Forcasting the response of a coastal ecosystem to persistent forcing: hypoxia in the Gulf of St. Lawrence

圣劳伦斯湾是加拿大东海岸的一个巨大生态系统，其健康状况在过去70年里一直在下降：生活和使用海底和深海水的生物获得的溶解氧一直在稳步下降。底层水起源于西大西洋的斜坡区。当这些水向陆地流动时，水柱和沉积物中的呼吸作用消耗氧气。底层的水与大气隔绝，因此失去的氧气无法被替代，除非从上面的水中缓慢扩散。今天，下圣劳伦斯河口的底层水严重缺氧，这种情况的定义是氧气水平低于饱和氧气水域的20%。低氧底层水(低于30%饱和度)也存在于安提科里海峡和埃斯基曼海峡的头部区域。我们将氧气水平下降的一半到三分之二归因于西北大西洋环流模式的变化，这决定了进入深海海湾环境的水团的含氧量。其余的归因于有机物质流向海底的流量增加。如果圣劳伦斯湾的氧气水平继续下降，今天的低氧水域可能会变得缺氧，海底环境可能会成为“死区”。即使我们没有达到缺氧的程度，氧气水平的逐渐降低也可能减少许多底栖和底栖动物的适宜栖息地的面积，包括鱼类和甲壳类动物。这项拟议的研究解决了能够理解和预测圣劳伦斯湾生态系统中低溶解氧水平的原因和影响的战略重要性。该项目还旨在制定补救战略，无论是针对通过圣劳伦斯河的营养物质排放的管理，还是直接向底层水域注入氧气。该建议结合了实地观察、实验室实验和建模，其目标是开发可用于预测未来氧气状况变化以及这些变化对生态系统过程和渔业资源的影响的工具。

The state of health of the Gulf of St. Lawrence, a vast ecosystem on the east coast of Canada, has been declining for the last 70 years: there has been a steady decrease in availability of dissolved oxygen to the organisms that live in and use the sea floor and the deep bottom water. The bottom water originates in the slope region of the Western Atlantic Ocean. As this water journeys landward, respiration in water column and sediment consumes oxygen. The bottom water is isolated from the atmosphere, so the lost oxygen cannot be replaced except by slow diffusion from the overlying water. Today, the bottom water of the Lower St. Lawrence Estuary is severely hypoxic, a condition defined by oxygen levels lower than 20% of what is found inwater saturated with oxygen. Hypoxic bottom waters (less than 30% saturation) also exist in the head regions of the Anticosti and Esquiman Channels. We attribute one half to two-thirds of the declining oxygen levels to changes in the circulation pattern in the northwest Atlantic ocean, which determines the oxygen content of the water mass entering the deep Gulf environment. The remainder is attributed to the increased flux of organic matter to the sea floor. If oxygen levels in the Gulf of St. Lawrence continue to decline, today's hypoxic waters may become anoxic and the bottom environment may become a "dead zone". Even if we do not reach anoxia, progressively lower levels of oxygen may reduce the area of suitable habitats for many species of demersal and benthic animals, including fish and crustaceans. The proposed research addresses the strategic importance of being able to understand and predict causes and effects of low dissolved oxygen levels in the Gulf of St.Lawrence ecosystem. The project further aims to develop remediation strategies, be they directed towards management of nutrient discharge via the St. Lawrence River or direct injection of oxygen into bottom waters. The proposal combines field observations, laboratory experiments, and modelling, and its goal is to develop tools that can be used to predict future changes in the oxygen regime and the effects of these changes on ecosystem processes and fisheries resources.