Aquatic macroinvertebrates of the Lower Amu Darya

The Amu-Darya is the largest river in Central Asia. In the lower course its width ranges from 400 m to 1.2 km, the depth reaches 10 m, and maximal flow velocity is 0.6-0.8 m/s. The river is characterized by a strong turbidity of water, whose transparency at the flood stage is almost zero. The bottom is predominantly sandy, highly mobile, having variable relief. In the medial zone one can observe clay bottom local sites and pebble and stone islets partially submerged in sand or clay. In rhypal, there are areas of soft silt (especially in small bays without currents), buckwheat and thickets of amphibious vegetation; there are virtually no real macrophytes. The main watershed of the Amu-Darya is located in the mountainous areas of the Pamir and Hindu Kush (Gorno-Badakhshan Autonomous Region of Tajikistan and Badakhshan province of Afghanistan); it is mainly glacierand snow-fed. Thus, it undergoes summer floods and winter (January-February) low flow, which is typical for most of Central Asian lowland rivers. Fauna and assemblages of benthic invertebrates of the Amu Darya have not been adequately studied, despite the fact that many of them are a valuable food resource for the endemic (Pseudoscaphirhynchus kaufmanni (Bogdanow, 1874), Pseudoscaphirhynchus hermanni (Kessler, 1877), etc.) and commercial fish species. Sponges (Rezvoj, 1926), mollusks (Lindholm, 1914, Zhadin, 1950), caddisflies (Martynov, 1914), Chironomidae (Pankratova, 1933, 1950, Shilova, 1955; Sagitov, 1971, 1983) of the Lower Amu-Darya were dealt with in a few studies, fragmentary information is also available on mayflies (Kluge, 1987). Nevertheless, general hydrobiological research was conducted only in the Amu Darya delta zone (Dengina, 1957), but exclusively the lake fauna was considered. The purpose of this work is to describe the fauna of macroinvertebrates and their assemblages in the lower course of the Amu Darya.


Introduction
The Amu-Darya is the largest river in Central Asia.In the lower course its width ranges from 400 m to 1.2 km, the depth reaches 10 m, and maximal flow velocity is 0.6-0.8m/s.The river is characterized by a strong turbidity of water, whose transparency at the flood stage is almost zero.The bottom is predominantly sandy, highly mobile, having variable relief.In the medial zone one can observe clay bottom local sites and pebble and stone islets partially submerged in sand or clay.In rhypal, there are areas of soft silt (especially in small bays without currents), buckwheat and thickets of amphibious vegetation; there are virtually no real macrophytes.The main watershed of the Amu-Darya is located in the mountainous areas of the Pamir and Hindu Kush (Gorno-Badakhshan Autonomous Region of Tajikistan and Badakhshan province of Afghanistan); it is mainly glacier-and snow-fed.Thus, it undergoes summer floods and winter (January-February) low flow, which is typical for most of Central Asian lowland rivers.Fauna and assemblages of benthic invertebrates of the Amu Darya have not been adequately studied, despite the fact that many of them are a valuable food resource for the endemic (Pseudoscaphirhynchus kaufmanni (Bogdanow, 1874), Pseudoscaphirhynchus hermanni (Kessler, 1877), etc.) and commercial fish species.Sponges (Rezvoj, 1926), mollusks (Lindholm, 1914, Zhadin, 1950), caddisflies (Martynov, 1914), Chironomidae (Pankratova, 1933, 1950, Shilova, 1955;Sagitov, 1971Sagitov, , 1983) ) of the Lower Amu-Darya were dealt with in a few studies, fragmentary information is also available on mayflies (Kluge, 1987).Nevertheless, general hydrobiological research was conducted only in the Amu Darya delta zone (Dengina, 1957), but exclusively the lake fauna was considered.The purpose of this work is to describe the fauna of macroinvertebrates and their assemblages in the lower course of the Amu Darya.

Materials and methods
Macrobenthos samples were obtained at the Pitnak-Qipchaq site (Fig. 1), from June 22, 2017 to June 27, 2017.Total of 23 samples were collected, both in the river itself and in the additional reservoirs, 17 of them were quantitative.The given characteristics of benthic assemblages are based on quantitative data.Samples of macrobenthos were collected with a spherical scraper with 0.5 mm mesh from all available biotopes up to 1 m depth.Samplings with grabber were taken in deep places from 1.5 to 10 m.One quantitative test corresponded to 5 scoops of soil with a scraper or one scoop with a grabber.Unfortunately, a considerable freshet (about 1.5 m) prevented a detailed examination of a few biotopes.

Results
In general, the fauna of macroinvertebrates of the Amu Darya is significantly depleted.A total of 50 species are identified.Some of these have been introduced into Central Asian waterbodies over the past 70 years (see Table 1).The least diverse communities of invertebrates are formed on sandy and clay soils On the clay substrate only single, randomly entered, specimens of chironomids were detected.Apparently, there is no permanent animal population here.Assemblages of sandy soils.Sand forms the bottom and condenses under a strong current (0.6-0.8 m / s), which makes it difficult to move inside it.Specialized chironomid larvae predominate here; they probably represent a particular undescribed genus, Chironomini gen? l. amudarjensis sensu Pankratova, 1980 (60-100% of the total abundance).The subdominants are the psammorheophilic larvae of Macropeza albitarsis Meigen, 1818 and Stylurus flavipes liniatus Bartenef, 1929 represented by first-instar larvae.The total biomass of benthos is quite small and does not exceed 0.2-0.4g / m 2 .With a decrease in the current to 0.1-0.2m/s and the appearance of some silting, a number of chironomid larvae join this assemblage.They are endemic Acalcarella nucus Pankratova, 1950, Demicryptochironomus vulneratus (Zetterstedt, 1838), Harnischia sp.In this case, the first species goes becomes dominant, amounting to 40% of the total abundance.Assemblages of muddy soils.Invertebrate complexes associated with silt are more species-rich; however, chironomid larvae (Tanypus vilipennis (Kieffer, 1918) constituting 25-60% of the total abundance, Paramerina sp. and others) remain the most abundant group.There are also the mayflies Caenis spp.(the most common is Caenis pseudorivulorum Keffermüller, 1960), as well as the larvae of the horseflies (Tabanidae: Chrysops sp.) and the biting midges (Ceratopogonidae: Palpomyia sp.) here.Local high numbers (100 and more specimens/m 2 ) are reached by the sludge worms Branchiura sowerbyi Beddard, 1892 occupying a dominant position in the community.In flow of slower current, above the muddy soils, one can foind numerous flocks of Mysidae comprised of the two species of the genus Paramysis, P. intermedia (Czerniavsky, 1882) and P. kessleri (Grimm, 1875).These are adventive species introduced into the Amu-Darya (and into other waterbodies and watercourses of Central Asia) in the late 60s and early 70s (Embergenov, Sagitov, 1980).The mysid flocks are quite numerous (20-50 specimens/m 2 and more) and serve as a valuable food resource for many fish species.Assemblages of rhypal biotopes (shore roots and other mosaic substrates).At a moderate (0.1-0.3 m/s) current, submerged amphibiotic vegetation is inhabited by actively swimming Baetis (Labiobaetis) desertus (Novikova et Kluge, 1987) previously known only from the Chu River in southern Kazakhstan (Novikova, Kluge, 1987); it constitutes 55-65% of the total abundance.The subdominants are represented by the specific Central Asian Heptagenia perflava Brodsky, 1930, previously recorded from the Lower Amu Darya (Kluge, 1987).They are less numerous than the previous species, accounting for 15-35% of the total community.The mayflies Baetis (Nigrobaetis) digitatus Bengtsson, 1912 and chironomids Polypedilum spp.occur singly.With a decrease in the current to 0-0.01 m/s, limnophilic larvae of a few dragonfly species, the mayflies Cloeon dipterum Linnaeus, 1761 and the mollusks Physella acuta (Draparnaud, 1805) and Radix auricularia (Linnaeus, 1758) also join this biotope.Furthermore, the beetles, Aulonogyrus concinnus Klug, 1834 and Gyrinus distinctus Aubé, 1864, form local large concentration in the rhypal zone.
Chironomini gen? l. amudarjensis sensu Pankratova, 1980 Amu Darya Pankratova (1933Pankratova ( , 1950)); In the area where the river runs near the ridge of Karatau, filter-feeding simulid larvae Psilocnetha sp.begin to contribute to this community.In Central Asia these insects are also known from the Syr Darya (South Kazakhstan) (Iankovskiy, Koshkimbaev, 1988).It is not entirely clear why they do not live upstream.
According to local residents, during low water this biotope is inhabited by the large shrimp Macrobrachium nipponense (De Haan, 1849) which was introduced in the Amu Darya in the 70s.We observed it only in the reservoirs of the Amu-Darya floodplain.
The surface of driftwood and other submerged wood objects is overgrown with colonies of bryozoans, mainly of the genera Plumatella and Victorella, and with the oligochaetes Nais spp.when silting.
Due to the significant freshet, we were unable to examine in detail the population of stony substrates.Small parts of the rocky ground were met by us only in the district of the village Pitnak.It was inhabited by larvae of caddisflies of the genus Hydropsyche and the chironomid larvae of the genus Rheotanytarsus.Apparently, population of this biotope is in fact significantly richer.Probably for the same reason, in our collections there are no known for the region rheophilic caddisflies Neureclipsis bimaculata (Linnaeus, 1758) (Martynov, 1914), mayflies Oligoneuriella, a number of chironomids (Sagitov, 1971(Sagitov, , 1983)), and sponges (Rezvoj, 1926).
Wet silt along the water's edge is inhabited by the amphibiotic amber snails Oxyloma and Succinea, less often by the pond snails Orientogalba viridis Quoy et Gaimard, 1833.Various larvae of dragonflies (for example, numerous larvae Anax spp.), Noteridae, Dytiscidae, large bivalve mollusks Sinanodonta, gastropods Gyraulus, Radix and Trochorbis have been found in floodplain waterbodies.A complete list of organisms from the Amu Darya floodplain waterbodies is shown in Table 1.Thus, the fauna of macroinvertebrates of the Lower Amu Darya River is rather depleted: only 50 species have been found.The reasons of this phenomenon are: 1) the predominance of fine sand among the bottom grounds; it is inconvenient for attachment, 2) general deficiency of organic food resources, 3) high turbidity of the river's waters (the suspended particles settle on the respiratory organs of animals, making breathing difficult).The richest communities of macroinvertebrates, in terms of both abundance and biomass, are formed on flooded woody objects, while the most depleted ones occur on washed sand and clay.

Table 1 .
Species of invertebrates found in the Lower Amu Darya macroinvertebrates of the Lower Amu Darya