Hydrobiologia (2020) 847:4063–4090 https://doi.org/10.1007/s10750-020-04400-0 (0123456789().,-volV)( 0123456789().,-volV) PRIMARY RESEARCH PAPER Longitudinal river zonation in the tropics: examples of fish and caddisflies from the endorheic Awash River, Ethiopia Gernot K. Englmaier . Daniel S. Hayes . Paul Meulenbroek . Yonas Terefe . Aschalew Lakew . Genanaw Tesfaye . Herwig Waidbacher . Hans Malicky . Alemayehu Wubie . Patrick Leitner . Wolfram Graf Received: 28 March 2020 / Revised: 14 August 2020 / Accepted: 29 August 2020 / Published online: 16 September 2020 Ó The Author(s) 2020 Abstract Specific concepts of fluvial ecology are differences in spatial species assemblage structure and well studied in riverine ecosystems of the temperate identified characteristic taxa of the observed bio- zone but poorly investigated in the Afrotropical coenoses by indicator species analyses. Fish and region. Hence, we examined the longitudinal zonation caddisfly assemblages were clustered into highland of fish and adult caddisfly (Trichoptera) assemblages and lowland communities, following the freshwater in the endorheic Awash River (1,250 km in length), ecoregions, but separated by an ecotone with highest Ethiopia. We expected that species assemblages are biodiversity. Moreover, the caddisfly results suggest structured along environmental gradients, reflecting separating the heterogeneous highlands into a forested the pattern of large-scale freshwater ecoregions. We and a deforested zone. Surprisingly, the Awash applied multivariate statistical methods to test for drainage is rather species-poor: only 11 fish (1 endemic, 2 introduced) and 28 caddisfly species (8 new records for Ethiopia) were recorded from the Gernot K. Englmaier and Daniel S. Hayes: equally contributing authors. mainstem and its major tributaries. Nevertheless, specialized species characterize the highland forests, Handling editor: Marcelo S. Moretti G. K. Englmaier (&) Y. Terefe Institute of Biology, University of Graz, Universita¨tsplatz Department of Biological Sciences, Ambo University, 2, 8010 Graz, Austria P.O. Box 95, Ambo, Ethiopia e-mail: [email protected] A. Lakew Á G. Tesfaye Á A. Wubie D. S. Hayes Á P. Meulenbroek Á Y. Terefe Á EIAR-National Fisheries and Aquatic Life Research H. Waidbacher Á P. Leitner Á W. Graf (&) Center, P.O. Box 64, Sebeta, Ethiopia Institute of Hydrobiology and Aquatic Ecosystem Management (IHG), Vienna, University of Natural H. Malicky Resources and Life Sciences, Gregor-Mendel Straße 33, Sonnengasse 13, 3293 Lunz am See, Austria 1180 Vienna, Austria e-mail: [email protected] D. S. Hayes Centro de Estudos Florestais (CEF), Instituto Superior de Agronomia, University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal 123 4064 Hydrobiologia (2020) 847:4063–4090 whereas the lowlands primarily host geographically At the same time, anthropogenic impacts on aquatic widely distributed species. This study showed that a ecosystems are rapidly increasing on a global scale combined approach of fish and caddisflies is a (Darwall et al., 2018; Sabater et al., 2018). The effects suitable method for assessing regional characteristics of deforestation, intensification of agriculture and of fluvial ecosystems in the tropics. other land-use changes, hydropower, river engineer- ing, and water pollution threaten aquatic biodiversity Keywords Africa Á Biodiversity Á Biogeography Á (e.g. Clausen & York, 2008; Fitzgerald et al., 2018; Species assemblages Á Freshwater ecoregions Á Hayes et al., 2018, 2019; Meulenbroek et al., 2019). Indicator species These trends can even have visible impacts on large- scale ecosystem processes (Darwall et al., 2018). The serial discontinuity concept was developed to address these pressures (Ward & Stanford, 1983, 1995). It Introduction suggests that dams and other anthropogenic stressors can disrupt the underlying natural gradient and cause In the early to mid-twentieth century, limnologists an upstream or downstream shift of species, as well as intensively described the longitudinal distribution of divide the river network into discrete zones. However, aquatic communities along rivers (e.g. Thienemann, natural influences such as lakes can also create 1925; Huet, 1949; Harrison & Elsworth, 1958; Illies, comparable patterns (Stanford et al., 1988). Hence, it 1961a, b; Illies & Botosaneanu, 1963). These studies is evident that, in contrast to the assumption of an enhanced the understanding of fluvial ecosystems, and uninterrupted gradient (Vannote et al., 1980), discon- the river continuum concept even became a frequently tinuities or transition zones constitute a significant tested hypothesis in applied fluvial ecology (Vannote component of faunal zonation (Statzner & Higler, et al., 1980). Since then, knowledge on riverine 1986). Moreover, certain functional process zones distribution patterns has been used, for example, to may repeatedly appear along a river and even form establish bioindication systems (e.g. Schmidt-Kloiber comparable patterns within an ecoregion. Beyond the & Hering, 2015) and river assessment criteria (e.g. ecoregional level, however, such patterns may be less Aarts & Nienhuis, 2003; Welcome et al., 2005). predictable (Thorp et al., 2006). Therefore, it must be Nevertheless, taxonomy and taxa differentiation of clarified if and to which extent zonation studies many biota, as well as their habitat preferences, conducted in tropical streams and rivers also reveal functional traits, and distribution patterns, are often such discontinuities reported for other systems still poorly understood (Balian et al., 2008). This is (Arau´jo et al., 2009), as well as if the ecoregion especially the case in tropical rivers (e.g. Gibon & regulates community zonation (Thorp et al., 2006). Statzner, 1985; Malicky & Chantaramongkol, 1993; Fish (Pisces) and caddisflies (Insecta: Trichoptera) Winemiller et al., 2008; Laudee & Prommi, 2011; are widely used to describe longitudinal changes of Skelton & Swartz, 2011; Ochieng et al., 2019). community structures (e.g. Harrison & Elsworth, Despite recent advances (Malicky & Chantara- 1958;Le´veˆque et al., 1983; Stanford et al., 1988). mongkol, 1993; Arau´jo et al., 2009), most conceptual These two organism groups provide the advantage of studies on river zonation were conducted in temperate relatively low sampling effort, the coverage of diverse regions of Europe and North America (Hawkes, 1975), habitat characteristics in the respective river stretches, which may limit the adoption of established concepts and a more profound taxonomic knowledge in com- into tropical regions (Arau´jo et al., 2009). Besides, the parison to other organism groups. Besides, both lack of knowledge of tropical rivers is a limiting factor groups have a high indicative power regarding envi- in assessing the integrity of these ecosystems. Hence, ronmental conditions. They are therefore implemented the scarcity of tropical studies impedes the assessment within the EU Water Framework Directive (2000/60/ of diversity and distribution patterns, as well as a EC) as faunistic biological quality elements to assess comparison to concepts and hypotheses of fluvial the ecological status of freshwater systems. Whilst fish ecology of temperate rivers (Ward et al., 2002; Thorp mostly respond to mesohabitat characteristics, caddis- et al., 2006). flies depend more on the availability of microhabitats. Particularly the latter is increasingly used for 123 Hydrobiologia (2020) 847:4063–4090 4065 ecological status assessment of African rivers (e.g. shared species of both adjacent communities should Dickens & Graham, 2002; Masese et al., 2009; Kaaya exist. et al., 2015; Lakew & Moog, 2015; Alemneh et al., 2019). However, ecological studies in riverine ecosys- tems of the Afrotropical region often rely on a high Materials and methods level of taxonomic resolution, such as family or genus level (e.g. Kaaya et al., 2015; Lakew & Moog, 2015; Study area and sampling sites Alemneh et al., 2019), despite the importance of including species-level information to enhance under- The Awash River catchment, with an area of standing of distribution patterns (Malicky & Chan- 112,696 km2, is home to approximately 14.9 million taramongkol, 1993). So far, however, species-specific people, making it one of the most important and studies throughout the Afrotropical realm mainly industrialized drainage basins in Ethiopia (Tesfaye & focused on taxonomy or a wide biogeographic context Wolff, 2014; Ministry of Environment, Forest and (e.g. Roberts, 1975) but rarely covered aquatic com- Climate Change, MEFCC, 2018). The Awash River munities in entire river systems (Harrison & Elsworth, springs in the Ethiopian Highlands at an altitude 1958; Payne et al., 2010). of [ 3,000 m. It flows for 1,250 km along the north- In this study, we explored the longitudinal zonation ern part of the Main Ethiopian Rift, where it finally of fish and adult caddisfly species in a long drains into saline Lake Abbe at the Ethiopian–Djibouti ([ 1,000 km, sensu Grill et al., 2019) tropical river border at an altitude of around 250 m (Tesfaye & in East Africa, including its major tributaries. We Wolff, 2014; Tadese et al., 2019). Most tributaries selected the Awash River in the Main Ethiopian Rift as originate in the highlands and join the mainstem river a case study because of several unique characteristics: from the West (Fig. 1). the river is an endorheic drainage which flows into the In the highlands, the mean annual precipitation Afar Depression, an arid region; it exhibits a distinc-