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

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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.