Student Investigator
Dustin Johnson, Graduate Student
Earth Sciences, UMaine
Synopsis
The aim of this project is to investigate the relationships and mechanisms involved with complexes of dissolved organic carbon (DOC), iron (Fe) and phosphorus (P) during exposure to UV radiation. A series of batch solutions varied in concentration of DOC, Fe, and P are exposed to UV-A radiation (315 – 400 nm) for equivalent periods of time. The components of the experiment are analyzed for chemical and physical changes that affect how P is cycled photochemically. DOC absorption of UV radiation is expected to decrease with time during irradiation. This indicates that the DOC structure is altered which may lead to the possible breakdown of complexes and subsequent liberation of ionic Fe and P to the water column. These experiments are designed to examine this fundamental aspect of P cycling in surface waters containing DOC and Fe.
Introduction
Decades of research have studied the importance of P as a limiting-nutrient and its impact on the trophic status of surface waters. P has also been shown to interact with DOC and Fe in aquatic environments. Surface waters that have DOC and are exposed to UV radiation will undergo photochemical reactions that affect the cycling of nutrients and trophic dynamics of surficial aquatic systems. Where substantial research has been dedicated to P cycling and its effect on biota, there has been little research on how P cycles photochemically in the presence of DOC and DOC-Fe complexes. This project is designed to evaluate the photochemical interactions of DOC and Fe with P in surface waters. This analysis will provide a better understanding of the relationships and mechanisms involved with the photochemical cycling of P and any relation this may have to trophic dynamics of surface waters.
Objectives and Hypotheses
- Quantify the effects of UV-A radiation (315 - 400 nm) on the fractionation of DOC-Fe-P and DOC-P complexes. I hypothesize that irradiation will break down the organic complexes liberating Fe and P from DOC.
- Examine Fe fractionation that may occur in relation to any P sequestration or liberation during irradiation. Organically-bound Fe will largely be converted to ionic Fe while some particulate Fe will also form. As ionic Fe forms it will bind and sequester P.
- Analyze the effects of UV-A radiation on DOC absorbance during and after irradiation. DOC absorption will decrease with prolonged exposure to UV-A radiation.
- Determine the mechanisms and relationships involved with P sequestration or liberation from DOC and DOC-Fe complexes during irradiation.
Methods
I intend to run 26 laboratory based batch experiments to understand how varying DOC and Fe concentrations affect P chemistry during exposure to UV-A radiation. DOC stock solutions will be prepared using Suwannee River fulvic acid and Milli-Q water in concentrations listed in Table 2. FeCl3 and K3PO4 salts will be added to the stock solutions and shaken for 24 hours prior to irradiation to homogenize. After homogenization, 500 ml aliquots will be placed in quartz crystal beakers for experimentation. Additionally, 500 ml aliquots will be prepared in opaque bottles as a dark control. Samples will then be placed in a Rayonet Photochemical Chamber Reactor and will be irradiated in the UV-A range. Aliquots will be sampled at 0, 15, 30, 45, 60, 90, and 120 minutes of irradiation time. Samples will be measured for DOC concentration using a DOC analyzer within 24 hours of sampling. Fe will be measured using flame atomic absorption spectroscopy while P will be measured colorimetrically.
Natural water samples will also be collected from the Third Machias Lake in Washington Co. Maine. This field location was chosen based on a preliminary study in December of 2008, which indicated that the surface water of the lake’s outlet has a DOC concentration of 0.93 mmol L-1, an Fe concentration of 4.05 μmol L-1, and a P concentration of 0.28 μmol L-1. The lake is very open with little to no vegetative cover allowing for full exposure to sunlight throughout the day. Samples will be collected in May, July, and October and run using the same experimental protocol as described above. This will allow for the investigation into the relationships between DOC, Fe, and P in laboratory and field based water samples when irradiated.
Impact of Project
P is a limiting-nutrient in most surface water environments and is thus important to understanding the trophic dynamics of surface waters. Increased concentrations of DOC and DOC-Fe complexes have been shown to affect the cycling of P in natural waters. These complexes potentially affect the bioavailability of P in the water column. Although P is involved in many chemical reactions in surface water environments, little work has been done to document how UV radiation affects P cycling in the presence of DOC and DOC-Fe complexes.
Expected Deliverables
- Annual report to the Mitchell Center
- Poster presentation at the 2010 Maine Water Conference
- Manuscript which will be submitted to Limnology and Oceanography
- Master’s Thesis
Table 2: Proposed Experiments
DOC (mmol/L) |
Fe (μmol/L) |
P (μmol/L) |
Treatment |
0.07 |
0 |
6.46 |
UV |
0.07 |
0 |
6.46 |
Dark |
0.07 |
5 |
6.46 |
UV |
0.35 |
0 |
6.46 |
UV |
0.35 |
5 |
6.46 |
UV |
0.35 |
5 |
6.46 |
Dark |
0.71 |
0 |
6.46 |
UV |
0.71 |
5 |
6.46 |
UV |
1.43 |
0 |
0.484 |
UV |
1.43 |
0 |
1.61 |
UV |
1.43 |
0 |
1.61 |
Dark |
1.43 |
0 |
3.22 |
UV |
1.43 |
0 |
6.46 |
UV |
1.43 |
0 |
12.91 |
UV |
1.43 |
3 |
6.46 |
UV |
1.43 |
5 |
0.484 |
UV |
1.43 |
5 |
1.61 |
UV |
1.43 |
5 |
3.22 |
UV |
1.43 |
5 |
6.46 |
Dark |
1.43 |
5 |
12.91 |
UV |
1.43 |
5 |
12.91 |
Dark |
1.43 |
7 |
6.46 |
UV |
1.43 |
7 |
6.46 |
Dark |
