Recovery time of macroinvertebrate community from Cd pollution in Longjiang River, Guangxi, China

Estimating recovery times from pollution incident is an important issue of targeted biomonitoring programs. In the present study, the impact and recovery of macroinvertebrate communities from a cadmium wastewater discharge in the Longjiang River, Guangxi, China, in early January 2012 were studied based on 83 samples collected in five surveys within 20 months after the incident. The pollution affected seriously the local aquatic biota, and consequently, the invertebrate abundance and species richness were reduced considerably. Twelve months later, the taxonomic number of macroinvertebrates began to increase. However, sensitive taxa remained rare. Twenty months later, the taxon richness and abundance of macroinvertebrates increased significantly compared to those in the previous four time points. To explore the possible time-scale over which pre-disturbance conditions might occur, we chose four different typical metrics of taxa richness (total taxa number, cumulative taxa number, taxa number per samples, and the Shannon-Wiener diversity index) and extrapolated modeled recovery trajectories. Target values for the four metrics were set at average values for sites from the nearby Lijiang River, which were used as a reference. Assuming a continued linear trajectory, the recovery times were estimated to be 52, 39, 39, and 31 months, respectively, which was roughly 3-5 years. This is consistent with results from recovery times from other studies of acute pollution cases, but contrasted strongly to the much longer recovery times associated with chronic pollution from groundwater contamination and mine-tailing runoff.