1. Introduction
Studying and protecting highland (i.e., high altitude) running-water biodiversity has gained growing interest over the world, because of more pristine conditions and higher sensitivity to environmental changes in highland rivers than in lowland rivers [
1]. The Qinghai-Tibet Plateau is usually called “the water tower of Asia” due to its active water cycle and because it serves as the headwaters for most of the largest Asian rivers. The plateau representing an ecologically interesting transition between the Palaearctic and Oriental regions, covers an area of about 2.3 million km
2 with an average altitude of over 4000 m a.s.l., and includes the topographically highest and least studied lotic ecosystems [
2]. Its environmental and ecological conditions have been the focus of attention for decades owing to its unique geographical location and ecological functions including water resource conservation and biodiversity protection [
3]. It has been expected to have novel relationships of macroinvertebrate communities with stream habitat variables since highly diverse and unique assemblages of aquatic Protozoa, Insecta, Hirudinea, Rotatoria, and Crustacea have been reported [
4,
5].
However, climate change and anthropogenic activities have significantly influenced the physical and ecological conditions of the headwaters on the plateau [
6]. Temperature and precipitation variations were regarded as the main driving forces for such changes [
7]. The plateau river ecosystems appeared to be suffering decreasing surface runoff, shrinking lakes and wetlands, and conflicts between ecological protection and social economic development under the increasing influences of global warming and human disturbance [
8]. Understanding the aquatic biodiversity, especially the relationships of biodiversity with major geographical and other environmental variables of the plateau is important for protecting the highland ecology and the headwater ecosystem functions of large Asian rivers.
Macroinvertebrates are among the most important components of the aquatic ecosystem and have been considered as useful indicators for analyses of relationships of biotic communities with stream habitat variables because of their confinement to the river bed, limited movement abilities, long life cycles, and sensitivity to environmental changes [
9]. Xu et al. [
5] reported the major features of the macroinvertebrate communities in several sections of the main stem and tributaries of the Yalutsangpo River. Jiang et al. [
2] analyzed the longitudinal patterns of macroinvertebrates in relation to environmental factors in the Niyang River, the largest first-order tributary of the Yalutsangpo. Pan et al. [
10] investigated the macroinvertebrates in the Sanjiangyuan river basin at the center of the plateau. These studies consistently indicated that the compositions of macroinvertebrate assemblages in the plateau were significantly associated with the high altitudes and the variation trends of biodiversity might be different from that of the lowland rivers. However, how the macroinvertebrate communities responded to altitude gradient and the other environmental variables associated with altitude was not clear from these three studies [
2,
5,
10].
According to the River Continuum Concept (RCC) theory, as a river changes along the altitude gradient from headwaters to the lower reaches, the physical geomorphological features change and thus structural and functional characteristics of stream communities are adapted to the most probable position or mean state of the physical habitat; and therefore, there will be a change in the relationship between the production and consumption (respiration) of the material [
11]. The hypothesis of this study was that the macroinvertebrate communities and their environmental variables varied with altitude, and a greater altitude gradient could attribute larger variations of macroinvertebrate community structure. The objectives were (1) to illuminate the differences in the macroinvertebrate communities and their environmental variables between the rivers with larger altitude gradients (highland rivers) and the rivers with smaller altitude gradients (lowland rivers); and (2) to document the influence of altitude gradient on macroinvertebrate assemblages and their environment variables.
We investigated the highland headwater rivers, including the Yalutsangpo River and the source region of the Yellow, Yangtze, and Mekong Rivers (called Sanjiangyuan in
Figure 1a) on the Qinghai-Tibet Plateau, and the lowland rivers: Songhua River, Juma River, and East River (
Figure 1a). Comparison based on statistical analyses was carried out to explore the variations in macroinvertebrate communities and stream habitat variables along the altitude gradient. With the intent to analyze in detail the missing difference in altitude, in particular between 3500–<1000 m a.s.l., more detailed sampling were carried out in the Yalutsangpo River basin (
Figure 1b), in which the altitude gradient was among the largest on earth. Further, based on the detailed samplings, the determinative physical variables affecting the macroinvertebrate assemblage composition were identified along the large altitude gradient.
4. Discussion
Altitude has been considered as the most important variable determining the living conditions of macroinvertebrates in plateau areas [
23]. Xu et al. [
5] showed that assemblage structures and functional feeding compositions of macroinvertebrates were closely related to altitude. This study indicated that the local biodiversity may not be significantly affected by altitude, while the high regional biodiversity of highland rivers was obviously associated with the high altitudes, and especially the high altitude gradients, which created the high habitat heterogeneity. Füreder et al. [
24] showed similar findings, that in high altitude regions great altitude gradients and the physical variables associated with the altitude, such as climate, riparian vegetation, and water temperature affected macroinvertebrate assemblages in terms of the functional feeding groups and biodiversity. Especially, water temperature and channel stability were regarded as the principle parameters determining the macroinvertebrate community [
25].
In this study, it was found that the environmental parameters of aquatic habitat, including dissolved oxygen; water temperature; and medium grain size, which essentially determined the channel stability, changed significantly as altitude decreased. Following the Yalutsangpo, as the altitude descended from 4000–5000 m to 3000–4000 m and then to 2000 m, the riparian vegetation changed from cold desert to arid steppe and then to deciduous scrub, which resulted in changes in taxa composition and the formation of a high regionally diverse macroinvertebrate community. For the central plateau headwaters, the Sanjiangyuan possessed a less diverse macroinvertebrate community compared with the Yalutsangpo, owing to its smaller altitude gradients and less variations in the altitude-associated environmental conditions [
26].
Milner and Petts’s conceptual model [
25] indicated that turbidity, discharge, sediment transport, and channel form all affected the channel stability and consequently influenced the biotic communities by influencing primary productivity, habitat suitability, attachment, and production of macroinvertebrates and fish. The CCA analyses of the Yalutsangpo (this study) and the Yellow River Source Region [
26] both confirmed that the diversity of the macroinvertebrate assemblage was positively correlated with the heterogeneity and stability of substrates. It was believed that coarse substrates provide a wide range of refuges and principal habitats for macroinvertebrates [
27]. In addition, our CCA analyses also indicated that water temperature was another determinative parameter influencing the assemblage composition, which was also consistent with the findings of Milner and Petts [
25]. As most of the source water in the Yalutsangpo was glacial in origin, the aquatic biota was adapted to persistent low temperature [
28]. Many psychrophilic taxa, such as Rhyacophilidae, Enchytraeidae, Ecdyuridae, etc. were indeed sampled in our investigation. Similarly, psychrophilic taxa were also commonly seen in the Sanjiangyuan basin with the average altitude higher than 3000 m [
26]. In general, as altitude decreased to less than 2000 m, water temperature increased because of less glacial origin water input, as a result, less psychrophilic taxa were present.
In addition to the variables influencing macroinvertebrates that were revealed in the CCA analyses, flow velocity was also suggested as a significant predictor of the community variance by Jiang et al. [
2] in their study of the Niyang River, the tributary of Yalutsangpo. Flow influences many important structural attributes of stream ecosystems such as substrate stability, habitat volume and channel morphology [
29]. Extremely high velocity (>3 m/s) can cause bedload movement, which destroys aquatic habitat [
14,
30]. Zhou et al. [
31] indicated that in the Yalutsangpo River the flow velocity and stream power increased as the altitude descended and consequently most of the taxa that preferred low or medium stream power conditions disappeared, while only the taxa that either have strong attachment or swimming abilities could survive. No macroinvertebrate was captured from the Yalutsangpo Grand Canyon during our study, confirming that the high current velocity restrained the aquatic biota.
Therefore, according to the macroinvertebrate biodiversity trends and the relationships of macroinvertebrates with stream habitat variables explored in this study, the following strategies for conservation of such highland headwater systems are proposed: (1) Donserving the high regional heterogeneity of aquatic habitat in different altitude regions, especially in 3000–4000 m, because the taxa composition varied among the different altitude regions and the high habitat heterogeneity supported high regional diversity of aquatic biota; (2) protecting suitable habitat conditions, including natural stream conditions, suitable flow velocity, and riparian vegetation conditions from anthropological disturbance to sustain suitable habitat conditions for diverse macroinvertebrate assemblages. Step-pool systems, stable gravel beds, and riparian wetland zones were suggested to be preserved or restored for suitable stream condition. Channelization, gravel mining, and riparian vegetation deterioration should be avoided, especially in the altitudes of 3000–4000 m. For the Yalutsangpo Grand Canyon, reasonable adjustments like increasing river bed roughness structures or knickpoints to reduce the extremely high flow velocity could be beneficial for aquatic biota.
Moreover, as in the remote areas, the plateau headwaters were nearly-pristine and well-preserved, and are supposed to have good water quality and little pollution. Owing to the harsh natural conditions of the high plateau, a gentle disturbance could result in a great deterioration in the eco-environment. For instance, as detected by Liu et al. [
26], the macroinvertebrate assemblages varied significantly from the headwaters to the tributaries and the main stem of the Yellow River source: fewer EPT taxa, more Chironomidae and more Oligochaeta were found in the headwater streams suffering livestock grazing and nitrogen enrichment owing to the low flow discharge of the headwater and low denitrification rate under the low temperature plateau condition. In summary, sustainable expansion of anthropological activities should pay more attention to conservation of the plateau aquatic ecosystem. Nevertheless, considering the ongoing climate change and the plateau being especially sensitive to climate change, the water temperature may change and cause disturbance to the aquatic ecosystem due to changes of water sources, e.g., may include more melting water. Such changes may cause a significant impact on the aquatic communities and should be studied further for better conservation.