Phosphate Oxygen Isotope Ratios as a Tracer of Nutrient Cycling in the Mississippi River and the Gulf of Mexico

EPA Grant Number: F13E20910

Title: Phosphate Oxygen Isotope Ratios as a Tracer of Nutrient Cycling in the Mississippi River and the Gulf of Mexico
Investigators: Mine, Aric Howard
Institution: University of Chicago
EPA Project Officer: Michaud, Jayne
Project Period: July 1, 2014 through July 1, 2015
Project Amount: $84,000
RFA: STAR Graduate Fellowships (2013) RFA Text |  Recipients Lists
Research Category: Fellowship - Environmental and Water Science , Academic Fellowships


The Mississippi River constitutes a significant flux of nutrients to the Gulf of Mexico where hypoxia is routinely observed. When coupled
with other nutrient measurements, the δ18O of orthophosphate, Pi, will yield a detailed account of nutrient dynamics within the Mississippi and extending into the gulf. This includes examination of how such systems evolve toward hypoxic conditions and their significance for ecosystems and coastal nutrient budgets. As riverine water enters the gulf, patterns in nutrient cycling, nutrient limitation and carbon fluxes may shift.


The proposed measurements on δ18Op in Mississippi River should enable tracking of Pi cycling, both within the river and as this river plume spreads into the Gulf. The sampling campaign will focus on measuring phosphate source isotopic compositions, their seasonal variation and the ways in which biological cycling modifies the isotopic composition along the river flow and into the Gulf. In conjunction with culture experiments, this work will provide a stronger mechanistic understanding of the links between measured δ18Op and Pi turnover rates and mechanisms, applicable both to the study of Gulf hypoxia and to all aquatic ecosystems.

Expected Results:

A Mississippi River Atchafalaya River Basin assessment of biological nutrient cycling will determine the coupling between nutrient availability, limitation and the fate of organic carbon. The δ18O value of P
i has substantial potential for tracking Pi turnover in an economically and environmentally significant region where excessive nutrient loading has led to devastating hypoxic events. This work will identify nutrients limit- ing and/or co-limiting primary production, determine rates of nutrient cycling and target strategies for mitigating hypoxia and nutrient loading.

Potential to Further Environmental/Human Health Protection

Identifying the effects of nutrient loading in the Mississippi River and Gulf of Mexico will provide insights toward agricultural runoff and widespread hypoxic events, which have devastating impacts on fisheries and water quality in the river and gulf. This work will provide a mechanistic link between nutrient dynamics in the Mississippi River and the Gulf of Mexico and evolution toward annual dead zones in the gulf.

Supplemental Keywords:

nutrients, phosphorus, stable isotopes

Progress and Final Reports:

  • Final