Submerged macrophytes establishment and growth on flooded nickel tailings

Nickel tailings were deposited between 1978 and 1988 in Falconbridge's New Tailings Area (NTA) located northeast of Sudbury, Ontario, Canada. The site was decommissioned and flooded in 1996 by constructing new dams and dredging, which formed the Upper Terrace (56 ha) and Lower Terrace (30 ha). Water covers minimize the oxidation of acid generating tailings but some oxidation and metal release may still occur. The water cover's effectiveness could be improved by establishing aquatic macrophytes to control tailings resuspension, remove metals from the water column and develop an organic layer to consume oxygen and support sulphate reducing bacteria. Nitrogen and phosphorus deficiencies are known to restrict terrestrial vegetation on mine tailings and to limit aquatic macrophytes growth in natural systems. Nutrient limitations may also inhibit submerged macrophytes establishment on the flooded tailings. To test this hypothesis, a growth comparison study and productivity surveys were conducted.;In the growth comparison study Potamogeton richardsonii was grown on Falconbridge nickel tailings in 1998 and 1999 at Falconbridge's New Tailings Area (NTA) and Rio Algom's Quirke Waste Management Area (QWMA). Growth on Quirke uranium tailings, Robinson Lake substrate and Depot Lake substrate was also tested. At each site, six 20 L pails per substrate were placed at a depth of 1 m. Ten Potamogeton richardsonii shoot segments, harvested from Robinson Lake, were sandwiched between sections of wire mesh and placed on the sediment surface in each pail. In mid August, after 60 days, the shoots and roots were harvested, dried and analyzed for nutrients. Substrate samples were analyzed for total and dissolved nutrients.;Potamogeton richardsonii shoot nitrogen and phosphorus concentrations were above levels that would be considered potentially growth limiting in the growth comparison study. However, concentrations decreased as biomass production increased indicating that, with time, deficiencies might occur. Biomass production was also influenced by year, site and substrate effect. At QWMA in 1998, dry summer conditions resulted in low water levels that restricted biomass production. At NTA total biomass and shoot/root biomass ratio were lower than at QWMA due to water column sulphur and total suspended solids. Biomass production was highest on Depot Lake substrate due to higher available nutrients and lower metals. Biomass production was similar on Falconbridge tailings and Robinson Lake substrate and lowest on Quirke tailings due to nutrient deficiencies.;In the Lower Terrace productivity surveys (1999, 2001 and 2003) submerged macrophytes biomass production was assessed on 121, 0.25 m2 plots established every 10 m along five transects. In each plot, water depth was measured and above ground biomass was harvested then sorted by species and air dried. Potamogeton pusillus shoots from 58 plots were prepared and analyzed for nutrients.;In the 1999 productivity survey, submerged macrophytes diversity was highest and Potamogeton pusillus and Chara vulgaris were the dominant species. Potamogeton pusillus shoot nitrogen and phosphorus were above the 1.3% and 0.13% concentrations that would be considered potentially growth limiting. Macrophytes establishment was influenced by the site's physical characteristics (size, shape, water depth) and the location where macrophytes introduction occurred. By 2003, Potamogeton pusillus was present in 117 of 121 plots with a mean biomass of 49.3 g/m2 but Chara vulgaris had died off, reducing total biomass production to half the 2001 levels. Shoot nitrogen and phosphorus had fallen below the 1.3% and 0.13% concentrations which indicate nutrient deficiencies.;Nitrogen and phosphorus deficiencies did not inhibit submerged macrophytes establishment but with time deficiencies reduced biomass production. It is likely that biomass production will continue to decline until equilibrium is reached. Fertilizer amendments in the Lower Terrace could improve submerged macrophytes productivity and the effectiveness of the water cover. The method of fertilizer amendment (water column verses substrate application) would need to be investigated to determine the optimum application method. Regular productivity surveys would document the changes in biomass production and species composition that will continue to occur with or without fertilizer amendments.