Plankton Species as Bio-Indicators of Water Quality in Treated Wastewater for the Purpose of Aquaculture Production.

Abstract

Effluents from the Mudor Treatment Plant (MTP), stored in ponds at the Water Research Institute (WRI) for irrigation by nearby vegetable farmers, were assessed to determine their potential for supporting aquaculture production. Physico-chemical parameters including temperature, pH, and dissolved oxygen (DO) were monitored both in-situ and ex-situ over a 24-hour period for two consecutive months, September and October 2023. Sampling intervals for the 24-hour monitoring were set at 6 am, 9 am, 12 noon, 5 pm, 8 pm, 12 midnight, 2 am, 4 am, and 6 am for the September period, and 6 am, 9 am, 12 noon, 5 pm, 8 pm, 12 midnight, 3 am, and 6 am for October. Temperature values ranged from 25.70 ±0.10°C to 28.23 ±0.31°C in September and 25.06 ±0.26°C to 27.37 ±0.14°C in October. pH levels fluctuated between 7.75 ±0.01 to 10.63 ±0.05 in September and 8.05 ±0.29 to 9.79 ±0.32 in October. Dissolved oxygen concentrations varied from 1.21 ±0.19 mg/L to 9.89 ±2.00 mg/L in September and 0.32 ±0.21 mg/L to 6.40 ±0.92 mg/L in October. Nutrient concentrations, measured in September at 12 noon and 3 am, indicated ammonia levels between 0.50 mg/L and 2.84 mg/L (noon) and 0.56 mg/L to 2.82 mg/L (3 am), while nitrite concentrations ranged from 0.002 mg/L to 0.216 mg/L (noon) and 0.002 mg/L to 0.221 mg/L (3 am). Plankton composition, identified in November, included 16 species; eight phytoplankton species and eight zooplankton species—across the four study ponds. Phytoplankton were represented by four classes: Cyanobacteria (37.5%), Chlorophyceae (37.5%), Rotifera (12.5%), and Euglenoidea (12.5%). The zooplankton population, however, consisted primarily of insect larvae, such as chironomids, stoneflies, dragonflies, Arachnida, Dystacta spp., ostracods, and polychaetes, with a lack of true zooplankton like copepods or calanoids. Statistical analysis, including Analysis of Variance (ANOVA), was conducted to investigate variations in physicochemical parameters across the ponds. Results showed significant differences in mean temperature (p < 0.05) for September (p=0.012) and- no significant differences for the month of October (p= 0.364). For Ph a significant difference (p < 0.05) was obtained for September (p= 0.002) while no significant differences (p < 0.05) were observed in October (p= 0.905). DO showed a significant difference (p < 0.05) for September (p= 0.00043) and in contrast no significant differences (p < 0.05) obtained in October (p= 0.551). For ammonia and Nitrite, both showed no significant differences (p < 0.05) with P values; (p=0.960) and (p=0.059) respectively. Diversity indices, including species richness (Margalef), species evenness (Pielou), and the Shannon-Weiner diversity index (H'), were used to assess plankton diversity. Margalef’s species richness suggested higher species composition in pond 2, followed by pond 1, pond 4, and pond 3. Pielou's evenness and Shannon-Weiner index also indicated higher diversity in pond 3, followed by pond 2, pond 1, and pond 4. Overall, the results indicated that these ponds' physico chemical parameters and nutrient levels were not optimal for aquaculture production. Specifically, the presence of elevated nutrient levels and the predominance of insect larvae over zooplankton species suggest that the ponds may require further intervention to improve water quality. This study underscores the need for further treatment of treated wastewater from the Mudor Treatment Plant to reduce nutrient loads and qualify it for aquaculture production.