Background

Littoral areas of the tropical oceanic and estuarine system contain a variety of unique ecosystems, and the prolific mangrove is one of them. Mangrove ecosystem comprises of vegetation which develop within the shore zone in humid and subtropical areas, and have distinct adaptations to live in this environment (Nisbet, 2007).

Mangroves are halophytes and have evolved mechanisms for salt resistance. They also provide important foraging grounds and habitats for fauna and other flora both on land and water (Kaizer, 2005). They play this role because they occur in the interface between the land and the sea (Numbere and Camilo, 2016). Mangroves have adventitious root system that grows above and outside the soil. These roots support other organisms such as oysters, algae and sponges which inhabit the surface (Spalding et al., 2010). 

The mangroves environment is a melting pot for organisms, which has improved the diversity of terrestrial, brackish and marine organisms. Birds rely on mangroves as wintering and settling sites along their intercontinental traveling routes. The mangroves serve as defenders of the coasts by preventing flooding and erosion from eroding the river banks. They are also said to save lives of coastal inhabitants during tsunamis. Mangroves are well-known to deliver diverse ecological services, which include; a breeding ground for fishes and crustaceans (Polidoro et al., 2010).

Forest conversion involves the removal of natural forests to meet the needs of mining of minerals, agriculture and pastureland for cattle. Deforestation is amongst the evil quartet of pollution, habitat loss, and invasive species that deplete mangrove forest population globally (Valiela et al., 2001). It is a significant force driving forest disintegration and loss with relative effects on biodiversity that often involves large adjacent areas to maintain their function (Long and Giri, 2011). The development of tree crop plantations (oil palm, Latex and cocoa) and the mining of gold are on the increase in the swamp forest of many West African countries owed to the overall costs of Palm oil, cocoa and gold. 

Mangroves are threatened globally by pollution, tsunami, aquaculture, deforestation, hydrocarbon pollution and invasive species (Numbere and Camilo, 2016; Numbere, 2018a). In Africa, mangroves are not only serving as refuges to flora and fauna but also function as homes to non-invasive species owed to poverty and over population. The greatest threat to the Niger-Delta mangrove is caused by crude oil exploration (Numbere and Camilo, 2016; Numbere, 2018b). 

The mangrove ecosystem in the Niger-Delta has remained under pressure for about a century now, since the intentional introduction of nypa palm (Numbere, 2018). The impact of Nypa palm has been evident on the inherent mangrove macrophytes (e.g. Rhizophora spp.). The influence of Nypa palm on plankton has been speculated, with the assertion that it has harmfully influenced populations of these organisms in its ranges of existence (Udoidiong and Ekwu, 2011).

The ability of Nypa palm to colonize areas outside its existing natural range had been reported from Trinidad (Bacon, 2001), Panama (Duke, 1991) and West Africa (Zeven, 1977) and Nigeria (Numbere 2018). Recent environmental impact assessments carried out for oil industries in the Niger delta observed that Nypa palms have invaded the mangrove areas of the Niger Delta especially around the Bonny and Imo Rivers and is causing long-term ecological damage (SGS Environment, 1995; Numbere, 2018a). It has been observed that once Nypa palm invade mangrove forest, the mangroves are out competed and the fishes that breed in them disappear (Aburto-Oropeza, 2009). It is thus hypothesized that dense Nypa palm colonization is affecting the breeding of fish in the Niger Delta thus contributing to the drop-in fish populations throughout the area (Sunderland and Morakinyo, 2002; Isebor et al., 2003).

The effect of invasive Nypa palm is easily seen on the native mangrove macrophytes species as soon as it is established and spread along sheltered creeks (Smith et al., 1999). It is possible that thick Nypa establishment disturbs fish breeding in the Niger Delta thereby resulting to the subsequent decline of fish fauna throughout the area (Sunderland and Morakinyo, 2002; Choosak et al., 2015). However, the effect of Nypa palm on plankton are yet unknown. Therefore, our study investigated the effect of Nypa palm and mangrove forest on the plankton community in Andoni River, Nigeria. The objective of the study includes: (1) To identify phytoplankton and zoo plankton species in different forest types i.e. mangrove forest, nypa palm forest, mixed forest and control, (2) To determine the species diversity of plankton in different forest types i.e. mangrove forest, nypa palm forest, mixed forest and control, (3) To determine the impact of mangrove and nypa palm on plankton community. 

1 Results and Discussion 

1.1 Physico-chemistry of study environment (Sites 1-4)

The physicochemical conditions and sensitivity to environmental changes of estuarine environments are very vital in determining the composition of every aquatic flora and fauna (Arimoro et al., 2008; Agbaire and Basaran, 2009). The pH result of this study ranged between 6.28 and 7.39, which is in line with other researchers (Hart, 1999; Woke and Wokoma, 2007; Komi and Sikoki, 2013). It also falls within the accepted limit of the Federal Ministry of Environment (FMENV). According to Woke and Wokoma (2007), pH is of ecological importance and that it has much to do with the physiology of aquatic lives. In support of this argument, the analysis of Variance (ANOVA) of the various stations revealed that there was no significant difference in pH across all stations (P > 0.05).

The temperature ranges between 25─33˚C across the various stations and have been described by previous studies as normal with the characteristic geography of the Niger Delta (Woke and Wokoma, 2007; Komi and Sikoki, 2013; Ansa et al., 2007). 

Salinity varied between seasons (P < 0.05), and ranges from 9.7-13.3 ppt. Mean salinity was higher in the dry season than in the wet season. This can be attributed to the diluting effect of river and rainfall water. Also, higher salinity in dry season is due to the concentrating effect of higher evaporation (Abowei, 2010). The mean salinity recorded differed from that of Komi and Sikoki, (2013) who reported higher values of 11.6 ± 0.51ppt - 22.8±0.37 ppt. This can be due to the flora cover around the sampling stations, time of sampling or incidence of rainfall which might have helped to attenuate the level of salinity.

The mean dissolved oxygen (DO) was similar across the stations. The mean dissolved oxygen range in the study area is between 3.2-8.0 mg/L, and in line with previous studies (Abowei, 2010; Komi and Sikoki, 2013). However, Francis et al. (2007) reported higher values of 5.0-12.30 mg/L in Andoni River. The solubility of oxygen is influenced basically by water temperature (Woke and Wokoma, 2007). This means at high temperature; oxygen level reduces while at low temperature oxygen level increases. The different stations in our study area had similar mean dissolved oxygen across the months, which may be caused by similar temperature range across the stations.

Total dissolved solids (Table 1) showed no statistically significant difference (p > 0.05).  Similarly, conductivity did not show significant difference between the stations (p > 0.05). The highest conductivity value (23.0 µS/cm) was recorded at station 2 while the lowest value (16.38 µS/cm) was recorded at station 1. The same is applicable for other parameters.

Table 1 Physico-Chemical Parameters of stations 1-4 in Asarama community in Andoni Local Government Area of River State, Nigeria indicates the range and the mean (i.e. Station 1= Control, Station 2= Nypa palm, Station 3= Rhizophora, Station 4= mixed)

1.2. Plankton abundance and population

The plankton assemblage of an aquatic environment is essential to its regular functioning, and sustenance of the food chain. While they institute the preliminary point of energy transfer, they are very sensitive to externally imposed changes in the environment (Eletta et al., 2005).

A total of 5529 plankton individuals were collected across the various stations. The break down indicates that a total of 755 zooplankton in seven classes and eleven genera were collected. Copepoda was the most abundant (577) 75.1% followed by Protista (130) 17.15% whereas Annelida had the least abundance with only one individual (0.13%) (Figure 1; Table 2). Amongst the copepodes Copepode nauplii dominated the four stations followed by Cyclopoida species. The Protists were represented by two genera, Parafavela and Tintinopsis with the former as the more dominant species. The Annelids were the least dominant taxon and was only represented by Polychaete larva.

Figure 1 Three dominant zooplankton species found at different sites in along Andoni River, Niger Delta, Nigeria. The bar graph shows that Copepods clearly outnumber the Protista and Annelid population. The number of organisms on y-axis is a multiplication of 100. Vertical lines show ± 1standard error of the mean

Table 2 Checklist of Zooplankton collected from Andoni River, Nigeria

Furthermore, a total of 4774 phytoplankton in six taxa, 29 species were collected across the stations. The Bacilliarophyceae (Diatoms) dominated with 4175 individuals, 18 species (87.45%). The Dinophycaea (Dinoflagellate) were next with 340 individuals, 4 species (7.14%). Chlorophyceae was the least abundant with 21 individuals at 0.44% (Figure 2; Table 3). The diatoms were dominated by Pleurosigma sp (1582) and Coscinodiscus sp. (1067), which were high in abundance across the stations all through the months of sampling. Dinoflagellates were dominated by Ceratium lineatum (154) and C. fuscus (181).

Figure 2 Three dominant phytoplankton species found at different sites in along Andoni River, Niger Delta, Nigeria. The bar graph shows that Bacilliarophyta clearly outnumber the Chlorophyceae and Dinophycaea population. The number of organisms on y-axis is a multiplication of 100. Vertical lines show ± 1standard error of the mean

Table 3 Checklist of Phytoplankton collected from Andoni River, Nigeria

All the plankton species encountered are identified to be frequently occurring in tropical riverine areas. The occurrence of an organism or species group in a particular environment has been explained by the ability of the species to adapt to the various ecological influences on the organism (Ndome et al., 2011). Emmanuel et al. (2008) reported similar dominance of diatoms in Calabar River, Nigeria. Uttah et al. (2008) reported that the predominance of diatoms (Bacillariophyceae) as a common feature of open waters indicating that the river exhibited some level of homogeneity in relative abundance of species of phytoplankton.

1.3 Diversity indices of species 

Based on the diversity index value derived in this study, the diversity of plankton in the four stations does not show any significant difference (Table 4). Station 1 (Control) had the least species richness (2.67) while stations 2, 3 and 4 were similar having the same species richness (3.48). Also, station 1 (Control) had the least abundance (1150 individuals or 20.8%). Station 4 (Mixed) was highest (1663 individuals or 30.08%). Stations 2 (Nypa palm) had 24% with 1308 individuals, while station 3 (Mangrove) had 25% with 1408. A multivariate test comparing the various stations and abundance showed no significant difference at P > 0.05. Thus, diversity was even across the stations. The highest evenness among species (0.099) was at station 4 (mixed) followed by station 3 (Rhizophora) (0.095) while the least evenness (0.089) was at station 1 (control).

Table 4 Stations Species Diversity for Plankton Note: H† = Shannon Weiner index of Diversity (1963); M‡ = Margalef’s Index (1967); J^ = Pielou’s Index of evenness (1969)

A comparison of abundance and number of taxa showed a significant difference (P < 0.05), which means the diversity of the species are not the same. They have significantly different abundance. The result indicates that there is uniformity across the four stations. This outcome disputes the earlier belief that the presence of nypa palm reduces species diversity and thus reduces its function as a spawning site for fishes and other aquatic organisms in mangrove forests. Hence, almost all the species of plankton encountered could be found at all the stations during the study (Neira et al., 2006).

Based on the results from the various stations, plankton was found to be more abundant in the mixed station (Station 4) and least abundant in the control. The presence of Nypa fruticans did not show any differences in physico-chemical parameters, this could be to the large size of the river and tidal flow. Again, its presence did not reveal any difference in the diversity and abundance of zoo and phyto-plankton in both native mangrove (Rhizophora) and invasive Nypa palm dominated areas as envisaged.

2 Materials and Methods

2.1 Study Area

The study area, is a section of the Andoni River (Figure 3) with a coordinate of 4°46’13.74 N and 7°28’ 6.42 E in Andoni Local Government Area of Rivers State, Nigeria. It serves as a major fish nursery in the Niger Delta region of Nigeria (Francis et al., 2007; Komi and Sikoki, 2013) which is attributed to the abundance of mangroves within the brackish water ecosystem. The Andoni locality has a climate which consists of a short dry season from November to March and a long-wet season which stretches from April to October. The area has six hours cycle of ebb and neap tides. The Andoni River is brackish water with an annual salinity range of 5─ 22% (Francis et al., 2007; Komi and Sikoki, 2013).

Figure 3 Map of Rivers State indicating Andoni Local Government area and the four sampling stations in the Niger Delta, Nigeria

Four sampling stations were selected along the river course and were located at Asarama, Ngo and Okrile communities.

2.2 Delineation of study sites

Station 1 is the open water, and lies between coordinates 7°28’.046 E and 04° 30’.838 N with an elevation of 14m. It is about 4 km from the Asarama/Ekruta Bridge. Station 2 lies along 04° 30’.938 N and 7°27’.377 E and an elevation of 8 m above sea level. This station is characterized by the dominance of Nypa fruticans. The station is located in Asarama about 3 km from the Andoni Bridge. Station 3 comprises of Ngo/Asarama Mangrove Island which lies between 7°27’.262 E and 4° 30’.787 N with an elevation of 7 m above sea level. This station is characterized by a high dominance of Mangroves (Rhizophora spp. and Avicenia spp.). Lastly, station 4 lies between 4°30’.997 N and 7° 28’.171 E and has an elevation of 8m. The station comprises of a mixture of mangroves (Rhizophora spp. and Avicenia spp.) and Nypa fruticans. Experimental design of sample location and collection pattern is illustrated in Figure 4.

Figure 4 Experimental design of sampling along Andoni River Niger Delta, Nigeria. S1 refers to open river, S2 refers to Nypa pal forest, S3 refers to Mangrove forest (i.e. Rhizophora and Avicennia) and S4 refers to mixed forest (Mangrove forest + Nypa palm foresst)

2.3 Plankton Sampling

Plankton samples were collected using the plankton nets (Verlencer and Desai, 2004). Tows were undertaken using 20 µm mesh plankton net, 1metre long and 30cm in diameter with a 250ml collecting jar at the bottom. Tow duration was kept to 5 minutes. A minimum of three tows were taken at each site. Once a tow was completed, the net was carefully backwashed with sea water so that captured organisms were caught in the collecting jar. The contents of the jar were poured into clean sample bottles and fixed with a solution of 4% formalin. Bottles were labeled clearly with date, site name, sample number and fixative. Samples were transported to the laboratory for identification and counting. Identification was made to the species and genus level.

2.4 Physicochemical Parameters

The temperature of the various stations was taken using mercury in glass thermometer. The probe was lowered into the water and temperature was taken in-situ. Water samples were collected and transported to the laboratory in a sample bottle. Samples were stored at 4°C before analysis. The physicochemical parameters (i.e. pH, conductivity, salinity and total dissolved solids (TDS) of the water were measured using Sper Scientific Benchtop meter (860033 Model).

2.5 Statistical analysis

Repeated measures ANOVA was done to determine the significant difference in species number and diversity at the different stations in different months. All analyses were done in R environment (R Core Team, 2014).The means and standard deviations of triplicate results were also calculated. Bar graphs were plotted to illustrate the differences.

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