Restoration of a Mine Pit Lake from Aquacultural Nutrient Enrichment



Minnesota, the land of 10,000 lakes, also has more than 4000 abandoned quarry pits and over 200 deep, exhausted iron ore pits. In the past 25 years the iron ore pits have gradually filled with groundwater and surface water, forming lakes on the Cuyuna, Mesabi, and Vermillion Iron Ranges in northeastern Minnesota. Most remain abandoned, but besides creating a small number of recreational parks and fisheries, the regional economic development agency promoted approximately 20 of the pit lakes for economic reclamation by using them for salmonid aquaculture. Intensive net-pen aquaculture was carried out from 1988 to 1995 in the Twin City–South and Sherman pit lakes on the Mesabi Range. A water quality controversy resulted over the potential for long-term degradation of the lakes and regional aquifer. The Minnesota Pollution Control Agency then mandated that aquaculture be terminated in Twin City–South in May 1993 and the lake restored to preaquaculture conditions by 1996. With no management other than artificial aeration for one summer, the lake rapidly recovered to near baseline water quality and returned to an oligomesotrophic (unproductive) status. Within 18 months the phosphorus budget was typical of reference pit lakes in the area and dissolved oxygen in bottom water remained above ∼4 mg O2/L without artificial aeration. Algal growth was low in 1993, due to light limitation from artificial mixing, but it remained low in 1994 without any management due to renewed phosphorus limitation. Inorganic nitrogen initially decreased faster than expected, at a rate similar to its increase during intensive aquaculture. More rapid reductions in water column nutrients might have occurred in 1993 by reducing aeration to allow anoxia in the lower hypolimnion, promoting denitrification and minimizing sediment resuspension, but this was precluded by water quality standards. The “natural” burial of solid wastes under inorganic sediment eroded from the basin walls effectively minimized transport of sediment nutrients to the overlying water. Fallowing for several years provided a simple, effective method for restoration of these pit lakes from aquacultural impacts. No change attributable to aquaculture was observed in the water quality of three nearby pit lakes, including a drinking water source. This fact suggests that there were few or no impacts from off-site migration of aquaculturally enriched water into the regional aquifer.