Ecological Assessment of a Tropical River Using Aquatic Insects Assemblage and Water Quality as Indicators

INTRODUCTION

Aquatic insects have constituted the most diverse and important component of the biota in the freshwater ecosystem. These organisms are important elements in the ecological dynamics of both lentic and lotic environments [1] Aquatic insects equally play a vital role in the cycle of materials as well as in the energy flow in the aquatic environment [2]. A very good number of insects are good indicators of environmental quality and this is a result of the fact that they have a wide range of sensitivity to environmental contamination [3]. For instance, members of the Ephemeroptera, Plecoptera, and Tricoptera (EPT) have been known to be the most sensitive to pollution, natural and anthropogenic influences in the environment. As such, they are considered an important component of community assemblages of aquatic insects [4]. Apart from these, the distribution, composition, diversity, high rate of reproduction, short time generation, and rapid ways of colonizing freshwater habitats by aquatic insects have made them useful as bio-indicators of the integrity of freshwater ecosystem [5]. Therefore, understanding the species composition and distribution in communities is crucial in the determination of the ecological status of water bodies [6].

The occurrence and distribution of aquatic insects are mainly regulated by factors such as Elevation, Flow velocity, Vegetation, and Physico-chemical parameters of the waterbodies [7]. Changes in these environmental variables provide useful information in the bio-monitoring and assessment protocols [4]. Therefore, the community structure of aquatic insects gives an insight into the types of ecological processes that regulate such assemblages and populations. A fairly good number of studies in which aquatic insects have been used as bio-indicators are available. Some important studies in which aquatic insects have been used as bio-indicators of water quality in Nigeria include; [8-10].  

The freshwater ecosystem is very sensitive to changes in the environment. As such, the importance of regular and intensive monitoring of water quality cannot be overemphasized [11]. The freshwater ecosystem needs to be preserved and protected as it does not only support a great diversity of life forms but it is also important to human and industries [12]. Therefore, monitoring is essential in knowing the current health status of freshwater ecosystems in an attempt to consistently ensure adequate water quality and quantity. Ara River is an important waterbody, especially to the inhabitants of Ara and its environs. The river, being the largest in Ara town provides potable water and serves other axillary functions such as provision of irrigation water and ground for fishing activities. However, in spite of the enormous importance of this waterbody to this community, no documented effort assessed the health status of the water to date. Hence, this study aims to assess the current health status of Ara River using the community structure of aquatic insects in relation to some environmental variables.

MATERIALS AND METHODS

Study area

The study was carried out on Ara River is located in Ara, Ejigbo Local Government Area, Southwestern Nigeria (Figure 1). The river is named after the town (Ara) because it is the largest river in the town. Ara covers between Latitude 07° 92.8' N to 07° 93.2' N and Longitude of 04° 31.2' E to 04° 31.7' E. Ara falls within the lowland tropical rainforest vegetation zone of Nigeria [13] characterized by emergent trees with multiple canopies and lianas most of which had since given way to secondary forest and derived savannah [14]. The area is surrounded by patchy forest lands, scattered residential buildings, and some farmlands. Some of the crops grown around Ara include; annual and perennial crops such as; cassava, cocoa, oil palm, and citrus crops. Ara River is an important source of water for domestic and agricultural activities. For this study, four sampling locations were selected as sampling points designated as SP1, SP2, SP3, and SP4 on the course of the river. The points were located close to the littoral zone of the river since aquatic insects are predominantly found around this part of the waterbodies.

Sampling procedures

Aquatic insects were sampled once monthly from January to June 2021 using a long handle D-frame net (500µm mesh). Hand-picking and direct search methods were also employed where necessary, especially around the shallow portions of the river. Sampling was usually carried out between 8 and 11 am. The collected insects were placed in a white tray for sorting and screening. The aquatic insects per sampling point were counted and preserved using 70% ethanol. Identification of the specimens was done to the possible lowest taxonomic level using standard taxonomic keys and guides such as [15] and a pictorial guide [16]. Environmental factors that influence the community assemblage of the aquatic insects at the study sites were investigated during the sampling period. Such environmental factors include pH, Air temperature (AT), water temperature (WT), water depth (WD), water velocity (WT), dissolved oxygen (DO), and conductivity (EC) [17]. The temperature was determined on-site with a Mercury-in-glass thermometer while a Hanna multi-probe meter (HANNA 9828) was used for measuring pH, dissolved oxygen, and electrical conductivity. The water current velocity of the stream was estimated by the displacement method in which a float, meter rule, and stopwatch were used over a distance of 10m [18].