Prevention of non-point source phosphorus exports through agricultural watersheds: perspectives from Quebec, Canada, and Vermont, USA

An overview is presented of agroecological studies undertaken in the Quebec portion of the primary watershed emptying into northern Lake Champlain's Missisquoi Bay. These cross-border waters, shared by the Province of Quebec and the State of Vermont, host recurring cyanobacterial blooms, which threaten potable water supplies and threaten the region's tourism and recreation activities. Given the primary contribution by phosphorus (P) loads to the degradation of Missisquoi Bay ecosystem, the governments of Quebec and Vermont have pledged to achieve a 41% decline in P loads reaching the bay. This objective particularly targets the agricultural sector, to which 79% of annual P loads reaching the bay are attributed. Applied research undertaken over the last decade has allowed: i. the development of a better understanding of non-point source P transport dynamics; ii. the fine tuning of diagnostics and water management tools at the scales both of the individual field (remote sensing) and of the whole territory concerned (hydrological modeling); iii. an assessment of the environmental consequences of watershed-wide concerted action by the farm managements. From an operational perspective, the main take-home message of these investigations has been to show that preeminent importance should be given to two main lines of agroecosystem defence: the control of both P sources and P transport. The first line of defence calls for long term management of soil P enrichment, as well as of the methods and of the timing of manure application, particularly with a view to minimize runoff-driven P losses. In this context, spatially-based assessment and presentation of P inputs and soil saturation rates become front-line tools in field-scale management. Their suitability has already been demonstrated for soils in Brittany, France. As a second line of defence, the limitation of P exports requires a reduction in overall volume and intensity of surface runoff through the implementation of low-erosion cultivation practices and field-scale drainage improvements. Development of this integrated and spatially-aware view of mass balances and of the landscape's influence on watershed hydrology will lead to the emergence of an overall, holistic audit of the fate of P over the watershed and best management practices which will bear fruit not only at the scale of individual farms but also at that of the entire watershed community