Controls on Nitrogen Inputs, Loads, and In-Stream Concentrations in The
Total Page：16
File Type：pdf, Size：1020Kb
CONTROLS ON NITROGEN INPUTS, LOADS, AND IN-STREAM CONCENTRATIONS IN THE ALTAMAHA RIVER, GEORGIA, AND BEYOND by SYLVIA CATHERINE SCHAEFER (Under the Direction of Merryl Alber) ABSTRACT Human activity has increased the availability of reactive nitrogen (N), an element of critical importance to life. The broad goal of this dissertation was to understand which sources of N to a watershed reach rivers. Nitrogen input budgets were calculated for 18 watersheds on the United States west coast. Fertilizer was the most important source of new N, with atmospheric deposition second. N export was best correlated with streamflow variations, which explained 66% of the variance. Including inputs explained an additional 16% of the variance. Riverine N export averaged 12% of inputs. Percentage export was also best related to streamflow. These results were likely due to the large range of streamflows across the study region, which may have overwhelmed other factors contributing to N export. An in-depth examination of the Altamaha River watershed (Georgia, USA) found that riverine N concentrations and cumulative loads were best related to population density, rather than input factors. Concentrations were highest in the upper watershed and lowest in blackwater streams and a sampling station downstream of a dam. Isotopic analysis of nitrate suggested that most in-stream nitrate is derived from sewage or manure. Where the concentration was particularly low, an atmospheric signal was observed, suggesting that background nitrate is of atmospheric origin. N in this system appeared to be lost primarily on the watershed surface, as only a small proportion of inputs reached the stream, N was transported downstream conservatively, and an isotopic mixing model fit the data well. The Altamaha is comparable to other watersheds worldwide in its nitrate concentrations and population density. A metadata analysis of medium-sized (2,000- 50,000 km2) watersheds, which are under-studied relative to large watersheds, revealed that nitrate concentrations are generally low (<25 µM). We found a relationship between - population density and riverine NO3 concentration, which varied among continents and latitudinal zones. A combination of population density, population density without access to improved sanitation, temperature, precipitation, slope, and fertilizer use, explained 45% of the variation in nitrate concentrations. The relationship had less explanatory power than in large watersheds, potentially due to increased variation in - NO3 concentrations with decreased watershed size. INDEX WORDS: nitrogen, nitrate, concentration, nitrate isotopes, watersheds, Net Anthropogenic Nitrogen Input (NANI), Altamaha River, population density CONTROLS ON NITROGEN INPUTS, LOADS, AND IN-STREAM CONCENTRATIONS IN THE ALTAMAHA RIVER, GEORGIA, AND BEYOND by SYLVIA CATHERINE SCHAEFER B.A., University of Virginia, 2001 M.S., University of Georgia, 2006 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2014 © 2014 Sylvia Catherine Schaefer All Rights Reserved CONTROLS ON NITROGEN INPUTS, LOADS, AND IN-STREAM CONCENTRATIONS IN THE ALTAMAHA RIVER, GEORGIA, AND BEYOND by SYLVIA CATHERINE SCHAEFER Major Professor: Merryl Alber Committee: Jay Brandes James T. Hollibaugh Charles Hopkinson C. Rhett Jackson Samantha Joye Electronic Version Approved: Julie Coffield Interim Dean of the Graduate School The University of Georgia August 2014 DEDICATION To Adrian, whose love and support has made this possible. iv ACKNOWLEDGEMENTS I must first and foremost thank my advisor, Merryl Alber, for her guidance on this project. Thanks also to the rest of my committee—Jay Brandes, Tim Hollibaugh, Chuck Hopkinson, Rhett Jackson, and Mandy Joye, for their advice and assistance. In particular, this project would not have been possible without the help of Jay Brandes, who was exceedingly generous with his time and equipment (and who keeps me updated on Florida Man!). Kim Hunter in the Joye lab and Emily Davenport in the Hopkinson lab helped me run samples and allowed me to use their equipment to do so. Joan Sheldon and Natalie McLenaghan let themselves be dragged sampling in sketchy rural south Georgia with me. Funding for this research was provided by NSF and the GCE-LTER. Moral support for this research was provided by members of the Alber lab past and present, including Caroline McFarlin, Joan Sheldon, Natalie McLenaghan, and Christine Hladik. Thanks also to my other friends, especially Heather “Epic Adventures” Reader and the members of the Classic City Band, for helping me keep my sanity. And last but certainly not least, thanks to my family and especially to my husband Adrian for their love and support through the years. v TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .................................................................................................v LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES ........................................................................................................... xi CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW .........................................1 1.1 The nitrogen cycle..................................................................................1 1.2 N stable isotopes and the N cycle ..........................................................4 1.3 Nitrogen in the watershed ......................................................................9 1.4 Measuring watershed N cycling ...........................................................13 1.5 Models of watershed nitrogen dynamics .............................................15 1.6 Dissertation overview ..........................................................................17 References ..................................................................................................18 2 WATERSHED NITROGEN INPUT AND RIVERINE EXPORT ON THE WEST COAST OF THE US ...........................................................................36 Abstract ......................................................................................................37 2.1 Introduction ..........................................................................................38 2.2 Methods................................................................................................39 2.3 Results ..................................................................................................46 2.4 Discussion ............................................................................................51 vi Acknowledgments......................................................................................56 References ..................................................................................................57 3 INVESTIGATING SOURCES OF NITROGEN IN THE ALTAMAHA RIVER, GEORGIA BASED ON CONCENTRATIONS AND STABLE ISOTOPES OF NITROGEN ...........................................................................73 Abstract ......................................................................................................74 3.1 Introduction ..........................................................................................75 3.2 Methods................................................................................................78 3.3 Results ..................................................................................................87 3.4 Discussion ............................................................................................95 3.5 Conclusions ........................................................................................108 Acknowledgments....................................................................................109 References ................................................................................................109 4 ANALYSIS OF NITRATE CONCENTRATIONS IN SMALL AND MEDIUM-SIZED RIVERS THROUGHOUT THE WORLD ......................159 Abstract ....................................................................................................160 4.1 Introduction ........................................................................................161 4.2 Methods..............................................................................................163 4.3 Results ................................................................................................167 4.4 Discussion ..........................................................................................170 4.5 Conclusions ........................................................................................175 Acknowledgments....................................................................................176 References ................................................................................................176 vii 5 CONCLUSIONS............................................................................................225 References ................................................................................................230 APPENDICES A NUTRIENT BUDGETER: A MATLAB-BASED PROGRAM TO SIMPLIFY THE CALCULATION OF WATERSHED NITROGEN AND PHOSPHORUS BUDGETS ..........................................................................232 viii LIST OF TABLES Page Table 2.1: General characteristics of western US watersheds for 1992 and 2002 .............65 Table 2.2: