Venlafaxine is an antidepressant drug present in freshwater ecosystems worldwide with detections of >2.0 μg/L. Its effects have been well studied in laboratory settings, and it has been observed to alter behavioural responses in fish. However, its potential effects have not been investigated in a whole ecosystem context where direct and indirect effects of chronic exposure could pose a risk to long-term ecosystem health. This thesis aimed to assess the potential risk of chronic venlafaxine exposure within a boreal lake ecosystem. In-situ enclosures (n=10, 2-m diameter, 1.5-m deep) were deployed in a lake at the IISD-Experimental Lakes Area, spiked weekly with venlafaxine at a target range of 4 ng/L to 100,000 ng/L for 10 weeks and monitored for various biotic and abiotic parameters. Enclosures contained native plankton and invertebrates and were each stocked with 5 finescale dace (Phoxinus neogaeus), a native small-bodied littoral fish. Few statistical differences were observed at environmentally relevant concentrations for most assessed endpoints. The concentrations tested were not acutely lethal to any aquatic biota. Indications of behavioural changes in fish may have caused indirect effects on invertebrate community structure, but at concentrations that are three orders of magnitude higher than what is considered environmentally relevant. This thesis integrates the whole ecosystem response to a commonly detected freshwater contaminant and found that venlafaxine at environmentally relevant concentrations does not pose a significant ecological risk to aquatic ecosystems.