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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF ENGINEERING, SCIENCE & MATHEMATICS School of Ocean and Earth Science DISSOLVED ORGANIC NITROGEN IN THE RIVER TEST AND ESTUARY. by Judith Melanie Homewood BSc. (Hons) Thesis for the degree of Doctor of Philosophy May 2005 Graduate School of the Southampton Oceanography Centre This PhD dissertation by Judith Homewood has been produced under the supervision of the following persons Supervisors Dr. Duncan A. Purdie Dr. Peter J. Shaw Chair of Advisory Panel Dr. Andrew N. Gooday Abstract UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING, SCIENCE & MATHEMATICS SCHOOL OF OCEAN & EARTH SCIENCES Doctor of Philosophy DISSOLVED ORGANIC NITROGEN IN THE RIVER TEST AND ESTUARY. By Judith Melanie Homewood BSc. (Hons) Studies of nutrients in rivers and estuaries have generally focused on dissolved inorganic nitrogen (DIN), although this accounts for only a part of the total nitrogen (TN) in these systems. Dissolved organic nitrogen (DON) has not yet been fully considered as a factor contributing to the nitrogen pool in aquatic systems. Advances in the analytical determination and characterisation of DON have identified that it is potentially biologically available to phytoplankton and bacteria. Very few studies of DON have been conducted on UK rivers and estuaries therefore little is known of the concentrations, proportion of TN, seasonal variations and DON load entering estuarine waters from riverine sources. An 18 month survey of the lower reaches of the River Test and upper estuary were conducted between July 2001 and December 2002 to investigate the temporal variation in nutrient concentrations. DON concentrations up to 152 µM were measured in saline samples and concentrations of up to 100 µM were recorded in freshwater samples from the River Test. DON was the second largest TN component, contributing up to 7 % of TN in the river and 13 % of TN in the upper Test estuarine waters. DON showed no apparent relationship with season, flow or salinity, whereas dissolved organic carbon (DOC) concentration showed some seasonal changes. Concentrations of DON were reduced by tidal salt marshes and sewage treatment works were a source of DOC to the river. The estimated DON load entering the Test estuary in 2002 was 1.9 x 104 mol km-2 yr-1, one order of magnitude lower than the nitrate load. The DON area-normalised load was in good agreement with other UK river systems. The assimilation of different molecular size fractions of DON and DOC by bacteria was investigated in water collected from the lower reaches of the river. A combination of ultrafiltration and bacterial bioassays showed that the DON and DOC pools were made up of a range of molecular size classes but these did not appear to influence the growth of freshwater bacteria. I List of Contents LIST OF CONTENTS ABSTRACT .....................................................................................................................I LIST OF CONTENTS....................................................................................................II LIST OF TABLES..........................................................................................................V LIST OF FIGURES...................................................................................................... VI DECLARATION OF AUTHORSHIP ...........................................................................X ACKNOWLEDGEMENTS.......................................................................................... XI LIST OF ABBREVIATIONS.................................................................................... XIII 1 CHAPTER ONE. INTRODUCTION.......................................................................1 1.1 NITROGEN CYCLE ..............................................................................................1 1.1.1 SIGNIFICANCE OF NITROGEN IN AQUATIC ECOSYSTEMS .............................................1 1.1.2 CURRENT TRENDS AND IMPLICATIONS OF THE GLOBAL NITROGEN BUDGET................3 1.2 DISSOLVED ORGANIC NITROGEN (DON) IN AQUATIC ECOSYSTEMS ..6 1.2.1 BACKGROUND AND CONTEXT ..................................................................................6 1.2.2 SOURCES OF DON...................................................................................................7 1.2.3 SINKS OF DON........................................................................................................8 1.2.4 VARIATIONS OF DON IN RIVERS AND ESTUARIES ...................................................11 1.3 BIOAVAILABILITY AND MOLECULAR SIZE COMPOSITION OF DOM.13 1.3.1 BIOAVAILABILITY OF DOM...................................................................................13 1.3.2 CHEMICAL CHARACTERISATION OF DOM ..............................................................14 1.3.3 SIZE-REACTIVITY CONTINUUM...............................................................................16 1.4 CHARACTERISTICS OF THE TEST RIVER AND ESTUARY......................17 1.5 THESIS AIMS AND OBJECTIVES....................................................................21 1.6 THESIS STRUCTURE.........................................................................................21 2 CHAPTER TWO. METHODOLOGY ..................................................................23 2.1 SAMPLING STRATEGY.....................................................................................23 2.2 SAMPLING AND STORAGE PROTOCOL.......................................................23 2.2.1 SAMPLE COLLECTION ............................................................................................23 II List of Contents 2.2.2 FILTRATION AND SAMPLE STORAGE .......................................................................26 2.3 ANALYTICAL PROCEDURES ..........................................................................26 2.3.1 NITRATE ...............................................................................................................27 2.3.2 AMMONIUM (INDOPHENOL BLUE METHOD).............................................................28 2.3.3 AMMONIUM (OPA METHOD) .................................................................................31 2.3.4 COMPARISON OF IPB AND OPA AMMONIUM ANALYSIS ..........................................33 2.3.5 DOC AND TDN ....................................................................................................33 DOC blanks.....................................................................................................................41 Oxidation efficiency ........................................................................................................42 Calibration consistency....................................................................................................45 Certified reference material..............................................................................................45 GANE community TDN intercalibration..........................................................................46 2.3.6 PON AND POC .....................................................................................................49 2.3.7 CHLOROPHYLL A...................................................................................................50 2.4 ENVIRONMENT AGENCY SAMPLING AND ANALYSIS PROCEDURES .50 3 CHAPTER THREE. TEMPORAL VARIATIONS IN THE INORGANIC AND ORGANIC NITROGEN IN THE RIVER TEST AND ITS ESTUARY .....................52 3.1 INTRODUCTION.................................................................................................52 3.2 METHODOLOGY................................................................................................53 3.3 RESULTS..............................................................................................................53 3.3.1 FIELD MEASUREMENTS ..........................................................................................53 3.3.2 NUTRIENT OBSERVATIONS .....................................................................................55 3.3.3 TOTAL NITROGEN AND TOTAL ORGANIC CARBON COMPOSITION ..............................71 3.3.4 NITROGEN LOADING OF THE TEST ESTUARY ...........................................................78 3.4 DISCUSSION........................................................................................................87 3.4.1 CONCENTRATIONS AND PROPORTIONS OF DON AND DOC .....................................87 3.4.2 TEMPORAL VARIATIONS IN DISSOLVED ORGANIC MATTER.......................................93 3.4.3 NITROGEN LOAD ENTERING THE TEST ESTUARY .....................................................96 3.5 CHAPTER SUMMARY .....................................................................................100