Causes and implications of changes in phosphorus delivery to Precambrian Shield lakes during the critical spring melt period

安大略前寒武纪盾湖的磷（P）水平在过去30年中有所下降，现在接近可能导致浮游植物群落组成和结构变化并影响渔业资源的浓度。尽管人类活动普遍增加（例如，在许多湖泊的分水岭。湖泊磷水平的下降与从陆地流域向湖泊输送的磷减少有关，在春季融雪期，磷浓度和出口下降最显着。由于春季融雪占加拿大水和元素年通量的50%以上，春季营养物质输送的变化对湖泊预算至关重要。此外，加拿大东部的气候变化预测包括冬季变暖，这可能会进一步影响从流域到湖泊的养分输送。本研究的目的是（a）确定的原因（S）的下降，春季P输出从流域湖泊的前寒武纪盾，和（B）确定的影响，在关键的春季期间的内部循环和浮游植物动态的P输送到湖泊。我们将通过对安大略中南部前寒武纪地盾流域（从大气中）和湖泊（从溪流中）的磷输送进行详细测量来实现这些目标，安大略环境部已对这些流域进行了超过30年的监测。详细的非生物和生物测量流域和湖泊内的地表水的质量和数量的高频监测，将使我们能够确定控制P出口和循环，并评估湖泊对未来变化的P输送。

Phosphorus (P) levels in softwater Precambrian Shield lakes in Ontario have declined over the past 3 decades, and are now approaching concentrations that could cause shifts in phytoplankton community composition and structure and impact fishery resources. These declines in lakewater P levels have occurred despite general increases in human activity (e.g., cottage use) in the watersheds of many lakes. Declines in lake P levels are associated with decreases in P delivery to lakes from their terrestrial watersheds, and P concentrations and export have declined most dramatically during the spring snow melt period. As spring snow melt accounts for over 50% of the annual flux of water and elements in Canada, changes in the spring delivery of nutrients are of critical importance for lake budgets. Furthermore, climate change projections for eastern Canada include winter warming, which may further impact nutrient delivery from watersheds to lakes. The objectives of this research are to (a) identify the cause(s) of declines in spring P export from watersheds to lakes on the Precambrian Shield, and (b) determine the impact of changes in P delivery to lakes on internal cycling and phytoplankton dynamics during the critical spring period. We will address these objectives by conducting detailed measurements of P delivery to watersheds (from the atmosphere) and lakes (from stream flow) at calibrated watersheds on the Precambrian Shield in south-central Ontario, that have been monitored by the Ontario Ministry of Environment for > 30 years. Detailed abiotic and biotic measurements within watersheds and lakes together with high-frequency monitoring of surface water quality and quantity will allow us to identify the controls on P export and cycling and evaluate the response of lakes to future changes in P delivery.