Stow Craig A, Qian Song S, Craig J Kevin
Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, South Carolina 29208, USA.
Environ Sci Technol. 2005 Feb 1;39(3):716-23. doi: 10.1021/es049412o.
The northwestern Gulf of Mexico shelf has been nicknamed "The Dead Zone" due to annual summertime (May-September) bottom-water hypoxia (dissolved oxygen < or =2 mg L(-1)) that can be extensive (>20 000 km2) and last for several months. Hypoxia has been attributed to eutrophication caused by increasing nitrogen loads, although directly linking hypoxia to nitrogen is difficult. While the areal extent of hypoxia has been shown to increase with Mississippi River flow, it is unclear whether this increase results from enhanced vertical water-column stratification or from eutrophication caused by river-borne nutrients. Disentangling the relative contributions of eutrophication versus stratification has important management consequences. Our analysis indicates that the top:bottom salinity difference is an important predictor of hypoxia, exhibiting a threshold, where the probability of hypoxia increases rapidly, at approximately 4.1 ppt. Using a Bayesian change-point model, we show that this stratification threshold decreased from 1982 to 2002, indicating the degree of stratification needed to induce hypoxia has gone down. Although this declining threshold does not link hypoxia and nitrogen, it does implicate a long-term factor transcending yearly flow-induced stratification differences. Concurrently, we show that surface temperature increased, while surface dissolved oxygen decreased, suggesting that factors in addition to nitrogen may be influencing the incidence of hypoxia in the bottom water.
墨西哥湾西北部大陆架因每年夏季（5月至9月）出现的底层水缺氧现象（溶解氧≤2毫克/升）而被昵称为“死亡地带”，这种缺氧现象可能范围广泛（超过20000平方千米）且持续数月。缺氧现象被认为是由氮负荷增加导致的富营养化所致，尽管将缺氧现象与氮直接联系起来并非易事。虽然缺氧的面积范围已被证明会随着密西西比河流量的增加而扩大，但尚不清楚这种扩大是由于垂直水柱分层加剧还是由河流携带的养分导致的富营养化所致。厘清富营养化与分层的相对贡献具有重要的管理意义。我们的分析表明，表层与底层盐度差是缺氧现象的一个重要预测指标，呈现出一个阈值，在大约4.1个千分点时，缺氧的概率会迅速增加。使用贝叶斯变化点模型，我们发现这个分层阈值在1982年至2002年间有所下降，这表明引发缺氧所需的分层程度已经降低。虽然这个下降的阈值并未将缺氧现象与氮联系起来，但它确实暗示了一个超越年度流量引起的分层差异的长期因素。同时，我们还表明表层温度升高，而表层溶解氧降低，这表明除了氮之外的其他因素可能正在影响底层水缺氧现象的发生率。
