Scientific Investigations Report
2007-5090 PDF (18 MB)

Abstract

Nutrient and sediment data collected at 115 sites by Federal and State agencies from 1993 to 2004 were analyzed by the U.S. Geological Survey to determine trends in concentrations and loads for selected rivers and streams that drain into the northwestern Gulf of Mexico from the south-central United States, specifically from the Lower Mississippi, Arkansas-White-Red, and Texas-Gulf Basins. Trends observed in the study area were compared to determine potential regional patterns and to determine cause-effect relations with trends in hydrologic and human-induced factors such as nutrient sources, streamflow, and implementation of best management practices. Secondary objectives included calculation of loads and yields for the study period as a basis for comparing the delivery of nutrients and sediment to the northwestern Gulf of Mexico from the various rivers within the study area. In addition, loads were assessed at seven selected sites for the period 1980-2004 to give hydrologic perspective to trends in loads observed during 1993-2004.

Most study sites (about 64 percent) either had no trends or decreasing trends in streamflow during the study period. The regional pattern of decreasing trends in streamflow during the study period appeared to correspond to moist conditions at the beginning of the study period and the influence of three drought periods during the study period, of which the most extreme was in 2000. Trend tests were completed for ammonia at 49 sites, for nitrite plus nitrate at 69 sites, and for total nitrogen at 41 sites. For all nitrogen constituents analyzed, no trends were observed at half or more of the sites. No regional trend patterns could be confirmed because there was poor spatial representation of the trend sites. Decreasing trends in flow-adjusted concentrations of ammonia were observed at 25 sites. No increasing trends in concentrations of ammonia were noted at any sites. Flow-adjusted concentrations of nitrite plus nitrate decreased at 7 sites and increased at14 sites. Flow-adjusted concentrations of total nitrogen decreased at 2 sites and increased at 12 sites. Improvements to municipal wastewater treatment facilities contributed to the decline of ammonia concentrations at selected sites. Notable increasing trends in nitrite plus nitrate and total nitrogen at selected study sites were attributed to both point and nonpointsources. Trend patterns in total nitrogen generally followed trend patterns in nitrite plus nitrate, which was understandable given that nitrite plus nitrate loads generally were 70-90 percent of the total nitrogen loads at most sites. Population data were used as a surrogate to understand the relation between changes in point sources and nutrient trends because data from wastewater treatment plants were inconsistent for this study area. Although population increased throughout the study area during the study period, there was no observed relation between increasing trends in nitrogen in study area streams and increasing trends in population. With respect to other nitrogen sources, statistical results did suggest that increasing trends in nitrogen could be related to increasing trends in nitrogen from either commercial fertilizer use and/or land application of manure.

Loads of ammonia, nitrite plus nitrate, and total nitrogen decreased during the study period, but some trends in nitrogen loads were part of long-term decreases since 1980. For example, ammonia loads were shown to decrease at nearly all sites over the past decade, but at selected sites, these decreasing trends were part of much longer trends since 1980. The Mississippi and Atchafalaya Rivers contributed the highest nitrogen loads to the northwestern Gulf of Mexico as expected; however, nitrogen yields from smaller rivers had similar or higher yields than yields from the Mississippi River.

Trend tests were completed for orthophosphorus at 34 sites and for total phosphorus at 52 sites. No trends were observed in about 57 percent of all phosphorus trend analyses attempted. Similar to nitrogen, no regional patterns could be confirmed because there was poor spatial representation of the trends sites. Flow-adjusted concentrations of orthophosphorus decreased at 10 sites and increased at 7 sites. Flow-adjusted concentrations of total phosphorus decreased at 6 sites and increased at 17 sites. It was understandable that trend patterns in total phosphorus did not follow trend patterns in orthophosphorus given that orthophosphorus loads accounted for about only 20-30 percent of the total phosphorus load at comparable sites. Trends in population data were inversely related to trends in flow-adjusted total phosphorus; therefore, trends in population were not considered a controlling factor to explain trends in total phosphorus. No relation was observed between phosphorus from fertilizer use and either orthophosphorus or total phosphorus trends. However, statistical results did suggest that increasing trends in both orthophosphorus and total phosphorus could be related to increasing trends in phosphorus from land application of manure.

There were more decreasing trends than increasing trends in phosphorus loads during the past decade, most of which were unique to the recent decade and not part of long-term decreases since 1980. Similar to nitrogen loads, the Mississippi and Atchafalaya Rivers contributed the highest phosphorus loads to the northwestern Gulf of Mexico as expected; however, phosphorus yields from smaller rivers were similar to or higher than yields from the Mississippi River. Trend analyses of suspended-sediment data were attempted at 39 sites. No trends were observed at about 71 percent of the sites. Remaining results indicated primarily decreasing trends in suspended sediment data. Most of the decreasing trends occurred on mainstem sites for the Mississippi, Arkansas, Red, and Atchafalaya Rivers, which are all regulated with reservoirs, locks and dams, and other erosion or flood-control structures that trap and prevent sediment from being transported downstream. Large decreases in suspended sediment in the Mississippi River Basin began in the 1950s when large reservoirs were constructed in the Missouri and Arkansas Rivers, which were considered the largest sources of sediment at the time. Because the Mississippi River and its major tributaries have continued to be modified and improved since 1990, it is suggested that declines in suspended sediment observed along the mainstem sites during the study period are related to ongoing watershed and channel modifications.

Version 1.0

Posted June 2008

• Report PDF (18 MB)