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的生态系统，其健康状况在过去70年中一直在下降：生活在海底和深海底层水中并利用海底和深海底层水的生物所能获得的溶解氧不断减少。底层水起源于西大西洋的斜坡区域。当这些水向陆地移动时，水柱和沉积物的呼吸作用消耗氧气。底层水与大气隔绝，因此失去的氧气无法补充，只能通过上覆水的缓慢扩散来补充。今天，圣劳伦斯河口下游的底层水严重缺氧，这种情况的定义是氧含量低于20%的饱和水中的氧含量。在Anticosti海峡和Esquiman海峡的顶部区域也存在着次表层的底层沃茨（饱和度低于30%）。我们将一半到三分之二的氧气水平下降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体的氧气含量。其余的则是由于有机物流入海底的流量增加。如果圣劳伦斯湾的含氧量继续下降，今天缺氧的沃茨可能会变成缺氧，底层环境可能会变成“死区”。即使我们没有达到缺氧，氧气水平的逐渐降低也可能减少许多底层和底栖动物物种，包括鱼类和甲壳类动物的适宜生境面积。拟议的研究解决了能够理解和预测圣劳伦斯湾生态系统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管理营养物排放，还是直接向底层沃茨注入氧气。该提案结合了实地观察、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.