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Seasonal distributions of organic nutrients on the Louisiana continental shelf and their implications for nutrient limitation and hypoxia formation

机译：路易斯安那州大陆架上有机养分的季节性分布及其对养分限制和缺氧形成的影响

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摘要

Nutrients delivered by the Mississippi and Atchafalaya Rivers to the Louisiana continental shelf cause local eutrophication and feed phytoplankton growth, resulting in a massive region of bottom water hypoxia each summer. Much of what is known about the drivers of this hypoxia is based on extensive datasets of inorganic nutrients. We measured dissolved organic and paniculate organic carbon, nitrogen and phosphorus in surface water along the Louisiana continental shelf between the Mississippi and Atchafalaya Rivers during three cruises in March, May and July 2004 to develop a more comprehensive understanding of interactions between nutrients and eutrophication. We specifically sought to determine if sufficient organic P is present to relieve the inorganic P limitation that occurs during the spring and early summer. DOP is added to shelf waters in situ, but drawdown of DOP concentrations below conservative mixing lines indicate DOP is also being used as a source of P for growth. Calculations of the amount of DOP potentially utilized towards primary production reveal there is not enough to relieve P-limitation. Total nitrogen to total phosphorus ratios were high, averaging 86, 72 and 51 in March, May and July, respectively. Additionally, correlations between DIN and total nitrogen (TN) were identical to those between DIP and total phosphorus (TP), indicating that the use of either TN:TP or DIN:DIP ratios is equally relevant in assessing nutrient limitation in this region, but DIN:TP ratios are clearly underestimates. Organic matter on the shelf is best correlated to TN and TP loads measured upstream in the Mississippi River one month prior. The combination of low organic P concentrations and high total N:P ratios indicated P-limitation of phytoplankton biomass on the Louisiana shelf during the spring and early summer of 2004. This P-limitation occurs during the period of hypoxia formation as a result of excess N loads delivered by the rivers, supporting dual controls of both N and P to reduce the hypoxia.
机译：密西西比河和阿查法拉亚河向路易斯安那州陆架输送的营养物导致了局部富营养化和浮游植物的生长，因此每年夏天都会导致大量的底部缺氧。关于这种缺氧的驱动因素，许多已知的信息都是基于大量的无机营养素数据集。在2004年3月，5月和7月的3次航行中，我们测量了密西西比河和阿恰法拉亚河之间的路易斯安那州陆架沿地表水中溶解的有机和颗粒状有机碳，氮和磷，以更全面地了解养分与富营养化之间的相互作用。我们专门寻求确定是否存在足够的有机磷，以缓解春季和初夏发生的无机磷限制。将DOP原位添加到架子水域，但DOP浓度低于保守混合线的下降表明DOP也被用作生长的P来源。计算可能用于初级生产的DOP量表明，没有足够的方法来缓解P限制。总氮与总磷之比很高，分别在三月，五月和七月分别为86、72和51。此外，DIN与总氮（TN）之间的相关性与DIP与总磷（TP）之间的相关性相同，这表明使用TN：TP或DIN：DIP比值在评估该区域的营养限制方面同样重要，但是DIN：TP比显然被低估了。架子上的有机物与一个月前在密西西比河上游测得的总氮和总磷负荷最相关。低有机磷浓度和高总N：P比值的组合表明2004年春季和初夏在路易斯安那州陆架上浮游植物生物量的P限制。由于过剩，P限制发生在低氧形成期间河流输送的氮负荷，支持氮和磷的双重控制以减少缺氧。

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