Modeling river-induced phosphorus limitation in the context of coastal hypoxia

Abstract

The urban development of coastal areas and the increased use of chemical fertilizers over the last century have led to a worldwide expansion of coastal eutrophication and a significant increase in the occurrence and intensity of human-induced coastal hypoxia. Proportionally, nitrogen load has often increased more severely than phosphorus load and phosphorus limitation became a common seasonal phenomenon in many eutrophic coastal systems. Phosphorus limitation may alter the magnitude, timing, and location of phytoplankton production with potential effects on hypoxia. Yet, because of the difficulty in observing these effects, limited work has been carried out to assess the influence of P limitation on hypoxia. Models are thus useful tools for simulating the effects of river-induced phosphorus limitation on coastal hypoxic systems. Modeling P limitation is important to better understand the processes controlling hypoxia, to improve the predictive skill of hypoxia prediction models, and to design and evaluate nutrient management strategies for hypoxia mitigation. Here, we review the effects of phosphorus limitation on a continuum of coastal hypoxic systems, contrasting the effects of P limitation on systems that are primarily one-dimensional (or "flow-through") like the Neuse River Estuary versus more dispersive open systems like the Mississippi River plume. We discuss modeling frameworks and techniques that are relevant in this context and summarize recent modeling work that quantitatively assesses the effect of phosphorus limitation on hypoxia development in the Mississippi River plume.