PHYSICO-CHEMICAL PARAMETERS, PLANKTON AND MACROINVERTEBRATES IN CALABAR RIVER AT OKOMITA, CROSS RIVER STATE, NIGERIA

Abstract

Calabar River is an important waterbody in South-south Nigeria that supports a thriving fishery in the surrounding communities. Middle Calabar River, at Okomita area experiences intensive sand mining, effluents and solid wastes input from industries, farmlands, markets, slaughter houses, dumpsites and human settlements. Pollutants from these sources could adversely affect water quality and resident biota. Studies on physicochemical parameters, plankton and macro-invertebrates have been carried out in Calabar area, downstream of Calabar River but not in Okomita. This study was carried out to investigate the physico-chemical parameters of surface water, diversity, abundance and distribution of plankton and macro-invertebrates in Calabar River at Okomita to ascertain its suitability for aquatic life and domestic uses. Surface water, plankton and macro-invertebrates sampling were carried out monthly from September, 2014 to August, 2016 at six purposively selected sampling stations. Physicochemical parameters including temperature, pH, Dissolved Oxygen (DO), hardness, conductivity, turbidity and metals were measured following standard methods. Plankton samples were collected with plankton net (55 µm mesh size). Macro-invertebrate samples were collected by kick sampling and with van-Veen grab (0.6 m2) and sorted with sieve (0.5 mm mesh size). The biota were identified using standard identification guides. Descriptive statistics, Student’s t-test, PCA, Shannon-Wiener’s species diversity and Evenness and ANOVA were used to analyse the data at α0.05. Surface water temperature (25.98±0.11ᴼC) and pH (7.84±0.06) were within NESREA and WHO recommended limits for aquatic life and drinking; iron (0.79±0.05 mg/L) and lead (1.12±0.03 mg/L) were higher, while DO (4.72±0.07 mg/L) and conductivity (22.11±0.77 µS/cm) were lower than the recommended limits. Water temperature, pH and turbidity were significantly different within stations and higher in the wet season. Bacillariophyceae (70.5%) dominated the phytoplankton population, while Dinophyceae (1.9%) was least. Rotifers (33.0%) dominated the zooplankton, while protozoans (2.6%) were least abundant. Insects (87.8%) dominated macro-invertebrates assemblage, while bivalves (1.4%) were least abundant. Significantly higher abundance of plankton was recorded in the wet season (phytoplankton, 67.5%; zooplankton, 52.7%), while higher abundance of macroinvertebrates was in the dry season (69.3%). Pollution-indicators were phytoplankton: Oscillatoria tenuis (2.4%), Surirella oblonga (2.4%) and Melosira granulata (2.2%); zooplankton: Philodina species (6.9%), Brachionus forticula (6.5%) and Lecane lunaris (5.6%) and macro-invertebrates: Enithares species (34.2%), Mesovelia furcata (8.9%) and Gerris species (7.2%). Species diversity in all the stations: 0.4–2.89 (phytoplankton), 1.31– 1.75 (zooplankton) and 0–2.50 (macro-invertebrates) indicate that the river was moderately polluted. Principal Components 1-4 accounted for 50.1% variations in physico-chemical parameters and biota abundance, and indicated that seasons (wet: -0.80 and dry: 0.80) significantly modulated physico-chemical parameters and biota abundance. The PCA also revealed that hardness (0.66); DO (0.67) and turbidity (-0.69) were principal determinants of plankton (blue-green algae, 0.49; diatoms, -0.59; rotifers, -0.50 and cladocerans, -0.60) and macro-invertebrates (insects, 0.65 and bivalves, 0.65) abundance. Abundance of pollution indicator biota, low diversity values and deviations of some physico-chemical parameters from recommended levels suggest that Calabar River at Okomita is under pollution stress and not suitable for aquatic life and domestic uses.