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

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.

Spatial and temporal variation in surface nitrate and phosphate in the Northern Gulf of Mexico over 35 years

“…For example, during NGoM hypoxic events, TDP is released into the water column from bottom sediments 7 . Though benthic DIP sources are likely important within the MAR watershed and in shallow, nearshore areas of the NGoM 69 , our results show a pattern of excess DIP at high salinities, that are typically found over deep water columns, further offshore.…”

“…A two-endmember mixing model for NGoM nutrient distributions is highly simplified and inputs from freshwater endmembers other than the MAR certainly contribute to NGoM nutrient concentrations 69 . Nevertheless, the high relative contribution of the MAR to NGoM nutrient loading 32 and the temporal correlations between MAR nutrient loads and NGoM concentrations shown in this study support our focus on the MAR as the dominant freshwater endmember.…”

“…DIP adsorbs to particles in freshwater, and is released with increasing salinity and discharge 69 , 75 . However, suspended-sediment-derived P is unlikely to explain the upward trend in NGoM DIP, because the percentage of P in MAR suspended sediment stayed relatively stable over time (Fig.…”

Distribution of streamflow, sediment, and nutrients entering Galveston Bay from the Trinity River, Texas, 2016–19

“…Total phosphorus concentrations increased proportionally with suspended-sediment concentrations, likely as a result of adsorption of dissolved phosphorus onto sediment particles. Because total phosphorus concentrations are normally closely tied to suspended-sediment concentrations in freshwater and estuarine systems (He and Xu, 2018), and because suspended-sediment concentrations are consistently lower at the Old River Lake site than at the Wallisville site, it would be reasonable to expect total phosphorus concentrations at the Old River Lake site to be less than those at the Wallisville site. However, during high-flow events, total phosphorus concentrations were similar at the Old River Lake and Wallisville sites.…”