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In cooperation with the U.S. Environmental Protection Agency Gulf of Mexico Program, Nutrient Enrichment Committee

Trends in Nutrient Inflows to the Gulf of Mexico From Streams Draining the Conterminous United States, 1972–93

U.S. Geological Survey
Water-Resources Investigations Report 96–4113

By David D. Dunn

This report is available as a pdf (7.09 MB)


Abstract

Trends are computed for nutrient inflows from 37 streams discharging into the Gulf of Mexico. The drainage areas of these streams represent about 86 percent of the drainage area to the Gulf from the conterminous United States. The period analyzed varies for each stream, but generally includes water years 1972–93. Stations included in this analysis primarily are part of the National Stream Quality Accounting Network operated by the U.S. Geological Survey.

Short-term trends for each station are indicated by LOWESS smooth lines superimposed on graphs of the relations between flow-adjusted residuals of concentration and time or load and time. Long-term trends were evaluated using Kendall’s tau and the slope of the Kendall-Theil robust line. Long-term trends for each station are indicated by Kendall-Theil robust lines superimposed on the aforementioned graphs. Annual loads are estimated with regression analysis and corrected for log-transformation bias with the Minimum Variance Unbiased Estimator. Trends in annual streamflow are presented to aid in the interpretation of trends in nutrient inflows.

Statistically significant, long-term increases in flow-adjusted residual concentrations of total nitrogen were detected at 19 stations, decreases were detected at 7 stations, and no significant trends were detected at 11 stations. Long-term increases in total nitrogen load were detected at 3 stations, decreases were detected at 4 stations, and no significant trends were detected at 30 stations.

Long-term increases in flow-adjusted residual concentrations of total phosphorus were detected at 7 stations, decreases were detected at 11 stations, and no significant trends were detected at 19 stations. Long-term increases in total phosphorus load were detected at 3 stations, decreases were detected at 12 stations, and no significant trends were detected at 22 stations.

The median yields (mean annual load divided by drainage area) of total nitrogen and total phosphorus were significantly lower (p < 0.05) for the 13 streams in Texas than for the 24 streams east of the Sabine River (Texas-Louisiana boundary).

Statistically significant trends in annual streamflow were detected at only four stations. However, annual streamflow influences trends in load, even when the streamflow trends are not statistically significant.

Table of Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Sources and Description of Data

Methods of Analysis

Computation of Flow-Adjusted Nutrient Concentrations

Computation of Nutrient Loads

Detection of Trends

Trends in Nutrient Inflows

Trends in Nutrient Concentrations

Total Nitrogen

Total Phosphorus

Trends in Nutrient Loads

Total Nitrogen

Total Phosphorus

Factors Affecting Trends in Nutrient Inflows

Data Limitations

Influence of Streamflow on Loads

Mississippi River Diversion to Atchafalaya River

Summary and Conclusions

References Cited

Figures

1–5.   Maps showing:
  1.   Locations of streamflow-gaging stations on the lower reaches of streams discharging into the Gulf of Mexico
  2.   Trends in flow-adjusted residuals of total nitrogen concentration for streams discharging into the Gulf of Mexico, 1972–93
  3.   Trends in flow-adjusted residuals of total phosphorus concentration for streams discharging into the Gulf of Mexico, 1972–93
  4.   Trends in estimated annual loads of total nitrogen for streams discharging into the Gulf of Mexico, 1972–93
  5.   Trends in estimated annual loads of total phosphorus for streams discharging into the Gulf of Mexico, 1972–93
6–42.   Graphs showing relation of total nitrogen and total phosphorus concentration to daily mean streamflow; and temporal trends in annual streamflow, in flow-adjusted residuals of total nitrogen and total phosphorus concentration, and in estimated annual total nitrogen and total phosphorus loads for:
  6.   Rio Grande, Tex. (RIO) (08475000)
  7.   Nueces River, Tex. (NUE) (08210000)
  8.   Mission River, Tex. (MSN) (08189500)
  9.   San Antonio River, Tex. (SAN) (08188500)
  10.   Guadalupe River, Tex. (GUA) (08176500)
  11.   Lavaca River, Tex. (LAV) (08164000)
  12.   Colorado River, Tex. (COL) (08162000)
  13.   Brazos River, Tex. (BRZ) (08114000)
  14.   Brays Bayou, Tex. (BRA) (08075000)
  15.   Whiteoak Bayou, Tex. (WHI) (08074500)
  16.   Trinity River, Tex. (TRI) (08066500)
  17.   Neches River, Tex. (NEC) (08041000)
  18.   Sabine River, Tex. (SAB) (08030500)
  19.   Calcasieu River, La. (CAL) (08015500)
  20.   Atchafalaya River, La. (ATC) (07381495)
  21.   Mississippi River, La. (MIS) (07373420)
  22.   Amite River, La. (AMI) (07378510)
  23.   Tangipahoa River, La. (TAN) (07375500)
  24.   Bogue Chitto, La. (CHI) (02492000)
  25.   Pearl River, La. (PRL) (02489500)
  26.   Pascagoula River, Miss. (PAS) (02479020)
  27.   Tombigbee River, Ala.(TOM) (02469762)
  28.   Alabama River, Ala. (ALA) (02429500)
  29.   Perdido River, Fla. (PER) (02376500)
  30.   Escambia River, Fla. (ESC) (02375500)
  31.   Yellow River, Fla. (YEL) (02368000)
  32.   Choctawhatchee River, Fla. (CHO) (02366500)
  33.   Chipola River, Fla. (CHP) (02359000)
  34.   Apalachicola River, Fla. (APA) (02358000)
  35.   Ochlockonee River, Fla. (OCH) (02329000)
  36.   Suwannee River, Fla. (SUW) (02323500)
  37.   Anclote River, Fla. (ANC) (02310000)
  38.   Hillsborough River, Fla. (HIL) (02304000)
  39.   Alafia River, Fla. (ALF) (02301500)
  40.   Little Manatee River, Fla. (MAN) (02300500)
  41.   Myakka River, Fla. (MYA) (02298830)
  42.   Peace River, Fla. (PEA) (02296750)

Tables

1.   Locations and drainage areas for selected streamflow-gaging stations on streams discharging to the Gulf of Mexico
2.   Period of record and number of nutrient analyses available for selected streamflow-gaging stations on streams discharging to the Gulf of Mexico
3.   Trends in annual streamflow and total nitrogen inflows for selected streamflow-gaging stations on streams discharging to the Gulf of Mexico
4.   Trends in annual streamflow and total phosphorus inflows for selected streamflow-gaging stations on streams discharging to the Gulf of Mexico
5.   Parameter estimates for total nitrogen load model
6.   Parameter estimates for total phosphorus load model
7.   Mean annual streamflow and nutrient inflows for the Mississippi River, Atchafalaya River, Old River Outflow Channel, and Red River, 1978–93

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