Using Benthic Algal Communities To Assess The Impacts Of Small Hydropowers On River Ecosystem

In order to explore the impacts of the small hydropowers (SHPs) on the river ecosystem, several researches based on benthic algae were carried out in Xiangxi River: 1) Changes of benthic algal communities following a low-head dam construction. 2) Developing a benthic algae based Index of SHP's Disturbance (B-ISHD) to assess the influence of the cascade small hydropowers on benthic algal communities in Xiangxi River system. 3) Effects of a small hydropower on benthic algal communities in different months. The main results are as follows:1. We chose a small hydropower station (named Luyuan hydropower station, run in August 2003), which is located on a tributary of Xiangxi River. Monthly sampling of 5 sites (JC09, JC08, JC05, JC03 and JC02) was carried out from February 2003 to August 2004 and a modified'Before-After-Control- Impact'(BACI) was used to study the responses of benthic algal communities downstream of the dam. Benthic algal sampling yielded 130 taxa (mostly to species levels) in 40 genera, which belonged to Bacillariophyta (107 taxa), Chlorophyta (11 taxa) and Cyanophyta (12 taxa), and diatoms were predominant with 69.81 % of the total taxa. Results indicated that some physical factors (flow velocity, depth and channel width) and 5 algal metrics (including the density of Cymbella individuals, % erect individuals, total density of diatoms, richness and Margalef diversity) were significantly affected by SHP, while other chemical factors (NH4-N, TN, SiO2, etc.) were not influenced. However, overall assemblage structure downstream remained similar to upstream control sites throughout the study by means of Non-metric Multidimensional Scaling ordinations (MRPP, p>0.05).2. In order to explore the impacts of the cascade development of small hydropowers on the river ecosystem, a total of 92 sites of 23 SHPs (5 sites were placed on each SHP, and site1 is located at 100m upstream the dam which formed by intake of SHP, site2 just upstream the dam, site3 just downstream the dam, site4 is about 50m upstream the SHP's outlet, while site5 is mounted at outlet of SHP) within Xiangxi River and other 4 reference sites (R1-R4) were sampled from October 20 to November 1, 2005.Periphyton sampling yielded 150 taxa (mostly to species levels) in 38 genera, which belonged to Bacillariophyta (133 taxa), Chlorophyta (8 taxa) and Cyanophyta (9 taxa), and diatoms were predominant with 78.05 % of the total taxa. Achnanthes linearis and Cocconeis placentula were the most abundant species, whose relative abundances were 60.82 % and 10.22 %, respectively.All the sites were divided into 5 groups based on their habitats. Group 1 was reference group including R1-R4 and site 1 of all SHPs; all SHPs'site 2 to 5 made up of Group 2 to 5, respectively. Then, we tried to comparing differences of physicochemical factors among them by one-way ANOVA, and the results indicated that physical factors (flow velocity) were the main factor affecting the aquatic ecosystem.91 potential attributes belonging to 13 categories were selected originally. Based on the selection methods, final 7 attributes (Chl a, the densities of 3 dominant genera, % Cocconeis individuals, Margalef diversity and richness) were selected to develop a benthic algae based Index of SHP's Disturbance (B-ISHD). Based on B-ISHD, we found that segment between the dam and the outlet was severely influenced by SHPs.3. In order to study the temporal impacts of SHP on benthic algal communities, we chose a typical station (Cangpinghe hydropower station) and carried out monthly sampling from November 2005 to June 2006.A total of 119 taxa (mostly to species levels) in 32 genera were identified, which belonged to Bacillariophyta (104 taxa), Chlorophyta (6 taxa) and Cyanophyta (9 taxa), and diatoms were also predominant with 61.90 % of the total taxa.Two-way ANOVA indicated that water flow velocity, width and depth were significantly different among not only different sampling dates but also sampling locations (p<0.05), while pH, turbidity (TURB), dissovled oxygen (DO), water temperature (WT) and ORP were only different among different sampling date (p<0.05). Benthic algae based Index of SHP's Disturbance (B-ISHD) was employed to assess the impacts of water abstractions on river ecosystem. All the sites were divided into 3 groups (G1-G3). Sites located upstream of dam were sorted into G1, and G2 included the sites between the dam and the outlet while sites downstream the outlet made up of G3. B-ISHD scores of three groups (G1-G3) showed a remarkable seasonal fluctuation (F values were 11.24, 17.14, 40.35, respectively and all the p values <0.0001). We set standard as: >75: excellence, 50-74: good, 49-25: fair, <24: poor, then, apart from November and June, which were good, all the B-ISHD scores of first group 1 (G1) are excellence; the B-ISHD scores of group 2 (G2) varied dramatically–from November 2005 to March 2006 it increased gradually (from fair to good) and then decreased until June 2006; the B-ISHD scores of group 3 (G3) were good or excellence except for November (fair). During the first 4-month sampling period, B-ISHD scores were significantly different among three groups (F values were 10.18, 17.51, 8.18 and 6.53, respectively; all the p values <0.05), but from March to June 2006 were not (all the p values >0.05). B-ISHD scores of two sampling occasions (November and June) were relative lower than others, which ranged between good and fair.The impacts of water abstractions on benthic algal communities were determined by the duration of drought. Even though in rainy seasons, the influences induced by SHPs on algal communities would be significant if the duration of drought was long enough (e.g. June 2006). On the other hand, if we kept connective of downstream and upstream reaches in dry seasons, impacts on algal communities of SHPs could also be ignored.