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 2 5 years the iron ore pits have gradually filled with groundwater and s urface 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 regio nal economic development agency promoted approximately 20 of the pit l akes 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 qu ality controversy resulted over the potential for long-term degradatio n of the lakes and regional aquifer. The Minnesota Pollution Control A gency 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, th e lake rapidly recovered to near baseline water quality and returned t o an oligomesotrophic (unproductive) status. Within 18 months the phos phorus budget was typical of reference pit lakes in the area and disso lved oxygen in bottom water remained above similar to 4 mg O-2/L witho ut artificial aeration. Algal growth was low in 1993, due to light lim itation from artificial mixing, but it remained low in 1994 without an y management due to renewed phosphorus limitation. Inorganic nitrogen initially decreased faster than expected, at a rate similar to its inc rease during intensive aquaculture. More rapid reductions in water col umn nutrients might have occurred in 1993 by reducing aeration to allo w anoxia in the lower hypolimnion, promoting denitrification and minim izing sediment resuspension, but this was precluded by water quality s tandards. The ''natural'' burial of solid wastes under inorganic sedim ent eroded from the basin walls effectively minimized transport of sed iment nutrients to the overlying water. Fallowing for several years pr ovided a simple, effective method for restoration of these pit lakes f rom aquacultural impacts. No change attributable to aquaculture was ob served in the water quality of three nearby pit lakes, including a dri nking water source. This fact suggests that there were few or no impac ts from off-site migration of aquaculturally enriched water into the r egional aquifer.