Water Quality Assessment of Lake Hawassa, Ethiopia, Using Macroinvertebrate and Diatom Based Multimetric Index
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Date
2019-04-03
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Addis Ababa University
Abstract
Lake Hawassa is one of the most threatened Rift Valley lakes because of human pressure like adverse land use in the watershed, urban development and rapid growth of industries. As a result, the social, economic and ecological functions of the lake are under question. Thus, there is a need for continuous follow-up and monitoring the lake ecology to maintain its water quality and biological integrity. Benthic macroinvertebrates and diatoms are often used for biological water quality assessment because they respond to different stressors and are sensitive to changes in aquatic environment. This study was conducted to assess ecological quality of Lake Hawassa by using diatoms and macroinvertebrate based multimetric index of biotic integrity. Both biological and the physicochemical data were collected from 12 sampling sites using standard procedures in the dry and wet seasons of the years 2015 and 2016. Data on selected physicochemical and habitat quality parameters [in-terms of percent disturbance score (PDS)] were used to categorize each sampling station.
Based on the PDS, sampling stations were classified ranging from as having no evident disturbance (0-25%) to highly disturbed status (75-100%). The PDS classified the 12 sites into three categories: four sites were categorized as having minimal disturbance (C1), five sites as moderate disturbance (C2) and three sites as high disturbance (C3). The physicochemical parameters showed a strong significant relationship along the three categorized sites (Kruskal-Wallis Test P<0.01). Except for dissolved oxygen (DO) and nitrite (NO2-), the other physicochemical variables had the highest mean record at highly disturbed sites while DO was mostly higher in minimally disturbed sites. Cluster analysis based on some selected physicochemical variables and PDS also showed a clear distinction between the three-disturbance levels.
From the 12 sites, 43 families of macroinvertebrates which belonged-to 14 taxonomic groups were collected and identified. The Order Hemiptera was the most diverse group followed by Odonata and Coleoptera. The most abundant families were Coenagrionidae, Physidae and Pleuroceridae which made up 19.53, 15.29 and 10.72%, respectively. These
families and Chironomidae, Planorbidae, Piscicolidae, Lumbriculidae, Libellulidae and Corixidae were more abundant in highly disturbed sites and showed a positive association with SRP, NO32-, NH4+, TP, EC and PDS (Spearman correlation P<0.05). Polymitarcyidae, Baetidae, Hydrophilidae, Notonectidae, Naucoridae, Belostomatidae and Galastocoridae were inversely related to SRP, NO32-, NH4+, TP and EC and showed a negative but significant correlation with PDS (P<0.05); and might be considered as indicators of minimally disturbed sites in the littoral area of Lake Hawassa.
A total of 105 diatoms species belonging to 39 genera were collected and identified from the 12 sites. The highest and the lowest species numbers were in minimally and highly disturbed sites, respectively, with 45 and 21 species recorded in the dry season. The genus Nitzschia was the most diverse group and contributed 15 species, followed by Gomphonema (8 species). Cymbella, Epithemia and Navicula genera comprised five species each and the other diatoms assemblages were represented by < 5 species. In terms of relative abundance Nitzschia frustulum Kützing (17.5%), Achnanthidium minutissimum Kutzing (9.4%), and Gomphonema parvulum Kutzing (8.4%) were the most abundant species. Most of the diatom species did not show significant temporal variation (P>0.05). Achnanthidium minutissimum, Aulacoseira muzzanensis, A. ambigua, Navicula tenella, Nitzschia frustulum, Staurosira brevistriata, and Staurosira construens species showed significant spatial variation (P<0.05). Diatom species Fragilaria capucina, F. tenera, Gomphonema gracile, G. olivaceum, G. parvulum, G. pumilum var. rigidum, Nitzschia frustulum, N. kuetzingioides, and N. palea were recorded from sites which had high concentration of nutrients and showed a strong significant association with them (P<0.01). Achnanthidium minutissimum, Aulacoseira ambigua, Cymbella cistula, C. kappii, C. turgidula, Encyonema muelleri, E. silesiacum, Encyonopsis microcephala, Navicula tenella and Ulnaria acus were more related to sites with relatively good DO. They were also negatively associated with TP, SRP, NO32- and NH4+ which indicates that they are good indicators of the minimally disturbed sites with low nutrient concentration. Analysis of similarities (ANOSIM) revealed significant variation on both diatom and macroinvertebrate assemblages among the three-clustered sites (P<0.01) which was supported by high values of dissimilarity percentage.
Out of 57 macroinvertebrate and diatoms candidate metrics, only 18 were selected as core metrics based on redundancy analysis (redundant metrics were rejected), response to environmental variables( metrics having significant relationship were accepted), percent discriminatory efficiency (metric > 50% were selected) and evaluation of the sensitivity of the metric of interquartile ranges (metrics with sensitivity value 2 or 3 were nominated). These core metrics were used to develop the multiassemblage multimetric index of biotic integrity of Lake Hawassa (MMIH). These MMIH index values were divided into quartile ranges of ecological quality classes such as 73-90, 55-72, 37-54 and 18-36, which were classified as very good, good, fair and poor quality, respectively. Based on these ranges, the MMIH index classified the sampling sites into three categories from very good (C1) to fair (C2) and poor (C3). The ecological quality ratio (EQR) further characterized the C1 sites as high quality, C2 sites as moderate quality and C3 sites as poor quality sites. Validation of the MMIH index using the independent data set of Lake Hawassa and Lake Ziway showed that the MMIH had good performance to discriminate between the reference and non-reference sites and evaluate the ecological quality of the clustered sites of both lakes. The MMIH index had inverse strong relationship with PDS which indicated that the MMIH index responded well to the habitat quality degradation of Lake Hawassa (n=9, r2=0.91, P=0.0003). Generally, it is concluded from this study that the MMIH index is robust and can clearly discriminate between sites exposed to different stressors. Thus, it can be applied by end-users as a practical tool for measuring the ecological quality of different sections of lake littoral. Therefore, it can be recommended to the concerned environmental authorities to develop a management strategy to rehabilitate the disturbed sites and to under-take regular monitoring program to maintain good habitat integrity of the Lake Hawassa ecosystem.
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Keywords
Disturbance Score, Littoral Zone, Management, Multiassemblages, Non-Reference, Reference Sites, Stressor, Validation