Community structure of rotifers and ecological assessment of water quality in Modaomen of the Pearl River Estuary
Rotifers are a key part of the zooplankton community and react to changes in water quality. Rotifer abundance and species composition reflect the conditions and nutritional status of bodies of water, and, therefore, form a basis for the evaluation of water quality. Rotifer community structure was investigated at three different sites in the Pearl River Estuary: S1, a stagnant water district; S2, the center of the river; and S3, the confluence of rivers at the Modaomen outlet. Samples were obtained twice monthly from July 2009 to January 2010. Water temperature, salinity, transparency, chlorophylla (chla), total nitrogen (TN), total phosphorus (TP) and other physicochemical factors were measured simultaneously. The comprehensive trophic state index, TLIc, was evaluated using abiotic environmental factors. Also ecological assessment of water quality was carried out through not only rotifer species composition but also by theE/O,QB/T and biodiversity indices. During the survey period, water temperature ranged from 12~29℃and salinity ranged from 0.11~7.9 PSU. The concentration of Chla, which ranged from 0.89~9.56μg·L-1, was positively correlated with water temperature (p<0.0001) and negatively correlated with salinity (p<0.01). The TLIc at the three sites ranged from 50-60, which suggested that water quality reached mild eutrophication. The degree of eutrophication descended in order from S1 to S2 and S3. During the survey period, 56 species of rotifer were recorded, of which 42 were pollution indicators. Rotifer species were highest at S1. The proportion of pollution indicator rotifer species of the total showed that the water in the Modaomen Estuary reached the eutrophication level, consistent with TLIc.Polyarthra vulgaris,Keratella cochlearis,K. tropica andTrichocerca pusilla were the dominant species. The rotifer abundances at S1, S2 and S3 ranged from 0~199 ind·L-1,1~54 ind·L-1and 0~48 ind·L-1, respectively, with mean abundances of 31 ± 54 ind·L-1, 13 ± 15 ind·L-1 and 11 ± 13 ind·L-1. The highest abundance was recorded at S1 in the late August; the abundances in the dry season were lower. Rotifer abundances were positively correlated with water temperature, TN, and Chla.(p<0.05). A significant negative correlation was detected between salinity and rotifer abundance (p<0.05). The physicochemical factors in the Modaomen outlet of the estuary changed dramatically and the diversity index of rotifers fluctuated widely, which indicated that the community structure of rotifers was unstable. Redundancy analysis showed that water temperature, salinity and Chla. were the most important factors influencing the seasonal variations of rotifer community structure. The ecological assessment of water quality indicated that the Modaomen outlet of the estuary had reached a level of mild eutrophication, in accordance with the comprehensive evaluation of trophic state based on abiotic environmental factors. The results show that a comprehensive evaluation combining ecological indicators and physical-chemical indicators is the most realistic way to assess water pollution.