Response of phytoplankton to ecological engineering remediation of a Canadian Shield Lake affected by acid mine drainage

Multivariate analyses were used to relate changes in water chemistry with those in the composition of the phytoplankton in two adjacent northern Ontario shield Lakes in Canada. One of the lakes is affected by both acid mine drainage (AMD) and a succession of remediation measures. The other, much larger lake is pristine and was monitored for both water quality and phytoplankton. The AMD lake, already contaminated at the start of the 17-year project, experienced further increases in Zn concentrations from around 10 mg L−1 to 35 mg L−1 and a pH decrease from 5 to 3 during the project. Over the same period, phytoplankton diversity declined from 52 taxa in 59 samples to 42 taxa in 74 samples. Only Ochromonas spp. and Chlamydomonas spp., apparently tolerant of a wide range of physical and chemical conditions, were consistently present in both lakes. Other taxa (Peridinium, Pinnularia and Euglena) were frequent in the AMD lake in the early stages of the project but then declined. Asterionella and Botrycoccus (although common in the pristine lake), were rarely observed in the AMD lake. Lepocinclis proliferated as metal concentrations increased and pH declined in the AMD lake but was rarely found in the clean lake.Such gradual adaptation or changes in phytoplankton populations have not been reported previously for such extreme chemical conditions. The multivariate statistical analysis (PCA, CCA, DCA) revealed that the highest number of taxa were found to be controlled by pH and Ni in the years 1986–1992.One-third of the taxa in that period were more closely associated with pH alone. In the late stages of the study (1999–2003) the predominant controlling factors appeared to be metals and conductivity. Most of the taxa followed the time/pH/metal axis, suggesting that occurrence changed with slowly changing water conditions. Those taxa which fell outside the time/pH/Zn/Cu/Al axis included mostly Chrysophytes, Cryptophytes, and Chlorophytes, although there were some Euglenophyceae and diatoms in this group. By the end of the study in 2003, the combined effect of the restoration measures brought about the development of an extensive periphytic underwater meadow, along with a slight improvement in water quality and an increase in the number of phytoplankton taxa.