Experimental freshwater cage aquaculture: Short term effects on carbon, nitrogen and phosphorus budgets and the metabolism of a boreal shield lake

Abstract

We conducted an experiment to determine the effects of cage aquaculture on the functional properties of a lake. We installed a commercial aquaculture operation into a lake located within a pristine headwater region of the boreal shield. We used a before-after control-impact design to evaluate the weight of evidence for causal effects. We quantified sinks, sources and exchanges of carbon, nitrogen and phosphorus to the lake and presented this information in a budget format. We then calculated retention rates for these chemicals and compared them to integral amounts of carbon, nitrogen and phosphorus accumulating in the water column. We also measured removal rates (sedimentation) of these elements from the water column and transformation rates (primary production) within the water column. We found that the amounts of nitrogen and phosphorus retained in the lake increased dramatically during farm operation. With the exception of an increase in particulate carbon, we did not observe an accumulation of these chemicals in the water column. We did however observe lake wide increases in carbon and nitrogen sedimentation. Primary production increased significantly in the aquaculture-impacted lake and the period of peak productivity shifted to later in the summer. We discuss our results in the context of consequences to aquatic life and source water and we provide some recommendations for management.