Applicability of bioassessment methods using benthic macroinvertebrates to evaluate the ecological status of highland streams and rivers in Ethiopia

Rivers and streams in Ethiopia are vital sources of drinking water and are targeted resources foreconomic growth of the country. Towards meeting the goals in water sanitation, biodiversity andsustainable use, water management needs to focus the decisions on the knowledge of the current waterquality in the targeted area. A critical input for management decisions on inland waters is: how is thequality of water bodies, is it improving or deteriorating? To assess the degradation of these resources byanthropogenic activities, developing and advancing monitoring systems at local and national level iscrucial. Thus the main objective of this study is to determine the diversity and composition ofmacroinvertebrates in relation with different pollution gradient and to test and adapt bioassessment toolssuch as scoring system for Ethiopia. The study was conducted in and around Addis Ababa, capital ofEthiopia. A total of 29 investigation sites were sampled. To minimize the natural variability of the biota,all sites were located in one ecoregion (Ethiopian Montane Grasslands and Woodlands). Investigationsites were pre-classified into five ecological quality classes (reference, good, moderate, poor and bad)based on rapid field bioassessment screening protocol. Except for two sites affected by pulse releasescaused by reservoir maintenance and siltation, the major stressor affecting the sampling sites was organicpollution. Multi-Habitat Sampling (MHS) approach consisting of 10 sampling units was performed.Physico-chemical parameters namely temperature, pH, conductivity, dissolved oxygen, biologicaloxygen demand and total phosphorus were measured.38 families of benthic macroinvertebrates consisting of 98 taxa were identified. Percentage of ET(Ephemeroptera, Trichoptera) taxa consist of 36, 27, 26, 18, 6 in quality class I, II, III, IV and Vrespectively while % of Diptera taxa consist of 38, 42, 44, 50 and 54 in the above quality class order.Metrics such as taxa richness, number of ET taxa, Margalef's diversity index and % of red chironomidsshowed high discriminating efficiency among pre-classified quality classes. Non-metric MultidimensionalScaling (NMS) indicate high similarity in species composition within pre-classified qualityclass rather than among quality classes. BMWP, NEPBIOS and SASS scoring systems showed highdiscrimination power and high correlation coefficient: 0.96, 0.97 and 0.98 at á = 0.05, n = 28 amongquality classes respectively. BOD values increased with increasing organic load whereas totalphosphorus increased from reference to quality class III and slight decrease was observed in quality classIV. Like BOD value total phosphorus was significantly high in quality class V. Conductivity poorlydiscriminates among the first four quality classes but it strongly discriminates other classes from qualityclass V. Water temperature and pH were in the same range in all quality classes. There is a clear shift inbenthic macroinvertebrates composition and structure as the level of anthropogenic impact increases.High diversity with relatively even distribution of benthic macroinvertebrates was observed in lessimpacted sites unlike only few tolerant taxa such as Chironomus sp. and Tubifex sp. with highabundances in highly impacted sites. Most of the calculated metrics agreed strongly with the preclassification,which indicates strong performance of rapid field screening protocol. Even if the size ofthe sample used in this study is too small to generalize, scoring system is promising bioassessment toolthat - after some adaptetion to local conditions - can be used for biomonitoring the rivers and stream inEthiopia