Diversity of Macrobenthic Invertebrates in Relations to Some Selected Water Quality Parameters in Lagos Lagoon, Nigeria Diversity of in Relations to Some Selected Water Quality

Species composition and diversity of benthic macroinvertebrate in relation to environmental variables are significant aspects of understanding the structure and function of coastal ecosystems. In this study, temporal and spatial variations in the distribution of macrobenthos in relation with environmental parameters were explored in nine stations along the Lagos Lagoon (May to December 2018). A total of 39 benthic macroinvertebrates belonging to 4 classes and 3 phyla were identified. Macrofauna were mainly composed of deposit feeders, dominated numerically by Molluscs. Diversity index analyses indicated that Simpson index varied from 0.71 in May to 0.90 in September, Shannon Weiner diversity values in the study area ranged from 1.43 to 2.46. Dominance index had the highest of 0.29 in May which reflects the complete dominance of few species and lowest with 0.10 in July. Margalef diversity index ranged between 1.60 to 3.99 and Evenness index also varied from 0.61 to 0.84. PCA had 82.74% variability associated with principal components 1 and 2. The Physico-chemical parameters of the water were significantly correlated (p<0.05) with Equitability index. The diversity were significantly influenced by environmental factors, thus the study revealed that the Lagos Lagoon is under anthropogenic pressure from organic and inorganic forms of pollutions. Macrobenthic Invertebrates Parameters in


Introduction
In recent times, biological diversity has received increased attention in relation to environmental variables within coastal ecosystems. Bottom sediment habitats cover most of the coastal environments and as such support a diverse array of macrobenthic communities that play important roles in ecosystem processes such as recycling nutrients, detoxifying pollutants, dispersion and burial, and secondary production. Thus, the diversity and relative abundance of these organisms are commonly used in biomonitoring, to measure the ecological health status of aquatic ecosystems [1].
Diversity within natural systems is important for their resilience to environmental change [2]. Therefore, species diversity, as an index measuring the number of species within a community, the proportional abundance of individual species, or both, is important for determining the status of biological communities. Quantifying the diversity of biological systems, and understanding the patterns and dynamics of biodiversity in ecosystems is of utmost importance for monitoring, towards the management and conservation of ecosystems [3]. Diversity indices are important statistical measure used to characterize richness (the number of species) and evenness (how uniform abundant species are in a sample) of the species in the community [4] and is used as a tool for determining the health and pollution of an ecosystem [5]. Consequently, different indices of diversity were taken into account to highlight their importance with respect to benthic community. Understanding the structure of the benthic faunal communities in relation to the impacts of pollution is an important part of monitoring changes in coastal ecosystems in Nigeria.
Water quality and benthos characteristics have been investigated in coastal ecosystems around the world [6] and indicate that the health of benthic communities is related to water quality conditions infringing communities, such as Lagoons which serve important purposes such as navigational pathways, migratory and spawning ground for aquatic life forms, and other social benefits [7]. The Lagos Lagoon is a brackish water body in Lagos which spans approximately 150.56 km sq [8] and empties into the Atlantic Ocean through the Commodore Channel to the west [9].
Environmental conditions like salinity, oxygen, temperature and nutrients influence the composition, distribution, and growth of biota [10]. Macrobenthic invertebrates of the Lagos lagoon have been extensively studied. Baseline information, as to the status of macrobenthic communities was done by [11]. Variation in the diversity and species composition of macrobenthic invertebrate communities in time and space has also been commonly explored [12,13]. Additionally, some benthos studies carried out in the Lagos in recent times have attempted to use macrobenthos as bioindicators of pollution [13][14][15]. Thus, this study aims to explore and understand spatial and temporal variation in macrobenthic diversity in relation to environmental variables in Lagos Lagoon.

Study Area
Lagos lagoon is a major part of the barrier Lagoon complex extends eastwards for about 200km from the Nigerian-Benin Republic border to the western limit of the transgressive mud coast of the Nigerian coastal zone. The Lagoon is located between latitude  Table 1).

Measurement of Physico-Chemical Characteristics
Mercury in-glass thermometer was employed in determining temperature. Dissolved oxygen, Biological oxygen demand, and turbidity were determined using a Smart spectrophotometer by LaMotte. Salinity was measured using a refractometer; while alkalinity was done by titration method. Total Dissolved Solids, pH and conductivity were measured with a multi-meter probe (Oakton™).

Statistical Analyses
Diversity indices such as Margalef's index for species richness,

Shannon-Wiener and Simpson's indices for species diversity, and the
Equitability index for evenness of the benthic macroinvertebrates were estimated using PAST 3.18, while variations in physicochemical parameters between stations and sampling periods were also explored using analysis of variance (ANOVA). All the other analyses were done using the R statistical software (R Development Core Team 2008): the response variables were subjected to the Shapiro-Wilk's normality test in order to determine the appropriate statistical tests to use [15,16]. Due to the non-normal distribution of the response variables (diversity indices), as determined by the Shapiro-Wilk's normality test, Generalized Linear Models (hereafter, GLMs) were fitted using the glm function (Thomas et al.

2015)
. These GLMs were used to explore the spatial and temporal variation in benthos diversity. We chose to use only two diversity indices (Shannon-Wiener diversity and species richness) as all the diversity indices are typically correlated. The GLMs were also used to investigate relationships between physicochemical parameters and benthos diversity.

Results and Discussion
The mean variation of the Physico-chemical characteristics of the water samples in the area across the period of sampling is shown in

Species Composition, Abundance and Diversity of Benthic Macroinvertebrates in Lagos Lagoon
A total of 39 benthic macroinvertebrates belonging to 4 classes and 3 phyla were collected from nine (9) stations in the Lagos Lagoon (Table 3) and percentage composition in Figure 2. The phylum Mollusca dominated with 89.26% and the least was Arthropoda with 0.67%. This dominance of molluscs may be typical of brackish water benthos [24]. From the study, the low number of benthic macroinvertebrates encountered could be due to some ecological imbalance arising from alterations of some important factors      [26].

Principal Component Analysis
In the PCA analyses, variables associated with principal

Relationship Between Benthic Fauna and Environmental Factors
The

Multivariate Statistical Analysis
The Cluster analysis revealed three distinct groups of the diversity indices of benthic macroinvertebrate which reflect differences in habitat types and influences to environmental factors within Lagos Lagoon (Figure 5a). The first group consists of (1) Fisher alpha and  * Correlation is significant at the 0.05 level (2-tailed).
** Correlation is significant at the 0.01 level (2-tailed). Furthermore, a model with interaction between TDS and sampling date was found to be significant (p<0.05) in predicting diversity (Diversity= TDS* sampling date). Thus, our study shows that the relationship between diversity and TDS will depend on the time of the year in which it is assessed(Abstract Figure 1). Although the diversity of macrobenthos didn't vary temporally, we record a temporal change in the relationship between diversity and TDS (and conductivity). Sampling date was found to have a synergistic effect on or an interaction with (Côte et al., 2016) the relationship between diversity indices and TDS. This suggests an indirect/ complex relationship between diversity and time; such that diversity may be predicted by temporal changes in these physicochemical parameters, in response to seasonal changes in salinity [14]. Our results suggest that DO and TDS (and conductivity), as water-chemistry parameters, are important for predicting the diversity of macrobenthic organisms in a spatially homogenous aquatic ecosystem such as the Lagos Lagoon.

Conclusion
This study provides insights on the impacts of environmental variables on the diversity of macrobenthic invertebrate communities of the Lagos Lagoon. The species composition and diversity were significantly influenced by environmental factors, thus the study revealed that the Lagos Lagoon is under anthropogenic pressure from organic and inorganic forms of pollutions.