Collaborative Research: Si, N, and P Limitation in the Northern Gulf of Mexico: Phytoplankton Responses to Increased N and P Delivery From the Mississippi River

多尔奇和纳尔逊在过去的30-40年里，许多河流中的硝酸盐和磷酸盐浓度大幅增加，而硅酸浓度要么保持不变，要么下降。这种影响在欧洲和北美耗尽农田的河流中最为普遍，似乎是由于土地利用的变化。邻近沿海海洋的这些变化的主要后果是富营养化和硅藻生长受到硅限制的可能性越来越大。现在已经在这些系统中的一些系统中观察到了从硅藻到非硅质分类群的主要植物区系转变，并在另一些系统中进行了预测。密西西比河及其沿岸的羽流就是这些现象的典型例子。自1960年以来，河口的硝酸盐浓度增加了一倍多，而硅酸浓度下降了30%，导致河流输送到墨西哥湾的溶解硅与溶解氮的比率几乎下降了75%。在1960年前，这一比率为~4(相对于硅藻营养需求而言，这意味着硅对氮的大量过剩)，而现在为~1(几乎正好与这些需求相平衡)。在很大程度上是这种平衡的结果，自1987年以来，在墨西哥湾沿岸的不同时间都表现出了硅、氮和磷的限制。总的规律是春季和上游低盐度水体中磷的限制最为普遍，夏季和下游高盐度水域中氮的限制更频繁，而硅在任何时候都可以成为系统中的限制。但年际差异和事件规模(洪水、风暴)扰动都足够重要，以至于很难辨别出一般的季节模式。化学计量营养平衡，再加上河流作为营养源的数量重要性及其时变性，在墨西哥湾沿海创造了一种有趣的、具有科学价值的情况。随着河流流量、海岸水流模式、天气(特别是主要风暴)以及热力层结和去层化的季节周期的变化，系统中的营养物质浓度及其比率随时间变化。这些变化可以选择性地使系统进入硅、氮或磷的限制。因此，主要营养物来源的时变性和由此产生的浮游植物在延伸的河流羽流中大量繁殖的相对限制，使该系统成为研究沿海海洋营养限制及其后果的天然实验室。这种情况，特别是与硅藻的硅限制有关的情况，似乎是历史上最近出现的现象，是由于河流中硅：氮和硅：磷比的变化造成的。在这个项目中，多尔奇和纳尔逊博士将对墨西哥湾沿海栖息地的硅、氮和磷的限制进行全面的研究。这个系统在养分输入方面经历了重大的人为变化，因为已经观察到三种主要养分中的每一种都受到限制。这项研究将包括在季节性和河流排放循环的关键时刻对硅、氮和磷限制程度的直接实验评估，根据测量的浮游植物对选择性释放硅、氮和磷限制以及所有可能的硅、氮和磷限制组合的反应。它将使用AVHRR和SeaWiFS数据以及正在绘制的温度、盐度和荧光地图，以确定河流羽流最大生物量范围内的采样点(那里的营养限制往往特别严重)，以及在生物量较低、羽流下游更咸的水域中的采样点。它将通过测量硅藻吸收32Si的速率来具体评估硅藻生长和生产力的硅、氮和磷限制，这是一个高度特定于硅藻的过程，以响应选择性的营养丰富。同样重要的是，这项研究将考虑从几个小时到三天的时间尺度上的营养限制，并将确定作为同一实验计划的一部分，在总体特性、硅藻特有特性和物种特有特性方面的变化。这将使我们能够评估受密西西比河外流影响的沿海水域的整体营养状况，从而产生第一个案例研究，根据这些研究可以检验关于沿海海洋富营养化驱动的营养限制的一般假设。

Dortch and Nelson During the last 30 - 40 years the nitrate and phosphate concentrations in many rivers have increased substantially while silicic acid concentrations have either remained constant or decreased. This effect is most prevalent in rivers that drain agricultural lands in Europe and North America, and it appears to be attributable to changes in land use. The main consequences of these changes in the adjacent coastal ocean have been eutrophication and an increasing likelihood that diatom growth is limited by Si. Major floristic shifts from diatoms to nonsiliceous taxa have now been observed in some of these systems, and predicted in others. The Mississippi River and its coastal plume provide a classic example of these phenomena. Nitrate concentrations at the river mouth have more than doubled since 1960 while silicic acid concentrations have decreased by 30%, resulting in almost a 75% decrease in the ratio of dissolved Si to dissolved N delivered to the Gulf of Mexico by the river. Before 1960 this ratio was ~4 (representing a large excess of Si over N in relation to diatom nutrient requirements) and now it is ~1 (almost exactly in balance with those requirements). Largely as a result of this balance, Si, N and P limitation have all been demonstrated at different time in the coastal Gulf of Mexico since 1987. A general pattern seems to have emerged in which P limitation is most common in spring and in the low-salinity waters of the upper river plume, N limitation occurs more frequently in summer and in higher- salinity waters down-plume and Si can become limiting anywhere in the system at any time. But inter-annual differences and event-scale (floods, storms) perturbations are both important enough to make a general seasonal pattern difficult to discern. The stoichiometric nutrient balance, in combination with the quantitative importance of the river as a nutrient source and its time-varying properties, has created an interesting and scientifically useful situation in the coastal Gulf of Mexico. Nutrient concentrations and their ratios change with time in the system in response to changes in river discharge, coastal flow patterns, weather (especially major storms) and the seasonal cycle of thermal stratification and de-stratification. These changes can selectively drive the system into Si, N or P limitation. Thus the time-varying character in the main nutrient source and the relative confinement of the resulting phytoplankton bloom within the extended river plume combine to make this system a natural laboratory for studying nutrient limitation and its consequences in the coastal ocean. This situation, especially as it pertains to Si limitation of the diatoms, appears to be a historically recent phenomenon resulting from the altered Si:N and Si:P ratios in the river. In this project Drs. Dortch and Nelson will undertake a comprehensive study of Si, N, and P limitation in a coastal habitat in the Gulf of Mexico. This system has undergone significant anthropogenic changes in nutrient input and because limitation by each of the three major nutrients has been observed. The study will include direct experimental assessment of the degree of Si, N and P limitation at key times of the seasonal and river-discharge cycles, based on the measured response of the phytoplankton to selective release from Si, N and P limitation and from all possible combinations of Si, N and P limitation. It will use AVHRR and SeaWifS data and underway mapping of temperature, salinity and fluorescence to locate sampling sites within the biomass maximum of the river plume (where nutrient limitation is often particularly severe) and in lower-biomass, more saline waters farther down the plume. It will specifically assess Si, N and P limitation of diatom growth and productivity by measuring the rate of 32Si uptake, a process that is highly specific to the diatoms, in response to selective nutrient enrichment. Of equal importance, this study will consider nutrient limitation on time scales ranging from a few hours to three days and will identify changes in bulk properties, diatom-specific properties and species-specific properties as part of the same experimental plan. This will enable us to evaluate the overall nutrient status of the coastal waters affected by the Mississippi River outflow, resulting in the first case study against which general hypotheses regarding eutrophication-driven nutrient limitation in the coastal ocean can be tested.