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

在过去的30年里，安大略省前寒武纪盾状软湖的磷水平已经下降，现在的浓度可能会导致浮游植物群落组成和结构的变化，并影响渔业资源。在许多湖泊的流域，尽管人类活动（例如，小屋使用）普遍增加，但湖水磷水平的下降还是发生了。湖泊磷水平的下降与陆地流域向湖泊输送磷的减少有关，其中春季融雪期磷浓度和输出下降最为显著。由于春季融雪占加拿大年水和元素通量的50%以上，因此春季养分输送的变化对湖泊收支至关重要。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)确定前寒武纪地盾流域向湖泊春季磷输出减少的原因；(b)确定春季关键时期湖泊磷输出变化对内部循环和浮游植物动态的影响。我们将通过在安大略省中南部前寒武纪地盾的校准集水区（来自大气）和湖泊（来自溪流）进行P输送的详细测量来实现这些目标，这些集水区已由安大略省环境部监测了30多年。在流域和湖泊内进行详细的非生物和生物测量，以及对地表水质量和数量的高频监测，将使我们能够确定对磷输出和循环的控制，并评估湖泊对未来磷输送变化的反应。

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.