Phosphorus Fluxes from Three Coastal Watersheds under Varied Agriculture Intensities to the Northern Gulf of Mexico

Songjie He,Y Xu

doi:10.3390/w10060816

Abstract

This study aims to evaluate recent total phosphorus (TP) and dissolved inorganic phosphorus (DIP) transport from three coastal rivers—the Calcasieu, Mermentau, and Vermilion Rivers—that drain watersheds with varied agriculture intensities (21%, 67%, and 61%, respectively) into the northern Gulf of Mexico, one of the world’s largest summer hypoxic zones. The study also examined the spatial trends of TP and DIP from freshwater to saltwater along an 88-km estuarine reach with salinity increasing from 0.02 to 29.50. The results showed that from 1990–2009 to 2010–2017, the TP fluxes for one of the agriculture-intensive rivers increased while no significant change was found for the other two rivers. Change in river discharge was the main reason for this TP flux trend. The two more agriculture-intensive river basins showed consistently higher TP and DIP concentrations and fluxes, as well as higher DIP:TP ratios than the river draining less agriculture-intensive land, confirming the strong effect of land uses on phosphorus input and speciation. Longitudinal profiles of DIP along the salinity gradient of the estuarine reach displayed characteristic input behavior. Desorption of DIP from suspended solids and river bed sediments, urban inputs, as well as stronger calcium carbonate and phosphorus co-precipitation at the marine endmember could be the reasons for such mixing dynamics.

Highlights

Excess nitrogen and phosphorus loads in rivers from both point sources and diffuse sources have been considered one of the main causes for eutrophication and harmful algal blooms of world’s coastal and lake waters [1,2,3] because these two elements strongly affect biological activity in aquatic environments
The total phosphorus (TP) and dissolved inorganic phosphorus (DIP) fluxes and yields from the Mermentau and Vermilion Rivers were clearly higher than those from the Calcasieu River, further confirming the strong effect of land uses on phosphorus input to estuaries in coastal river basins
This study investigated total phosphorus and dissolved inorganic phosphorus loadings from three coastal rivers that drain watersheds under varied agriculture intensities (i.e., 67%, 61%, and 26%)

Summary

Introduction

Excess nitrogen and phosphorus loads in rivers from both point sources and diffuse sources have been considered one of the main causes for eutrophication and harmful algal blooms of world’s coastal and lake waters [1,2,3] because these two elements strongly affect biological activity in aquatic environments. Abiotic phosphorus inputs decline over time because phosphorus weathers out of rocks, whereas nitrogen inputs from the atmosphere do not change much as soil develops. Such declining phosphorus inputs during pedogenesis has been termed by Walker and Syers [4] as a “terminal steady state” of phosphorus deficiency. It is interesting to know if pedogenic dynamics of phosphorus is reflected in river waters that drain watersheds with different dominant land uses. Such information can be useful for predicting long-term riverine phosphorus transport from large drainage areas to their receiving basins

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