminiferal density across the estuary. Hayward & Hollis (1994) investigated foraminiferal assemblages in estuaries of New Zealand and reported that salinity and tidal energy were major influencing factors on foraminiferal distribution. The authors observed that as salinity increased, estuarine foraminifera increased in diversity, decreased in abundance of agglutinated taxa and increased in abundance of calcareous taxa. Quilty & Hosie (2006) studied the distribution of modern foraminifera and influencing factors in Swan River estuary, Western Australia. These authors correlated higher numbers of foraminifera closer to the estuary mouth with more marine influence.

The distribution patterns of foraminifera in Nigerian brackish water settings have been documented. Asseez et al. (1974) distinguished two broad biofacies for the Ogun River estuary, namely the upper estuarine facies of Ammobaculites and the lower estuarine facies comprising agglutinated and calcareous taxa with a preponderance of Ammonia beccarii (Linnaeus). The authors considered salinity and rate of sedimentation as the major factors influencing the distribution of all species. Fajemila et al. (2020) recognized 42 species comprised of porcelaneous, hyaline perforate and agglutinated taxa in Lagos Lagoon sediments. The authors associated high diversity of marine taxa in the harbor areas with favorable salinity and pH conditions. Philips et al. (2020) studied factors determining benthic foraminiferal distribution in the shallow-water coastal environments of the Southwest Nigerian Sector of the Gulf of Guinea. The authors related the non-recovery of foraminiferal species in the Badagary and Yawa coastal creeks to the low saline nature of the environment and associated high diversity of species in Lagos Lagoon to favorable factors, such as salinity, pH, and substrate. Dublin-Green (1990) carried out a baseline study on the distribution pattern of foraminifera in the Bonny estuary of the Niger Delta, Nigeria. The author related the abundance of agglutinated forms in the upper and middle portions of the estuary to the low pH of sediment. Similar observations were made by Ramanathan (1981) for the Cross River estuary. Foraminiferal assemblages in modern brackish water settings are important tools in paleoenvironmental reconstruction and bio-monitoring of the environment (Alve & Nagy, 1986; Murray, 1991).

Qua-Iboe River estuary is a coastal plain estuary of the Niger Delta region. Estuaries of the Niger Delta region of Nigeria are the focus of many integrated studies in view of their economic importance, especially in industrial related activities (Dublin-Green, 1990). The estuary is economically significant for recreation, seafood trading, maritime transportation, fishing, and oil-related activities and has close proximity to an oil company effluent treatment facility. The impact of hydrocarbon pollution in the Niger Delta region has significantly contributed to the degrading health of its coastal ecosystems. Antia & Santa (2013) carried out a study on the geochemistry of the Niger Delta region. They observed higher hydrocarbon concentration in the eastern coastal swamps of the Niger Delta (e.g., Qua-Iboe River estuary, Cross River, Calabar River, Great-Kwa River) despite higher frequency incidents of oil spills in the western coastal swamp areas (e.g., Forcados, Andoni, and Nun estuaries). The enriched concentration of total hydrocarbons in mangrove sediments of estuaries such as Qua-Iboe and Cross River may be attributed, in addition to local inputs, to the eastward flowing longshore currents transporting oil spills to the eastern coastal swamps as soon as they are flushed out of the western coastal swamps (Antia & Santa, 2013). Langer et al. (2016) carried out a study on shallow-water nearshore benthic foraminiferal assemblages in Gabon, West Africa. They associated the high abundance and diversity of benthic foraminiferal taxa with coastal sites that were unimpacted by offshore drilling.

The distribution of foraminiferal assemblages in sediments of the sub-tidal environment of Qua-Iboe River estuary have been sparsely documented despite numerous commercial activities. Earlier studies on foraminiferal assemblage distribution in Qua-Iboe estuarine system have been documented by Essienumoh (1987). The author reported that salinity and tidal current intensity were major influences on foraminiferal abundance and distribution in the estuary. Harry et al. (2017) investigated estuarine oceanographic effects on benthic foraminifera along the adjoining shorelines of Qua-Iboe, Eastern Obolo, and Uta-Ewa/Opobo River estuaries at well-spaced intervals based on samples collected between August and December 2014. With increased urbanization, it is imperative to document the distribution of foraminiferal assemblages and to examine the present ecological status of the estuary channel.

This study provides information on the spatial distribution of foraminifera throughout the stretch of the estuary, a detail lacking in literature. Relationships between foraminiferal assemblages and physicochemical parameters within the estuary have been statistically analyzed. Understanding the present-day patterns of foraminiferal assemblages and environmental conditions within the estuary will enhance paleoenvironmental reconstruction of its ancient analogue. Information from this study will aid in the assessment of future patterns of foraminiferal assemblages and ecological variations within the estuary.

Qua-Iboe River estuary is influenced by marine processes due to its open boundary with the ocean. Fluvial, tidal, and wave processes are active in sediment deposition and distribution within the estuary channel (Antia et al., 2012). The volume of fresh water and sediment discharged into the estuary by fluvial current varies with seasonal changes. Rainy and dry seasons are prevalent in the study area. The rainy season generally spans April through September and is characterized by high freshwater discharge into the estuary. The dry season typically occurs around the months of October through March and is usually characterized by low fresh water influx into the estuary. Salinities as low as 0–0.5 ppt are recorded in the upper estuary. This marks the upstream limit of the channel (Dalrymple et al., 1992).

The Qua-Iboe River estuary is a typical meso-tidal estuary with a tidal range of 2–4 m (Antia et al., 2012). Flood and ebb tidal currents are active in sediment deposition and distribution within the channel. An average surface tidal current velocity of 1.2 m/s has been recorded for the estuary (Antia et al., 2012). Strong ebb-directed flow observed in Qua-Iboe River estuary has been attributed to high precipitation rates and inputs by adjoining rivers and creeks (Antia et al., 2012). The tidal energy is generally observed to dissipate towards the banks. Wave processes are minimal upstream but more evident at the mouth of the estuary where bottom water salinity increases to 32 ppt. The major types of vegetation within the study area are Nypa palms and mangrove swamps.

Sediment samples for the present study were collected in November 2018 from 55 sampling stations in Qua-Iboe River estuary. A total of 55 surface sediment samples from 0–10 m water depth (Fig. 1) was obtained with the aid of a Van Veen Grab sampler from a boat. The top 1–2 cm of the surface sediments were scraped off the grab sampler. Bottom water parameters including temperature, pH, salinity, dissolved oxygen, and water depth were measured concurrently at sediment sampling stations (Appendix 1). Water temperatures and pH were measured using a Hanna HI 98130 multi-parameter sensor probe device. Bottom water salinity was measured using a salinity refractometer, while dissolved oxygen was measured using a waterproof portable dissolved-oxygen meter (HI 198193). All sample stations were geo-referenced using a GPS instrument. Water depths were measured in-situ using an echo-sounder, and readings were used to produce a bathymetric map (Fig. 1). Sediment samples obtained from each sampling station were immediately transferred into plastic bags and refrigerated for further analysis.

Approximately 20 g of sediment sample were disaggregated with tap water and washed over a 63-μm sieve. The sand fractions (>63 μm) retained on the 63-µm mesh sieve were air-dried. The mud fractions (<63 μm), which passed through the 63-µm mesh sieve, were collected as suspension and evaporated in an oven at a temperature of 40°C. The dried mud residue (<63 μm) and sand fractions (>63 μm) were weighed using an electronic scale. The percentages of gravel, sand, and mud fractions were computed. The mean grain size of sediment was calculated based on Folk & Ward (1957). Grain size data is shown in Appendix 2.

Foraminiferal tests were picked and identified from the sand fractions retained on >63-μm (i.e., 500, 250, and 125 μm) meshes. Every individual was picked and counted. Foraminiferal individuals were counted from each sample and identified using a binocular reflective microscope. Living foraminifera were not differentiated from dead assemblages. The total foraminiferal assemblage (living + dead) were used for analysis in this study. Broken tests (more than 50% remains) where the proloculus was visible were analyzed. Identification of taxonomy at the genus and species level were based on Loeblich & Tappan (1987), eol.org, mikrotax.org, and marinespecies.org.

The number of foraminifera per 10 g of dry sediment was determined for each sample. Since the sediment mass of each sample collected varied from one station to another, we standardized all sediment mass to uniformity (10 g), for ease of comparison. This was done mathematically by determining specimen count divided by the sediment mass for each sample. The result was then multiplied by 10.

Q-mode cluster analysis was performed on foraminiferal species using Statistica v.10 (StatSoft, Inc, Tulsa, Oklahoma) to identify different groups, based on similarity, composition and abundance of foraminiferal assemblages.

Diversity in the estuary was generally low. Qua-Iboe River estuary is a major recipient of offshore hydrocarbon pollution and petroleum exploitation wastes (Antia & Santa, 2013). The low species diversity of foraminifera in Qua-Iboe River estuary are similar to those of other brackish water settings within the Niger Delta (Ramanathan, 1981; Dublin-Green, 1990; Ukpong et al., 2015). Essienumoh (1987) recognized 43 foraminiferal species comprising 41 benthics and two planktics for Qua-Iboe River estuary. A total of 10 foraminiferal species comprising nine benthics and one planktic were documented in the present study. The observed decrease in species diversity over the years may be due to the effect of pollution acting as an inhibiting factor for the proliferation of certain foraminiferal taxa.

Qua-Iboe River estuary channel is a shallow coastal plain estuary that has a depth range of 0 to 10 m (Fig. 1). The maximum depth of the channel (10 m) is located towards the upper estuary. Some parts of the channel have been artificially modified by human activities. Tidal creeks including Stubbs, Douglas, and Ukpenekan creeks discharge fresh water and sediment into the estuary. The upper estuary has a depth range of 2 to 10 m. The lower portions of the estuary have water depths ranging from 0 to 4 m. Sands are prevalent within the channel (Fig. 3). The bottom sediments of the upper reaches of the estuary consist of silt, fine sands, and medium-grained sands. The middle and lower portions of the estuary channel are characterized by fine to very fine-grained sands. Sand bars are evident around the mouth of the estuary.

The pH values of bottom water at the various sampling stations varied from slightly acidic to slightly alkaline. Bottom waters were slightly acidic to neutral (6–7) in the upper and middle estuary, becoming slightly alkaline (7.7) in the lower estuary. The pH values of bottom waters generally increased downstream (Fig. 4).

The salinity profile of bottom waters (Fig. 4) shows an increasing downstream trend. Bottom water salinity increased from 0 ppt at the upper ends of the estuary to 32 ppt at the estuary mouth.

Dissolved oxygen concentration of the estuary bottom waters showed a near uniform trend (1.5–2 mg/l) at the upper ends of the estuary with a sharp increase in the downstream direction (Fig. 4). Strong variations in dissolved oxygen levels (0.2–6.7 mg/l) were observed in the middle and lower estuary.

Some dominant or common species of the study area are discussed below. The species considered are those species that constituted >5% of the total assemblage.

Ammonia beccarii (Linnaeus, 1758). Ammonia beccarii is the second most abundant species of the study area, constituting 31.1% of the total foraminiferal assemblage. This species is typical of coastal areas (Charrieau et al., 2018). The species A. beccarii has been found to occur in shallow subtidal or intertidal estuarine environments (Murray, 1991) and can tolerate a wide range of environmental conditions. In Qua-Iboe River estuary, A. beccarii was prevalent in the lower estuary, decreasing upstream (Fig. 6). Similar observations have been previously made for Qua-Iboe River estuary (Essienumoh, 1987) and for Bonny estuary (Dublin-Green, 1990). In the study area, A. beccarii was recovered at a maximum water depth of 6 m. The predominance of A. beccarii in the lower estuary was associated with warm temperatures (28–31°C), fluctuations in dissolved oxygen levels (1–6 mg/l), brackish to marine water conditions (10–32 ppt), slightly acidic to slightly alkaline bottom waters and fine-grained substrate. Ramanathan (1981) reported an abundance of A. beccarii in the Cross River estuary with bottom water salinities of 6–33.5 ppt.

Pseudononion japonicum (Asano, 1936). Pseudononion japonicum was restricted to the mouth of Qua-Iboe River estuary (Fig. 6), constituting 10.6% the total foraminiferal assemblage. It was found to occur in shallow depths (0–4 m) of the lower estuary. The occurrence of P. japonicum in the lower reaches of the estuary was associated with warm temperatures (29–31°C), high salinities (18–32 ppt), slightly acidic to slightly alkaline waters, low levels of dissolved oxygen (2–4 mg/l) and fine to very fine-grained sands.

Cribroelphidium excavatum (Terquem, 1875). Cribroelphidium excavatum was frequently encountered within the lower reaches of the estuary (Fig. 6) at a maximum water depth of 6 m. It constituted 6% of the total foraminiferal assemblage. The distribution of C. excavatum within the lower portions of the tidal estuary was associated with warm temperatures (28–31°C), increased salinity (18–32 ppt), slightly acidic to slightly alkaline waters, dissolved oxygen levels of <6 mg/l and fine to very fine-grained substrate. The species C. excavatum is typical of subtidal/lower estuarine environments (Hayward & Hollis, 1994). The distribution of C. excavatum in the lower reaches of Bonny estuary has been reported by Dublin-Green (1990).

Biotope 1b occurs in the mid-estuarine environment, extending downstream from the distal end of Biotope 1 (Fig. 8). It has a maximum water depth of 4 m and is found in brackish waters (1–10 ppt) with slightly acidic to neutral pH (6–7). This biotope occurs in fine to very fine-grained sands and consists of foraminiferal species including A. mexicana and A. beccarii. The agglutinated species A. mexicana is the dominant species of this biotope, constituting 98% of the total foraminiferal assemblage.

Biotope 2 occurs within the subtidal/lower estuarine environment and has a maximum water depth of 4 m (Fig. 8). It is found in brackish to marine waters (10–32 ppt) with slightly acidic to slightly alkaline pH (6–8) and dissolved oxygen varying from 1 to 5.9 mg/l. The substrate is mainly fine to very fine-grained sands. Foraminiferal species commonly found in this biotope includes A. beccarii, Pseudononion japonicum, and C. excavatum. Biotope 2 is distinguished from Biotope 1b by the abundance of calcareous species.

The occurrence of planktonic species (G. ruber) in the middle and lower estuary is indicative of tidal strength. Tidal energy is important in the distribution of dead foraminifera in mesotidal and macrotidal estuaries (Wang & Murray, 1983). Wang (1992) noted that exotic specimens are transported from the open sea into the estuary by tidal currents either as bed load or in suspension within the water column. The predominance of calcareous taxa at the lower estuary may be due to marine processes which predominate the marine end of the estuary.

There is a strong relationship between salinity and the distribution of foraminiferal species in Qua-Iboe River estuary. Agglutinated forms are abundant in the middle estuary where brackish water conditions (1–18 ppt) prevail. The lower estuary, where salinity reaches up to 32 ppt, is characterized by a predominance of calcareous tests. Similar distributions have been observed in other estuaries of the Gulf of Guinea (e.g., Asseez et al., 1974; Dublin-Green, 2004; Fajemila et al., 2020). Wu & Wang (1989) and Hayward (1993) noted that as salinity increased in littoral areas, agglutinated species were replaced by calcareous ones. An increase in salinity downstream may be related to the penetration of marine waters into the estuary. Species diversity was highest at the estuary mouth where open marine conditions prevail.

Strong variations in dissolved oxygen levels were observed at different stations occupying the middle and lower reaches of the estuary (Fig. 4), and as such, the influence of dissolved oxygen on foraminiferal distribution could not be accurately determined. Variations in dissolved oxygen levels may be related to tidal mixing of estuarine waters with marine waters. High foraminiferal species diversity and abundance coincided with stations where dissolved oxygen levels were >2 mg/l (Fig. 5). The variation in pH seems to be associated with tidal fluctuations. This trend follows similar observations in other estuaries globally (e.g., Cunha & Dinis, 2002; Debenay et al., 2002; Moreno et al., 2005).

The distribution of mixed faunas including Rotaliina (calcareous) and Textulariina (agglutinated) within the estuary can be attributed to variations in pH from slightly acidic to slightly alkaline. This is in accordance with the view of Apthorpe (1980) that mixed faunas occur when the pH of the substrate is between 6.5 to 7.5. Agglutinated species are predominant in the middle reaches of the estuary, characterized by slightly acidic bottom waters. Calcareous hyaline species are abundant in the lower estuary, which is typical of neutral to slightly alkaline waters.

Temperatures were generally warm, ranging from 27–31°C. The highest temperature values (30–31°C) were obtained at the estuary mouth. Temperatures documented for the study area are suitable for foraminifera to thrive (Bradshaw, 1957). Foraminiferal species were abundant downstream of the estuary associated with increased salinity and a prevalence of fine to very fine-grained sediments (Figs. 3, 5). Wang (1992) established a correlation between foraminiferal test size and sediment grain size and noted that benthic foraminifera preferred fine-grained bottoms, as finer sediments yielded higher abundance of foraminifera. The general characteristics of foraminifera such as test composition and patterns of distribution agree with earlier studies carried out by Essienumoh (1987) in Qua-Iboe River estuary. Similar distribution patterns have been observed in other estuaries of the Niger Delta region (Ramanathan, 1981; Dublin-Green, 2004).

The distribution of foraminiferal assemblages in Qua-Iboe River estuary were related to environmental factors, such as salinity, pH, water depth, temperature, dissolved oxygen, tidal energy, and substrate. The non-recovery of foraminiferal species in the upper estuary may be attributed to freshwater conditions. Foraminiferal diversity and abundance increased as salinity increased in the seaward direction. Salinity variations seem to be related to the interaction between fluvial and marine water conditions in the estuary. Agglutinated forms strongly dominated the middle estuary where brackish water conditions prevailed. Calcareous foraminifera were mainly restricted to the lower estuary, which is associated with open marine conditions. The distribution of mixed faunas including agglutinated and calcareous taxa may be attributed to the variations in pH from slightly acidic to slightly alkaline waters. The presence of planktonic species in the middle estuary may be associated with increased tidal energy. Foraminifera were mostly recovered from the low energy banks associated with shallow depth (<4 m) and high deposition of fine-grained sediment. Water temperatures were generally conducive for foraminifera to thrive. Strong fluctuations in dissolved oxygen levels were observed at most stations within the middle and lower estuary.

The Q-mode cluster analysis based on similarities in foraminiferal assemblages and ecological parameters revealed two biotopes, representing distinct sub-environments. The agglutinated species, A. mexicana was the most abundant species in the middle estuary. Arenoparrella mexicana can tolerate varying salinities and can thrive in low oxygen conditions. The calcareous species, A. beccarii was the most dominant species of the lower estuary. Ammonia beccarii can thrive in a wide range of environmental conditions. The present result suggests a correlation between foraminiferal distribution and existing coastal environmental conditions. Salinity had the strongest influence on the distribution pattern of foraminifera in modern sediments of Qua-Iboe River estuary.

Earlier investigations on foraminiferal distribution in the estuary have been compared to present studies, and results show a decreasing trend in species diversity over the years. The observed decrease in species diversity may be attributed to the effect of pollution acting as an inhibiting factor for the proliferation of certain foraminiferal species. Scientific data generated from this study can be used as a guide in paleoenvironmental studies.

This work was supported by the Institution Based Research Grant of the Tertiary Education Trust Fund, University of Calabar, Nigeria. The authors are grateful to Late Professor Seighard Holzloehner and staff of the Institute of Oceanography, University of Calabar for providing technical support in the field. Emmanuel Iroka, Eko Uno and Emmanuel Oyo are thanked for their assistance in the laboratory. Special thanks to Late Professor John Murray for providing help with some species nomenclature and to Prof. Dr. Wolfgang Kuhnt for foraminifera imaging. The authors are grateful to two anonymous reviewers and Editor-in-Chief Marci Robinson for their very insightful comments on the manuscript. Appendices 1–4 can be found linked to the online version of this article.