RAPID: Unprecedented Hypoxia in Cape Cod Bay

这是对南科德角湾（CCB）氧动力学的快速反应研究，目的是更好地了解导致史无前例的连续第二个夏季大面积海底缺氧的物理过程。在关键的9月和10月，将进行综合测量，以解决重要的时空尺度，以便更好地了解该地区的锋面动态，这被认为是控制底部氧气浓度的关键作用。沿海水域的缺氧通常与大量人为营养物输入或强烈而持续的上升流有关。南部CCB都没有，但连续第二个夏天出现了大面积的低氧海底水域。这种发展中的缺氧事件提供了一个独特的机会来研究导致这种新型缺氧状态的物理和生物地球化学过程。有机碳在局部区域的浓度，也接受较少的氧气输入，因为减少混合预计是因果机制。这项研究的结果将通报给马萨诸塞州海洋渔业部和马萨诸塞州捕虾人协会。这项研究将有助于促进与这些团体的合作，目的是评估缺氧是否可能在未来一年中持续存在，并随着区域地表水变暖而变得更加严重。这项研究的结果将有助于制定缓解战略，以尽量减少经济损失和对龙虾渔业的危害。据推测，在CCB和其他没有持续强上升流或大量人为养分输入的系统中，有机碳的产生不足以在夏季引起广泛的缺氧，但当汇聚运输集中了足够数量的有机碳以耗尽底部氧水平时，可能会发生局部缺氧。在CCB，缺氧预计将与发生在季节性斜斜与海底相交处的强底密度锋一起发展。在这个位置，紧邻海床的强烈热分层抑制了湍流，通过垂直混合阻止了通风，而锋面积累的有机物质增强了这个位置的呼吸作用。为了在夏季条件下保持这个底部锋，需要频繁的放松风/下流风。与底部锋相关的耗氧机制与在更常研究的上升流区域导致缺氧的机制根本不同。然而，在其他具有强热分层和可变风强迫的环境中，它可能是重要的。这项跨学科研究将解决驱动CCB缺氧的机制，并深入了解为什么低氧条件在这个经济重要地区似乎更频繁地发生。该团队将1)大约每5天进行一次船载采样，以解决温度、盐度、溶解氧、叶绿素荧光和浊度的完整三维分布，以及这些变量如何响应风的强迫；2)部署3个底部着陆器，在N-S样带上跨越缺氧区，连续记录底部温度、盐度、DO和水柱循环；3)利用水动力-生物地球化学耦合模型对该区域进行数值模拟，以更好地了解导致低底氧的基本过程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This is rapid response study of oxygen dynamics in southern Cape Cod Bay (CCB) in order to better understand the physical processes that are contributing to an unprecedented second consecutive summer of extensive bottom hypoxia. Comprehensive measurements will be made during the critical months of September and October, in order to resolve the important spatial and temporal scales needed to better understand the frontal dynamics of this region, which are hypothesized to play a key role in controlling bottom oxygen concentration. Hypoxia in coastal waters is typically associated with either large anthropogenic nutrient inputs or strong and persistent upwelling. Southern CCB has neither, yet extensive hypoxic bottom waters have developed for the second consecutive summer. This developing hypoxia event provides a unique opportunity to examine the physical and biogeochemical processes responsible for this novel hypoxiaregime. Concentration of organic carbon in a localized area that also receives less oxygen input because of reduced mixing is expected to be the causal mechanism. Results from this research will be communicated to both the Massachusetts Department of Marine Fisheries and the Massachusetts Lobstermen’s Association. This research will help foster collaborations with these groups with the goal of assessing whether hypoxia is likely to persist in the coming year and become more severe as regional surface waters warm. Results from this research will help develop mitigation strategies to minimize economic losses and harm to the lobster fishery.It is hypothesized that in CCB, and other systems without persistent strong upwelling or large anthropogenic nutrient inputs, the production of organic carbon is insufficient to drive widespread hypoxia during the summer, but localized hypoxia can occur when convergent transport concentrates organic carbon in sufficient quantities to deplete bottom oxygen levels. In CCB, hypoxia is expected to develop in association with a strong bottom density front that occurs where the seasonal pycnocline intersects the seafloor. At this location, strong thermal stratification immediately adjacent to the seabed suppresses turbulence, preventing ventilation via vertical mixing and the front accumulates organic material that enhances respiration at this location. In order to maintain this bottom front during summer conditions, frequent relaxation/downwelling winds are needed. The mechanism of oxygen depleting associated with a bottom front is fundamentally different from what drives hypoxia in more commonly studied upwelling regions. However, it may be important in other environments with strong thermal stratification and variable wind forcing. This interdisciplinary research will resolve the mechanisms that drive hypoxia in CCB and provide insight into why low oxygen conditions appear to be occurring more frequently in this economically important region. The team will 1) conduct ship-based sampling roughly every 5 days to resolve the full three-dimensional distribution of temperature, salinity, dissolved oxygen, chlorophyll fluorescence and turbidity and how these variables respond to wind forcing; 2) deploy 3 bottom landers that span the hypoxic region on a N-S transect to continuously record bottom temperature, salinity and DO and water column circulation; and 3) conduct numerical simulations of the region using a coupled hydrodynamic-biogeochemical model to better understand the basic processes that result in low bottom oxygen.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.