The assemblage characteristics of benthic macroinvertebrates in the Yalutsangpo River, the highest major river in the world

Abstract

Aquatic ecosystems of highland rivers are different from those of low altitude rivers because of the specific topography and environmental parameters associated with high altitudes. Yalutsangpo, the upper course of the Brahmaputra River, is the highest major river in the world, flowing from west to east across Tibet, China and pouring into India. Macroinvertebrates were sampled from Yalutsangpo and its tributaries, the Lhasa, Niyang, and Parlong Tsangpo Rivers, from October 2009 to June 2010, to study characters of the highland aquatic ecosystem. Altogether, 110 macroinvertebrate taxa belonging to 57 families and 102 genera were identified from the basin. The biodiversity and composition of macroinvertebrate assemblages were strongly affected by altitude gradients. Local diversity represented by taxa richness and the improved Shannon-Wiener index were high at altitudes of 3,300–3,700 m, among which suitability of habitat was higher due to the better integrated environmental conditions of water temperature, dissolved oxygen, and aquatic vegetation, etc. Macroinvertebrates were grouped into shredders, scrapers, predators, collector-filterers, and collector-gatherers according to their feeding behaviors. It was found that the distributions of the functional feeding groups varied with habitat altitudes. Shredders were present at altitudes of 2,900–4,400 m, while scrapers mainly inhabited altitudes of 3,500–4,500 m, and collector-filterers preferred 3,500–4,000 m.

Even though the local taxa richness was not high at each site, the taxonomic composition and density of the assemblages varied greatly among the different sites, resulting in much higher regional diversity compared to the lowland river with similar flow and substrate conditions. The regional cumulative taxa richness of Yalutsangpo decreased and more families were lost as the altitude increased. However, some families that were newly present as the attitude increased were essential for sustaining the high regional biodiversity. The ordination diagram obtained from Detrended Correspondence Analysis indicated that altitude, river pattern, riverbed structures, bank structures, and flow conditions were the main factors that influenced the macroinvertebrate assemblages in the Yalutsangpo basin.

Keywords

Yalutsangpo basin / high altitude / macroinvertebrate / local diversity / regional diversity

Introduction

Describing and understanding patterns in biological diversity along major geographical gradients is an important topic in ecology because geographical gradients affect aquatic assemblages (Jacobsen, 2004). However, most previous studies of aquatic ecology and the impact of environmental parameters were conducted mainly in lowland rivers in China (Duan et al., 2011; Pan et al., 2011, 2012). In the present study, investigation of aquatic ecology was conducted in the river located at the highest altitude in the world, the Yalutsangpo River, which is the upper course of Brahmaputra River. The Yalutsangpo basin is bordered by the Himalayas in the south and the Gangdisê and Nyainqêntanglha Mountains in the north. It is 1,200 km long and flows from west to east in the South Tibet Valley. In East Tibet, the river passes through the world’s largest and deepest canyon, the Yalutsangpo Grand Canyon. Leaving the canyon, the river flows to India.

The aquatic ecology of the Yalutsangpo is unique compared to mountain rivers typically located at altitudes less than 2,000 m a.s.l., due to the sharp gradients in environmental conditions, such as altitude, climate, riparian vegetation, and water temperature (Füreder et al., 2001). As the altitude of the river descends from 4,500 m a.s.l. to 3,500 m a.s.l. and then to 2,000 m a.s.l., the riparian vegetation changes from cold desert to arid steppe and then to deciduous scrub, respectively. Temperature and precipitation throughout the Yalutsangpo basin also vary greatly along these varying altitudes. The annual precipitation of the upstream of the basin is less than 300 mm, while in the downstream of the Grand Canyon, it is 4,000 mm (Liu et al., 2007). The average annual air temperature of the downstream is at least 10°C higher than the upstream (Song et al., 2011).

The Yalutsangpo and its tributaries are one of the most remote and undisturbed aquatic environments. They are generally much less influenced by pollution from agriculture and sewage water than rivers in other parts of China. However, these rivers are especially sensitive to climate change. It was found from monitoring during 1961-2005 that the magnitude of warming over the Yalutsangpo basin since the 1970s exceeded that of the Mt. Qomolangma region, Tibetan Plateau, and the global average. The magnitude of increasing precipitation and the potential for decreasing evapotranspiration were also greater in the Yalutsangpo basin; as a result, the source of the river water would be changed (You et al., 2007). As most of the water source in the high altitude rivers is glacial in origin, the aquatic biota are adapted to persistent low temperatures (Skjelkvåle and Wright, 1998), and would react very sensitively to the change of water source. Therefore, it is critical to study the characteristics of aquatic ecosystems of river basins located at high altitudes.

Benthic macroinvertebrates are important components of river ecosystems and have been considered as the most useful and convenient indicators for assessment of the aquatic ecology because of their confinement to the river bed, limited movement abilities, long life cycles, and sensitivity to water pollution and environmental changes (Milbrink, 1983; Rosenberg and Resh, 1993; Smith et al., 1999). They play a significant role in trophic dynamics by cycling nutrients and providing food for higher trophic levels. Their feeding function reflects the different types of material the organisms ingest and their predominant feeding behaviors (Smock, 1983). Generally, they were grouped into five functional feeding groups: shredders, scrapers, collector-filterers, collector-gatherers, and predators according to their feeding habits (Plafkin et al., 1989; Barbour et al., 1999).

Based on a series of previous studies, there is an increased knowledge of characters of macroinvertebrate assemblages in rivers along different altitudes. Jacobsen (2004) indicated that family-level identification of macroinvertebrates can facilitate interpretation of sources and sinks of biodiversity along geographic gradients. Macroinvertebrate assemblages in remote highland lakes across Europe showed that their response to altitude gradients varied with latitude (Fjellheim et al., 2009). The occurrence of macroinvertebrate families and species in glacier-fed rivers is determined by the maximum water temperature and channel stability (Milner et al., 2010). Reduced water temperatures from increased contributions of glacial meltwater, and decreased channel stability from changed runoff patterns reduce the diversity of macroinvertebrate assemblages in glacier-melt dominated rivers (McGregor et al., 1995). A preliminary investigation of macroinvertebrates in Yalutsangpo River showed that their number density was much lower and the assemblage composition was quite different from that observed in low altitude rivers (Zhao and Liu, 2010). However, the distribution of macroinvertebrate assemblages in the Yalutsangpo basin at different altitudes is yet to be studied. In this study, the main objectives were to: (i) describe the characteristics of macroinvertebrate assemblages in the Yalutsangpo basin; (ii) assess the aquatic ecological status by evaluating alpha and beta diversities of macroinvertebrate assemblages; (iii) analyze the impact of altitude gradients on the macroinvertebrate assemblages; and (iv) identify the important environmental parameters structuring macroinvertebrate assemblages in the basin.

Sampling sites and study methods

Field investigations and macroinvertebrate samplings were conducted from October 2009 to June 2010 in the Yalutsangpo basin. Figure 1 displayed the whole river basin and the sampling locations in the main channel of Yalutsangpo and its tributaries: Nianchu, Lhasa, Niyang, and Parlung Tsangpo Rivers. The sites S6, S9, S11, S12, and S13 were located in the main channel of Yalutsangpo. S3 was in the upstream tributary Nianchu River. S1, S4, and S8 were in the mid-stream tributary Lhasa River. S2, S10, and S14 were in the mid-downstream tributary Niyang River. S5 and S7 were in the downstream tributary Parlung Tsangpo.

At each sampling site, three replicate samples were collected from the bed surface to a bed depth of about 15 cm using a kick-net (1 m × 1 m area, 420 μm mesh). The sampling area for each replicate sample was 1/3 m², giving a total sampling area of 1 m². Macroinvertebrates were sorted immediately from sediment and preserved in 75% ethanol in the field, then identified and counted under a stereoscopic microscope in the lab. Specimens were identified mostly to family or genus level (Morse et al., 1994; Liang and Wang, 1999), and were weighted separately using an electronic balance. Their functional feeding groups were classified according to Duan et al. (2011). Environmental parameters were measured at the sampling sites. Water depth was measured with a sounding lead or a steel ruler. Water velocity was measured at 60% of the water depth from the river bed with a propeller-type current meter (Model LS 1206B; Nanjing Automation Institute of Water Conservancy and Hydrology, China). Dissolved oxygen and water temperature were measured with a hand-held oxygen meter (HACH HQd-40).

Taxa richness S and the improved Shannon-Wiener index B have been considered as the most important indices for evaluating alpha diversity for the local sampling site; density D and biomass W were also widely used (Duan et al., 2009). Therefore, these indices were chosen to assess the aquatic ecology and analyze the impact of altitude gradients on the biodiversity in this study. Taxa richness S is defined as the number of taxa that occurred in each sample. Density D is the total number of macroinvertebrate individuals per unit area, in ind·m^-2. Biomass W is the total wet mass of macroinvertebrates per unit area, in g·m^-2.

The improved Shannon-Wiener index B is defined as

(1)B=-(\ln N){\displaystyle \sum _{i=\mathrm{1}}^S(n_i/N)}\ln (n_i/N),

where N is the total number of macroinvertebrates at each site, and n_i is the number of individuals of the i^th taxon (Wang et al., 2009). This index contains information of both taxa richness and density. The higher the value of B, the higher the alpha diversity of the community.

Moreover, beta diversity β was evaluated for assessment of the regional diversity for the whole basin. It is defined as

(2)\beta ={\displaystyle \frac{M}{{\displaystyle \frac{\mathrm{1}}{S}}{\displaystyle \sum _{i=\mathrm{1}}^S{m}_i}}},

in which M is the number of the selected sampling sites, among which the macroinvertebrate compositions are the most varied. m_i is the number of sites where the i^th taxon occurred (Wang et al., 2012). For a given M, the higher the value of β, the higher the heterogeneity of the community.

Macroinvertebrates were also sampled from Juma River, which is a mountain river basin in the suburbs of Beijing with an altitude less than 300 m a.s.l. The sampling sites in Juma shared similar substrate composition and flow conditions to those in Yalutsangpo. Alpha and beta diversity indices of macroinvertebrate assemblages were compared for both of the basins. In the calculation of the beta diversity index, eight sites with the most varied compositions of macroinvertebrates were selected, thus M=8.

Furthermore, cumulative taxa richness was calculated to analyze the impact of altitude on the regional taxa distribution. Cumulative taxa richness S_n was introduced and calculated as follows: S₁ was the taxa richness at site S1; S₂ was the total taxa richness in the samples of S1 and S2; S_n was the total taxa richness in all of the samples of S1, S2,…,Sn. As listed in Table 1, S1 had the highest altitude, S2 had the second highest altitude, and Sn had the n^th highest altitude. The taxa that occurred at S2 but not at S1 were tallied as dS₂, then S₂=S₁+ dS₂. Similarly, the taxa that occurred at Sn but not at S1, S2,…, S(n-1) were tallied as dS_n, then S_n=S_(n-1)+dS_n. In this way, the cumulative loss and gain of families can be calculated as the altitude increased.

In addition to the analysis of the impact of altitude on macroinvertebrate assemblages, taxonomic distribution along the other main environmental gradients was determined using a method of indirect gradient analysis, Detrended Correspondence Analysis (DCA) (Hill, 1979). Software CANOCO for Windows Package 4.5 (Ter Braak and Šmilauer, 2002) was used for the DCA. Clusters of samples in ordinate space were identified by giving a relatively objective community ordination of the matrix of species × samples.

Results and discussion

Table 1 lists the environmental parameters, including altitude (H), water depth (h), flow velocity (v), dissolved oxygen concentration (DO), water temperature (T), and coverage rate (VC) and height (VH) of riparian vegetation for all sampling sites. Alpha diversity indices S, B, D, and W are also given in Table 1.

Composition of macroinvertebrate assemblages

Altogether 110 macroinvertebrate taxa belonging to 57 families and 102 genera were identified from the whole basin. Among them were 1 Turbellaria, 1 Nematoda, 16 Oligochaeta, 4 Hirudinea, 7 mollusks, 1 Arachnida, 1 Crustacea, and 79 Insecta. The total taxa richness was much higher than that previously reported by Zhao and Liu (2010). The proportions of Oligochaeta, Hirudinea, Gastropoda, and Insecta were 14.5%, 3.6%, 5.5%, and 70%, respectively. Figure 2 shows density compositions of the taxonomic groups Arthropoda, Mollusca, Annelida, and other rare groups at all sites. Arthropoda was the most dominant group in the whole basin. Mollusca and Annelida were mainly in the sites with attitudes lower than 3,700 m a.s.l. The proportions of Annelida or Mollusca were extremely low at the sites with altitudes higher than 4,000 m a.s.l. The rare groups including Platyhelminthes and Nematoda were only sampled from the sites with attitudes higher than 3,500 m a.s.l.

Figure 3 shows density compositions of the five functional feeding groups: shredders, scrapers, collector-filterers, collector-gatherers, and predators. Collector-gatherers and predators inhabited almost all sites. Scrapers mainly inhabited the sites with altitudes of 3,500-4,500 m a.s.l. Shredders mainly inhabited the sites among 2,900-3,600 m a.s.l and 4,000-4,500 m a.s.l. Trees and bushes grew between 2,900-3,600 m a.s.l., adequately supplying fallen leaves for the shredders, i.e., Chrysomelidae. In addition, alpine algae and plateau meadow grew between 4,000-4,500 m a.s.l. sufficiently supplying food for the shredders, i.e., Tipula sp., Pteronarcidae. Collector-filterers inhabited the sites between the altitudes of 3,500-3,900 m a.s.l. Predators existed in all sites, with an increased density proportion with altitude. Tomanova et al. (2007) found that the density proportions of collector-gatherers, shredders, and scrapers were clearly related to altitude. Macroinvertebrates are constantly moving, and the distance they cover depends on the sparsity of their food source distribution (Boyero, 2005). In this study, the food sources for shredders, scrapers, and collectors relied on vegetation distribution, which are affected by altitudes; thus, the distributions of the three groups varied with altitudes. Alternatively, living animals are the primary food source for predators; thus, predators are distributed at all altitudes.

As listed in Table 1, the highest taxa richness and improved Shannon-Wiener index were at S7 (36 and 15.7, respectively) and S10 (33 and 20.4, respectively), and the lowest taxa richness and improved Shannon-Wiener index were at S5 (8 and 3.1, respectively). The taxa richness in the upstream tributary Nianchu was lower than that of the main channel of Yalutsangpo. The average taxa richness in the mid- and downstream tributaries Lhasa, Niyang, and Parlong Tsangpo were higher than in the stem Yalutsangpo. Nevertheless, the two highest densities of macroinvertebrates were 2,440 ind·m^-2 at S9 and 2,415 ind·m^-2 at S14, respectively, while the three lowest densities were 46 ind·m^-2 at S13, 186 ind·m^-2 at S4, and 192 ind·m^-2 at S5, respectively.

Figure 4 compares the alpha and beta diversity indices of macroinvertebrate assemblages in the Yalutsangpo and Juma basins. Both had similar alpha diversity indices; however, the beta diversity was much higher in Yalutsangpo than in Juma. It is indicated that the heterogeneity of macroinvertebrate assemblages and the regional diversity of aquatic ecosystems are much higher in highland rivers than in lowland rivers.

Impact of altitude gradients on biodiversity of macroinvertebrate assemblages

Altitude is considered the most important variable for determining the living conditions of macroinvertebrates in plateau areas (Čiamporová-Zaťovičová et al., 2010). In Europe it was found that taxa richness of macroinvertebrates generally decreased as altitude increased (Brittain and Milner, 2001). However, it was reported that there is an increased number of macroinvertebrate families in the rivers at high altitudes in South America (Henriques-Oliveira and Nessimian, 2010). In this study, it was found that the alpha diversity indices were clearly related to altitudes.

Figure 5(a) shows taxa richness S as a function of altitude H. S increased as H increased in the range of 2,900-3,500 m a.s.l., while S decreased and maintained low values in the range of 3,700-4,900 m a.s.l. Figure 5(b) shows the improved Shannon-Wiener index B as a function of altitude H, which is similar as the relationship between S and H, in that B fluctuated with H. The highest value of B occurred at approximate altitudes of 3,500 m a.s.l., indicating that both density and taxa richness of macroinvertebrate assemblages reached their highest values at these altitudes.

Figure 6 shows the density D and biomass W as functions of altitude H. Similar to the relationship between B and H, the density and biomass fluctuated with altitude, shown as: 1) increase with altitude increases from 2,900 to 3,500 m a.s.l.; 2) decrease with altitude increases from 3,500 to 4,000 m a.s.l.; and then 3) increase with altitude increases from 4,000 to 4,500 m a.s.l. The high density and biomass at the high altitudes were due to the extreme dominance of the few high altitude tolerant taxa, such as Physa Draparnaud, Hippeutis sp., Rhyacodrilus stephesoni, Hydrachnidae, Baetidae, Ecdyuridae, Glossosomatidae, etc. The maximums of D and B also occurred around 3,500 to 3,700 m a.s.l. To be specific, the variations of D and B were very high when the altitudes were among 2,900 and 3,300 m a.s.l. It was considered that the super flow velocity of the mid-downstream sites restrained the density of macroinvertebrate assemblages.

Figure 7 displays cumulative taxa richness S_n as a function of attitude H. Cumulative taxa richness decreased as the altitude increased. Similar tendencies were found in northern Ecuador, South America (Jacobsen, 2004) where S_n increased slowly from 4,500 to 4,000 m a.s.l., and then increased quickly from 4,000 to 3,500 m a.s.l. The richness increased slowly from 3,500 to 2,900 m a.s.l. Coincidently, 3,500 to 4,000 m a.s.l. is where the vegetation belt varies from high-land steppe to complex vegetation with grasses, herbs, shrubs, and woods. Moreover, the main water source for rivers in regions higher than 4,000 m a.s.l. comes from melting snow and ice, whereas at altitudes lower than 3,500 m a.s.l. rainfall is the main water source (Füreder et al., 2001). Milner et al. (2010) stated that there is a large variance in macroinvertebrate assemblages found in rivers with different water sources. For example, the winter stonefly Capniidae was only found at altitudes ranging from 3,500 to 3,700 m a.s.l. Nevertheless, the river patterns were different for rivers with altitudes below and above 3,500 m a.s.l. in the Hindu Kush-Himalayas area (Lu et al., 2011). Therefore, the sharp slope between 3,500 to 4,000 m a.s.l. shown in Fig. 7 was caused by the variation of food resource availability and changes in geomorphology and water resources at different altitudes.

The relationship between the cumulative loss and gain of families and altitude were analyzed. As shown in Fig. 8, the cumulative loss of families increased linearly from around 10 to 50 as the altitude increased from 3,000 to 4,500 m a.s.l., while the cumulative gain of families was less than 10 and rarely changed with the altitude. This resulted in the reduction of the cumulative taxa richness shown in Fig. 7. Furthermore, the regional richness composition of insect families and non-insect groups was compared for different altitude ranges to show the changes of assemblage composition as the altitude increased (Fig. 9). For the whole region, the most family-rich insect orders were Plecoptera, Diptera, Trichoptera, and Epemeroptera. These orders did not vary systematically with altitudes, but most seemed to peak at intermediate altitudes. The regional taxa richness of Plecoptera and Diptera were highest at 3,500 to 4,000 m a.s.l., and Epemeroptera peaked from 3,000 to 3,500 m a.s.l. The regional taxa richness of Trichoptera peaked from 4,000 to 4,500 m a.s.l. Odonata, Coleoptera, Hemiptera, Megaloptera, and Entomobryomerpha account for a minimal amount of family richness. However, they are scattered throughout regions of different altitudes and are essential for sustaining high regional biodiversity.

Effect of environmental parameters on macroinvertebrate assemblages

Many alpine areas have sharp gradients in altitude, and therefore climate, riparian vegetation, and water temperature, all of which affect macroinvertebrate taxa composition (Jeník, 1997; Füreder et al., 2001). In this study, DCA was performed to explore distributions of taxa among the sampling sites and the indication of environmental gradients. A DCA analysis of the 110 taxa identified in the 14 samples was carried out using Canoco 4.5. The densities of taxa at the 14 sampling sites were used as input data. Figure 10 displays the ordination diagram of the sampling sites, which were sorted into five groups. In each group, the sampling sites share similar composition of macroinvertebrate assemblages. Eigenvalues of Axes 1 and 2 of the DCA were 0.685 and 0.481, respectively, and explained 26.3% of the overall variance of the taxa data.

The sampling sites characterized by water temperature were mainly distributed along Axis 1. The sites with water temperatures below and above 10°C were separated on the left side and right side of Axis 2, respectively. Milner and Petts (1994) characterized glacier-fed rivers as having water temperatures below 10°C, and indicated that macroinvertebrate assemblages in glacier-fed rivers were different from those in rainfall streams. Therefore, the varying effects of water temperature on macroinvertebrates may be caused by different sources of river water.

As a result from altitude differences, and thereby climate differences, there are major changes along alpine rivers in both in-stream and riparian conditions, which strongly affect macroinvertebrate assemblages (Burgherr and Ward, 2001). The present study indicated that bed structures and river patterns affected sorting of the sampling sites along Axis 1. The sites characterized by step-pools were plotted on the left of Axis 2, while the sites characterized by braided rivers were on the right of Axis 2. Flow velocity also affected sorting of the sites along Axis 2. The sites with suitable flow velocities (0.3-0.8 m·s^-1) were usually sorted around or below 0, while the sites with unsuitable flow velocities (<0.3 or>0.8 m·s^-1) were sorted far higher than 0. Beauger et al. (2006) indicated that the riverbed tends to be filled and is not very productive with flow velocities below 0.3 m·s^-1; therefore, taxa richness is low. In addition, the flow velocities above 1.2 m·s^-1 constrain most living materials and organisms.

Furthermore, riparian conditions, including riparian vegetation and bank structures, also affected the sorting of sites. For instance, the site S3 shared physical conditions similar to S6, S8, and S9. All were in a braided river with stable cobble beds and suitable flow velocities for macroinvertebrates. The only difference is that S3 had concrete levees while the other three had natural river banks. As a result, S3 was sorted far away from the group of S6, S8, and S9. The main reason was considered that the concrete levees of S3 destroyed the connectivity and integrity of macroinvertebrate habitat and consequently changed the assemblage composition.

Summarizing, the explanation of the DCA diagram indicated that water source, flow velocity, bed structure, river pattern, and riparian conditions all played important roles in structuring macroinvertebrate assemblages in the Yalutsangpo basin. Step-pool bed structures, natural stream banks, and suitable flow velocities were the most important parameters for sustaining the aquatic ecosystem.

Conclusions

The taxa composition and density of macroinvertebrates were very heterogeneous in the Yalutsangpo basin, although taxa richness varied slightly among different sites. Notably, the taxa composition of each site was only a small portion of the regional taxa composition for the whole basin. Therefore, the beta diversity of the basin was relatively higher compared to the low-altitude river with similar habitat conditions and alpha diversities.

Altitude strongly affected the macroinvertebrate assemblages. The taxa richness and improved Shannon-Wiener index were both high at altitudes of 3,300 to 3,700 m a.s.l., as a result of the availability of multiple food and high heterogeneity in habitat conditions. Taxa and functional feeding composition of macroinvertebrates fluctuated with altitudes. More and more families were lost while a few distinctive families were gained as the altitude increased. The functional feeding group “predators” existed in all altitudes, and their density proportion increased as the altitude increased. The scrapers mainly inhabited regions at altitudes of 3,500 to 4,500 m a.s.l. The shredders mainly lived at altitudes of 2,900 to 3,600 m a.s.l. by feeding on fallen leaves of trees, and at 4,000 to 4,400 m a.s.l. by feeding on alpine algae and plateau meadow. The collector-filterers inhabited regions at 3,500 to 3,900 m a.s.l.

In addition, macroinvertebrate assemblages were also affected by water source, flow velocity, bed structures, river patterns, and riparian conditions. Stable stream beds, natural banks, and suitable flow velocities are essential for sustaining high regional biodiversity of the Yalutsangpo basin.

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