Spatial Distribution of Macrozoobenthos in a Large and Deep Impoundment: Xin’anjiang Reservoir, Zhejiang Province

Abstract

The horizontal and bathymetrical distribution pattern of macrozoobenthos in Xin’anjiang Reservoir, a large, deep reservoir in Zhejiang Province, China, was investigated from 2007 to 2009. Macrozoobenthos was sampled and environmental variables were measured at five sites along the upper to lower reaches (S1 at upper reaches, S3 and S4 at middle reaches, and S8 and S9 at lower reaches) in 2007 to 2008. Benthic macroinvertebrates were also collected over three transects in 2009, i.e., T1, T4, and T8, traversing S1, S4, and S8, respectively, and 10–15 sites were set along each transect. Totally, 24 taxa from 5 classes were recorded. Oligochaeta predominated in the macrozoobenthic community in terms of important value (IV), density, and standing crop, among which Limnodrilus hoffmeisteri was prevailing. The annual average density and biomass were 793.8 ± 92.1 ind. m−2 and 2.25 ± 0.32 g m−2, respectively. There were no clear differences in density and biomass between seasons, but significant horizontal differences revealed a maximum in the upper reaches and minimum at the lower reaches. Both density and biomass negatively correlated with water depth and Secchi depth (SD); density positively correlated with total nitrogen (TN) and total phosphorus (TP) of mixed water samples from the water column (MWSWC) and with TP of local water samples at the bottom (LWSB). Biomass positively correlated with TP of MWSWC. Only water depth (from eight physical and chemical variables) was selected by a stepwise regression model to explain the variation in density and biomass of macrozoobenthos in 2007 to 2008. Bathymetrically, both density and biomass increased with water depth, peaked just below the thermocline, and then decreased gradually. This numerical bathymetric change in Xin’anjiang Reservoir conformed to the patterns of vertical distribution of zoobenthos in oligo- and mesotrophic lakes. The relationship of density and biomass with the main environmental variables, the bathymetric distribution pattern of macrozoobenthos, and the two biological indices (Wright index and Carlander’s biotic index) all typified Xin’anjiang Reservoir as oligo- and mesotrophic, with the upper reaches mesotrophic or moderately polluted and the middle and lower reaches oligotrophic or slightly polluted. Three other biological indices (King, Goodnight–Whitley, and Shannon–Weaver) were unsuitable to assess water quality.