Assessment of the Biotic Integrity and Water Quality of Lake Ziway Using Benthic Macroinvertebrate and Diatom Based Multimetric Index
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Date
2019-05-04
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Addis Ababa University
Abstract
Human-induced stressors (urbanization, agriculture, and industrial effluent) are common around Lake Ziway. As a result, the lake is threatened and ecological functions of the lake are in question, and this requires urgent monitoring. Therefore, this study was conducted to assess the ecological quality of Lake Ziway using a composite diatom and macroinvertebrate multimetric index. Physicochemical and biological data were collected from 9 sites in the littoral zone between September 2015 and April 2016 in 4 rounds. Candidate references were proposed through a priori selection criteria. Physicochemical parameters were different among sites, except for pH, soluble reactive phosphorus (SRP), and NH4+. pH showed no significant change among sites as well during the last two decades. DO, EC, Total Phosphorus (TP), and NO2, showed variation among the sites as the mean values ranged from 3.51±0.27 to 9.33±0.30 mg/L, 405±0.5 to 592±75.6 μS/cm, 0.14±0.02 to 0.51±0.45 mg/L, and 0.03±0.02 to 0.22±0.21 mg/L, respectively (p<0.01). Lake Habitat Quality Assessment score (LHQAs) displayed significant difference across site categories (p=0.003). References displayed highest average LHQAs values, while stressed sites exhibited least values. Thirty-four invertebrate families which belonged to 12 orders and 39 diatom species which belonged to 24 genera were identified. Hemipterans contributed (25.77%) abundance followed by Snails (18.57%) and Bivalves
(17.29%). The diatom communities were dominated by Achnanthidium sp., Ulnaria ulna, and Encyonopsis microcephala with percent abundance of 12.5, 11.7, and 10.3, respectively. There was a significant variation in number of diatom species among sites (p=0.003). Diatom species in references (37) with 94% was higher than test sites (31) confined 79%. Hydropsychidae, Philopotamidae, and Polymitarcyidae showed higher percent abundance in reference sites, whereas Hirudinidae, Chironomidae and Oligochaeta can be considered reliable indicators of highly-stressed sites typified by low concentration of DO (3mg/L). The association of Chironomidae, Physidae, and Hirudinidae with NO2 and NO3 was strong and positive (Spearman correlation p<0.05). Polymitarcyidae and Hydropsychidae showed strong but negative association with most nutrient variables. Cymbella kappii, Gomphonema augur, and Ulnaria ulna were positively related to SRP and TP; and Aulacoseira granulata with pH. The most abundant species at the test sites were Ulnaria ulna and Nitzschia sp. which tolerate organic pollution and high nutrient load. Shannon diversity (H′) values of invertebrates and diatoms were different among sites (Kruskal- Wallis t p<0.05), and a considerable large H′ values were observed in references (2.69 and 2.09), and low values were noted in multiple stressors, (1.97 and 1.89), respectively. For developing Multimetric Index of Lake Ziway (MMIZ), 32 macroinvertebrate and 18 diatom metrics were evaluated for interquartile range overlap (discrimination efficiency), response to water quality parameters (spearman correlation test) and redundancies within candidate metrics (box-and-whisker plots). Out of 50 candidate metrics tested, 16 met the three criteria and were compiled for the multimetric index development using discrete scoring system. Therefore, four levels of discriminatory bio-criteria for water quality were eventually
obtained: 16-32, poor; 33-48, fair; 49-64, good; and 65-80, very good. The MMIZ showed that water quality of the reference sites were very good, 1 site from the intermediary stressors was good, whereas 2 sites from the intermediary and 1 site from the multiple stressors were fair, and 2 sites from the multiple stressors were poor. Ecological Quality Ratio (EQR) was calculated by dividing MMIZ value by the median value of reference, and five classes were developed to rate the ecological quality status as very good ( ≥ 0.96), good (≥ 0.72 < 0.96), moderate (≥ 0.48 < 0.72), poor (≥ 0.24 < 0.48), and bad (< 0.24). Two sites were rated high quality; 2 sites good quality; 3 sites moderate quality; and 2 sites poor quality. The MMIZ showed positive correlation with lake habitat quality scores (r=0.959, p<0.01), which indicated good responsiveness of the multimetric index to variables of stressors. Validation of MMIZ using independent data set of Lakes Ziway and Hawassa indicated the MMIZ had good performance to discriminate between reference and non-reference sites. Therefore, the study indicated the potential use of a multimetric index to assess and monitor the habitat integrity and water quality in similar ecoregions in Ethiopia.
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Keywords
Benthic Diatoms, Benthic Macroinvertebrates, Bioassessment, Lake Ziway, Metrics, Multimetric Index, Physicochemical, Stressors, and Water Quality