Turner R Eugene, Rabalais Nancy N, Justic Dubravko
Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
Environ Sci Technol. 2008 Apr 1;42(7):2323-7. doi: 10.1021/es071617k.
A 20+ year data set of the size of the hypoxic zone off the Louisiana-Texas coast is analyzed to reveal insights about what causes variation in the size of the hypoxic zone in summer, the accumulation of carbon storage in sediments, and pelagic and sediment oxygen demand. The results of models support the conclusion that some of this variation can be explained by a higher sedimentary oxygen demand, which may be larger than water column respiration rates in summer. Proxies for organic loading to sediments reveal that carbon losses continue after accumulation, and results from other studies indicate that sediment oxygen demand is directly related to surface water phytoplankton production, which has increased because of higher nutrient loading from the Mississippi River watershed. The potential size of the hypoxic zone for a given nitrogen load has increased as a result and has doubled from 1980 to 2000. The development of widespread hypoxia after the early 1980s and its consequences could, therefore, be considered a shift to an alternate ecosystem state. The Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico goal of reducing the size of the hypoxic zone to an average of 5000 km2 by 2015 becomes more difficult to achieve for every year there is no significant reduction in nutrient loading. The decisions made to reduce the size of the hypoxic zone must incorporate these nonlinear responses and, we think, err on the side of caution in assuming that existing management efforts are sufficient to restore water quality on this shelf. The legacy of a higher sediment respiratory demand following eutrophication should apply to other coastal systems.
对路易斯安那州 - 得克萨斯州海岸缺氧区面积长达20多年的数据集进行了分析，以揭示有关夏季缺氧区面积变化、沉积物中碳储存积累以及海洋和沉积物需氧量的影响因素。模型结果支持这样的结论：这种变化的部分原因可以用较高的沉积需氧量来解释，而沉积需氧量在夏季可能大于水柱呼吸速率。沉积物有机负荷的替代指标显示，碳在积累后仍会继续损失，其他研究结果表明，沉积物需氧量与地表水浮游植物产量直接相关，由于密西西比河流域营养负荷增加，浮游植物产量有所上升。因此，在给定氮负荷下，缺氧区的潜在面积有所增加，从1980年到2000年增加了一倍。所以，20世纪80年代初之后广泛出现的缺氧现象及其后果可被视为向另一种生态系统状态的转变。墨西哥湾北部减少、缓解和控制缺氧行动计划的目标是到2015年将缺氧区面积平均减少到5000平方公里，但如果营养负荷没有显著降低，那么每年实现这一目标都会变得更加困难。为减少缺氧区面积所做的决策必须考虑到这些非线性反应，而且我们认为，在假设现有管理措施足以恢复该大陆架水质时应谨慎行事。富营养化后沉积呼吸需求增加的遗留问题应适用于其他沿海系统。
