White J R, Fulweiler R W, Li C Y, Bargu S, Walker N D, Twilley R R, Green S E
Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
Environ Sci Technol. 2009 Aug 1;43(15):5599-604. doi: 10.1021/es900318t.
High nitrogen (N) loading to coastal aquatic systems can be expressed as increased algal production and subsequent low dissolved oxygen. In April, 2008, predictions for extreme flood stage for the Lower Mississippi River triggered the opening of the Bonnet Carré Spillway, a major release valve for the river. The spillway diverted approximately 8 km3 of water over one month of operation into Lake Pontchartrain with a concomitant 10000 t of NO3-N. Satellite imagery, physical, water quality, and chlorophyll a (chl a) measurements show that the Mississippi River plume mixed with < 40% of the lake during this time, and much of the nutrient load was transported to the coastal ocean. Nitrate, dissolved reactive phosphorus (P), and dissolved silica (Si) concentrations were 4.8, 5.0, and 3.2 times higher, respectively, within the river plume when compared with those of the lake water. Despite the high nutrient concentrations within the river plume, phytoplankton biomass, evidenced by chl a concentrations, was low. Much of the nutrient load appeared to bypass the lake and was transported to the coastal ocean during the opening of the diversion. The potential removal of a total of 7.6% of the N load from the Mississippi River during the one month of flood level flow may have been a contributing factor in the lower than predicted hypoxia zone off the Louisiana coast during the summer of 2008.
沿海水生系统的高氮负荷表现为藻类产量增加以及随之而来的低溶解氧。2008年4月，对密西西比河下游极端洪水水位的预测促使邦尼特卡里泄洪道开启，这是该河的一个主要泄洪阀。在一个月的运行期间，泄洪道将约8立方千米的水引入庞恰特雷恩湖，同时带来了10000吨硝酸氮。卫星图像、物理、水质和叶绿素a测量结果表明，在此期间，密西西比河羽流与该湖不到40%的水体混合，大部分营养负荷被输送到了沿海海域。与湖水相比，河羽流中的硝酸盐、溶解活性磷和溶解硅浓度分别高出4.8倍、5.0倍和3.2倍。尽管河羽流中的营养浓度很高，但以叶绿素a浓度为证的浮游植物生物量却很低。在泄洪开启期间，大部分营养负荷似乎绕过了湖泊，被输送到了沿海海域。在一个月的洪水流量期间，密西西比河总氮负荷可能有7.6%被去除，这可能是2008年夏季路易斯安那海岸缺氧区低于预测水平的一个促成因素。
