Diversity and distribution patterns of benthic insects in streams of the Aurès arid region (NE Algeria)

Aquatic ecosystems are dynamic, complex and very important elements of the landscape, because they are characterized by high biological productivity, rich biodiversity, and a multitude of ecological processes and functions (Higler 2009). Dryland regions represent over one third of the world’s land area, characterized by a low and variable rainfall pattern, yet sometimes containing a surprising array of aquatic habitats (Jenkins et al. 2005). The presence or absence of water defines much of the ecology of semi-arid and arid landscapes (Stafford Smith, Morton 1990), while rivers and their flows mediate much of the ecological variability (Walker et al. 1995).

Intermittent streams are among the main features of arid and semi-arid regions (Comin, Williams 1994). They are ubiquitous in areas with Mediterranean climate (Gasith, Resh 1999). Physicochemical and biological factors are among the most constraining environmental factors for insect fauna in the majority of these streams (Williams 1996). The patterns of water disappearance are of major importance, regardless of whether they are predictable (i.e. part of a stable cycle) or unpredictable (i.e. associated with vagaries of local climate; Wiggins et al. 1980; Delucchi 1988). Thus, the dry period duration reduces the habitat size. Temperature changes result in subtle changes in water density, which may affect smaller instars of insects, while ionic concentration tends to rise significantly as temporary water dries up, which may affect pH and perhaps have direct osmotic consequences for insects (Williams 1996). Oxygen will be further affected by changes in water temperature, which will increase with disappearance of water in summer-dry water bodies (e.g. autumnal ponds; Wiggins 1973) and decrease in winter-dry water bodies (e.g. summer ponds; Daborn, Clifford 1974).

Thermal fluctuations in the Mediterranean area are sometimes brutal and irregular heavy rainfall is concentrated in short periods of time. The streams in the region are characterized by an irregular hydrological flow (Arab et al. 2004). The hydrological pattern is characterized by a peak of water discharge in spring and autumn and severely low water levels in summer (Giudicelli et al. 1985). In arid regions with low and irregular rainfall, rivers remain dry for long periods of time due to high evaporation during periods of low water flow. The first flood following the period of low water is generally in the form of violent storms that cause heavy runoff. The latter corresponds to flood flow, brings new alluvium and causes a lot of damage that can disturb the living environment of aquatic organisms (Joly 2006).

Studies of benthic invertebrate communities, their structure and spatial distribution are of paramount importance for understanding the functioning and management of natural hydrosystems and for assessing their ecosystem health state (Dakki 1992). Macroinvertebrates, such as insects, are good bioindicators due to their sedentary status, great diversity and variable tolerance to pollution and habitat degradation (Moisan, Pelletier 2008). Perennial and intermittent flow regimes are often perceived as categorically different, flow intermittence varies continuously, and biotic communities vary along gradients of intermittence. For example, taxonomic richness in different aquatic groups continues to decrease as the severity of intermittence increases, which suggests a lack of clear thresholds in the response of communities to flow intermittence (Darty et al. 2014).

In North Africa, streams (wadis) are characterized by an extensive range of physical conditions, including severe flooding and droughts (Pires et al. 1999), irregularity of flows and strong hydrological fluctuations (Giudicelli et al. 1985). The Aurès area in Algeria, corresponding to the eastern part of the Atlas Basin, is characterized by a semi-arid Mediterranean climate. Anthropogenic activities in the region, such as agriculture and sewage discharge, contribute to the degradation of water quality. Low water levels, especially in summer, reduce the self-purification capacity of streams, making them vulnerable to pollution, while aquatic macroinvertebrates are severely affected, especially by summer drought. Some sections of streams may become partially or completely dry and lotic macroinvertebrates can survive by aestivating in cool moist microhabitats (Samraoui 2009), while lentic species prevail during high flow periods, and rheophilic species complete their life cycle and become dominant.

The objective of the study was to document the poorly-known freshwater invertebrate fauna of North Africa, especially insects, and to identify relevant environmental factors (natural or anthropogenic) that shape the structure of freshwater communities in arid-land streams. We have tested two hypotheses:

The current survey provides an initial insight into the composition of insect assemblages, representative of the Aurasian wadis, and some of the environmental determinants that appear to affect the structure of these assemblages.

A total of 42 benthic insect taxa were recorded during our inventory, confirming the low qualitative and quantitative taxa diversity revealed in previous studies on North African wadis, which, like other Mediterranean rivers, are exposed to a wide range of hydrological fluctuations and extreme physical conditions (Arab et al. 2004; Lounaci et al. 2000; Belaidi et al. 2004). As a result, intermittent streams generally harbor fewer species than permanent streams (Del Rosario, Resh 2000). Despite the difference in the sampling effort, the taxonomic richness observed in this semi-arid region was lower compared to that reported from the more humid climate of North Algeria with higher annual precipitation, e.g. from the Kabylie region inventoried by Lounaci (2005), or the Soummam streams inventoried by Zouggaghe, Moali (2009). The low taxonomic richness and abundance also found in different arid and semi-arid bioclimatic regions of Algeria (Bebba et al. 2015; Sellam et al. 2017) can be explained by unstable environmental conditions, particularly the frequent lack of flow, insufficient flowing-water levels and high summer temperatures (> 35°C), leading to prolonged periods of drought, with a negative impact on biodiversity that tends to be low (Arab et al. 2004). This phenomenon was also observed in temporary streams of all regions around the world: California, the Mediterranean Basin, Chile, South Africa and SW Australia (Williams 1996; Bonada 2003). In terms of abundance, insect assemblages were dominated by Diptera, Ephemeroptera and Trichoptera. Diptera were much more dominant taxa, as was the case in other arid Mediterranean rivers in North Africa (Sellam et al. 2017). Pires et al. (2000) reported that Diptera were also dominant in intermittent streams in Portugal, because of their ability to tolerate drought conditions and their efficient recolonization mechanisms. Simuliidae were among the most abundant family found in W. El Ma and W. Yabous with larvae adhered to stable substrates such as stones and rocks, probably due to their filtering habit (Tachet et al. 2010) and tolerance to moderate levels of pollution (Augusto, Marcos 2010).

W. Chaaba, which is located in the integral zone of the NPB, is characterized by high biodiversity compared to other wadis, whereas the fauna of W. Hamla, W. Chelia and W. Yabous was less diverse. This difference in the diversity between the sites is probably due to the anthropogenic disturbances of the aquatic ecosystem, as some of the sites were exposed to domestic pollution. Similarly, Azrina et al. (2006) and Cereghino et al. (2002) noted that species richness is sensitive to human impact on aquatic ecosystems, particularly aquatic insects, which are often good indicators of environmental conditions in streams and are therefore very sensitive to pollution. Species richness can be a good descriptor of the human impact on streams (Compin, Cereghino 2003).

The distribution of benthic insects is closely associated with environmental factors. Their composition and abundance are determined by physicochemical and ecological characteristics of the physical environment. The results of the research on the distribution of different insect groups are consistent with the fact that the observed response of the total insect abundance could largely reflect the abundance of the dominant order (Diptera). The abundance of Diptera, which was mainly dominated by Simuliidae, was higher in the urban area with a stone substrate. The substrate type was found to be a strong determinant of invertebrate distribution (Halwas et al. 2005). Relatively coarse-grained and highly structured stable substrates attract benthic fauna, because they represent sites of minimal disturbance during floods and provide refuges where recolonization can occur after flooding (Lancaster, Hildrew 1993; Rempel et al. 1999). Moreover, the identification of the human impact as the primary factor affecting the abundance of insect groups could also be explained by the fact that Diptera were the most abundant order, being more resistant to pollution (Augusto, Marcos 2010).

Our results indicate that Baetis sp. and Acentrella sp. occurrences in urban areas are correlated with the substrate size and flow velocity. Due to the differences in the substrate composition (characterized by relatively high cover of stones and coarse sand) and moderate flow velocity in urban areas, the distribution of the two above genuses can be explained by the habitat suitability for these groups. Baetidae are eurytopic and tolerant taxa, currently found in large numbers in lowland and piedmont streams of northern regions of Algeria. Changes in the sediment heterogeneity may be very important when explaining the variation in the taxonomic composition of macroinvertebrates along the residential gradient (De Sousa et al. 2008). The nature of the sediment substrate and the current velocity also seem to influence the structure of macroinvertebrate communities in North African humid and arid streams (Loanaci et al. 2000; Arab et al. 2004; Chaib et al. 2011; Sellam et al. 2017). The structure of benthic communities in semi-arid streams in Portugal was associated with low stability and high resilience, with species having strong capacity to recolonize streams subject to large scale floods or affected by drought (Cortes et al. 2002).

Brachycentrus sp. was found in the Chelia and Yabous mountain wadis, characterized by relatively high cover of rocks. As a consequence, fewer habitats are vacant for stenothermal taxa, causing lower taxonomic diversity due to the lack of potential habitats. In the same way, the upstream-downstream species replacement found in other streams (Dakki 1992; Arab et al. 2004; Arigue et al, 2016) did not or scarcely occurred in the Aurès arid streams. However, the particular high-altitude climatic characteristics of North Africa induce an adaptation of some headwater taxa (Giudicelli 1984; Giudicelli et al. 1985). The most abundant taxa that do not belong to any of the four groups identified in the RDA analysis can therefore be considered: i) ubiquitous as they are located in the middle of the RDA plane, or ii) with a narrow spatial distribution, highly affected by one or several environmental factors.

Environmental factors in these streams directly or indirectly affected the macroinvertebrate assemblages, which indicate that macroinvertebrates are useful indicators of water quality. Biotic communities could suffer from irreversible changes resulting from anthropogenic shifts from perennial to intermittent flow regimes (Darty et al. 2014). Populations may become even more fragmented under dry conditions and local extinctions could be more likely for some poorly adapted taxa that may be eliminated from newly intermittent rivers (Phillipsen, Lytle 2013). The invertebrate fauna of these intermittent streams is sensitive to severe environmental conditions and can therefore be used as a valuable monitoring tool in the assessment of freshwater aquatic environments. Given the short period of our study, it would be unreasonable to consider the presented conclusions as definitive or to relate the obtained results to other periods of the year, while further studies could indicate how insects respond to seasonal and annual changes in the structural characteristics of the environment. This will allow to predict how changes in habitat conditions may affect assemblages of insects.