Phosphorus budget and productivity of an experimental lake during the initial three years of cage aquaculture

An experiment was conducted at the Experimental Lakes Area (ELA; Ontario, Canada) to investigate the effect of rainbow trout (Oncorhynchus mykiss) cage aquaculture on the phosphorus budget and productivity of a small Precambrian Shield lake during the first three years of fish farming. Annual inputs of phosphorus to the experimental lake from aquaculture waste (67-100 kg) exceeded combined annual inputs from inflows and direct atmospheric deposition (418 kg). Compared with a reference lake, hypolimnetic dissolved oxygen concentrations in the aquaculture lake decreased by 36% and hypolimnetic ammonium and total phosphorus increased by 120% and 35%, respectively. However, compared with an epilimnion-fertilized lake, hypolimnetic dissolved oxygen in the aquaculture lake was 255% greater and epilimnetic suspended carbon and chlorophyll a was lower by 64% and 85%, respectively. Epilimnetic suspended carbon and hypolimnetic dissolved oxygen concentrations during aquaculture were more similar to concentrations measured in a hypolimnion-fertilized lake. Although the experimental lake remained oligotrophic during the initial three years of aquaculture, hypolimnetic dissolved oxygen decreased each year and hypolimnetic ammonium and total phosphorus increased each year, which suggests that the lake did not reach a steady state.