Study On Macroinvertebrate Ecology Of Potholes In Mountain River

The pothole is a product of the interaction between river flow and riverbed, which is a constituent part of bed micro-topography and also is a special habitat in the river ecosystem. Potholes occur in impermeable bedrock, which are well-defined and easily controlled with short food-chain. Thus, the pothole often serves as a model system in ecological studies. Macroinvertebrates are an important component of river ecosystems, and are also an indispensable connection link in energy flow and matter cycle. The effects of environment factors on community structure of macroinvertebrate are very complex: there are numerous environment factors and the effects of such environment factors on different macroinvertebrate groups are variety. Simultaneously, the relationship between environment factors and macroinvertebrate community structure as a research hotspot is one of the most basic and important research subjects. In this paper, the macroinvertebrates of potholes in a typical mountain river(Wubu River in Chongqing Municipality) were studied through field investigations and in situ experiments, including the current situation and the characteristics of pothole shape, physical and chemical factors in the water of potholes, composition and distribution of the macroinvertebrate within potholes, influence of environmental factors on community structure and distribution of macroinvertebrates in potholes, difference between macroinvertebrate in potholes and benthic macroinvertebrate. The main contents and research results are summarized as follows:(1) In this study, 179 potholes in total(including long axis, short axis, and depth of the potholes) were analyzed in two sampling points in the Wubu River. The results showed that most potholes are of ellipse shapes, while a few of them are of suborbicular type and extreme ellipse shapes. The proportion of 1.0-1.2 long-to-short axis ratio of the potholes is the highest, and diameter and depth are mostly distributed within 10-20 cm. The direction of long axis is mostly distributed within 290°-330° and 50°-80°. In most of cases, the frequency of pothole oblateness is 0.1-0.4. According to the results of the depth and width ratio of the potholes, deep type and shallow type potholes are the majority, whereas the proportions of extremely deep and extremely shallow pothole are low. The long axis and short axis, diameter and depth of the potholes are strongly(log) positive correlated. The results indicated that the pothole shape is closely related to jointing, lithology, hydraulic erosion, suspended sediment entrained within the vortices in mountain rivers.(2) Ten physical and chemical factors of 80 potholes were measured for four seasons, and then were compared with that of the surrounding river surface waters. The results indicated that the pH value and conductivity of river surface waters were significantly higher than in pothole habitat, whereas opposite results were obtained with the content of total phosphorus, total nitrogen and ammonia nitrogen. In the potholes, the concentrations of total nitrogen content of water, ammonia nitrogen(spring, autumn and winter) and total phosphorus(autumn) decreased with the increasing area of the potholes. However, pH value(winter and spring) and conductivity(winter) increased with that of the potholes. Water temperature, the contents of total dissolved phosphorus and orthophosphate content in summer were found higher than those found in other seasons; pH value in autumn was higher than that of other seasons. Principal components analysis(PCA) on water parameters of potholes habitat indicated that water temperature, sulphate content, conductivity, total phosphorus content, pH value, nitrate nitrogen and ammonia nitrogen content could represent the habitat characteristics of potholes. The results showed that the concentrations of physical and chemical factors changed with spatial and temporal change, and was influenced by seasons and pothole area in pothole habitat of mountain rivers.(3) Biodiversity of macroinvertebrate in 280 pothole habitats was investigated for four seasons. The results showed that the frequency of pebble, boulder and algae increased with the increase of pothole area, i.e. the larger pothole contained the more complexity pothole habitat. A total of 43 macroinvertebrate species were identified in pothole habitat with an average of 7.3 species in per pothole. The greatest number of species was found in summer(36 species) with an average of 7.2 species in per pothole, whereas the lowest number of species was found in winter(28 species, 5.9 species in per pothole). In both autumn and spring, 32 species were found in the potholes, but the average number in per pothole differed: 8.5 species and 7.5 species, respectively. Shannon-Wiener index was found highest in spring, which is significantly higher than that of the remaining seasons. Margalef richness index was recorded significantly higher in spring than in winter and summer. Significant higher macroinvertebrate biomass was obtained from summer, in contrasted to that of winter and spring. The density of macroinvertebrate within potholes varied with time, the peak of density present in autumn while the valley value appeared in winter. The Shannon-Wiener index, Margalef richness index and number of individuals of macroinvertebrate increased with the increase of potholes area, and there is no obvious relationship between biomass and the potholes area. Collector-gatherer, predator and scraper are the dominant functional feeding groups in pothole habitat. Principal components analysis(PCA) well indicated the spatial and temporal distribution patterns of macroinvertebrate community structure in the potholes. Potholes with larger area clearly harbor more species, and both the richness and abundance of species are closely related to seasons. The results of Redundancy Analysis(RDA) showed that the water temperature, pothole area and water depth are important environmental factors that influence the community structure of macroinvertebrates in the potholes of mountain rivers. Season, pothole area, habitat heterogeneity, water environment conditions and capacities of dispersal are important environmental factors that influence the community structure of macroinvertebrates in the potholes of mountain rivers.(4) Macroinvertebrate and water samples were collected in 30 potholes in Wubu River to analyze the physicochemical properties, the composition and distribution of macroinvertebrates and the effects of environmental factors on the macroinvertebrate community structure. A total of 8111 specimens were collected and were identified as 31 species, belonging to 14 orders and 25 families and with an average of 9.27 species and 270.37 individual per pothole. Caenis sp., Baetis sp., Cinygma sp. and others(8 species in total) were recorded as occurrence frequency > 50 %. The average valued of Shannon-Wiener index, Margalef richness index and Pielou evenness index were 2.06, 1.55 and 0.65 per pothole, respectively. Collector-gatherer was the dominant functional feeding group in pothole habitat, which was significantly higher than collector-filterer, scraper and shredder; shredder is the least and is distinct fewer than that of other functional feeding groups. Both macroinvertebrate richness and abundance were found significantly positive correlated with pothole area. The results of Redundancy Analysis(RDA) showed that the dissolved oxygen, total phosphorus and nitrate nitrogen concentration were main environmental factor that influence the community structure of macroinvertebrates in the potholes. The results indicated that the community structure, distribution and characteristics of functional feeding group of macroinvertebrates are closely related to the concentrations of physical and chemical factors, and the contents of total phosphorus, nitrate nitrogen and dissolved oxygen are main environmental factor that influence the community structure of macroinvertebrates in the potholes of mountain rivers.(5) Field investigation on the composition and distribution of river benthic macroinvertebrate community were also carried out, and the results were compared with those obtained from pothole habitat. The results demonstrated that in river benthic habitat, Cinygma sp. and Leptophlebiidae were common dominant species during the whole sampling seasons, additionally, Baetis sp., Caenis sp., Limnoperna fortunei, Camptochironomus sp. were common dominant species in river benthic habitat in different seasons. Collector-filterer, collector-gatherer and scraper were the dominant functional feeding groups in river benthic habitat, and shredder was the least. Some species were dominated in both habitats, but the species richness of macroinvertebrate in pothole habitat was higher than that of river benthic habitat. The functional feeding groups differed between river benthic and potholes habitat. The analysis of Similarities showed that these of two communities was different(R = 0.415, P < 0.05). Cluster analysis and non-matric multidimensional scaling analysis also illustrated that these macroinvertebrate community structures differed between two habitats(stress < 0.1). The results present above indicated that the presence of pothole habitats in mountain rivers not only provide habitats and refuges for aquatic macroinvertebrates, but also the presence of pothole habitats was help to increase and protect macroinvertebrate diversity of mountain rivers.