photochemical alteration of dissolved organic nitrogen in the surface ocean of the north atlantic
Post on 31-Dec-2015
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Background: A DiscrepancySargasso sea water has
<30nM ammoniumBarely measurable
Ammonia effluxes from the surface of the oligotrophic oceanic gyre
The measured efflux is significantly higher than the efflux calculated by Henry’s law
(Johnson et al., 2008, Lipschultz 2001)
NO3-
NH4+
DON NH4+
ParticlesNH4HSO4
(NH4)2SO4
NH4NO3
NaNO3
NH3(g)
Rain [NH4+ ] = 3.5 μM
NH4+ in the Nitrogen cycle
4 μM <30 nM
Oceanic concentrations from the Bermuda Atlantic Time-series Study (BATS)
Lipschultz 2001, Knapp et al., 2005
Project goalTo test the hypothesis that the photo-
oxidation of the high concentration surface DON pool contributes to ammonia efflux from the subtropical surface ocean
Testing the hypothesisDesign an apparatus and/or system to
demonstrate the conversion of dissolved organic nitrogen (DON) into ammonium.
Test the closed system at Princeton and test the open system at the Bermuda Institute of Ocean Sciences.
MethodsAcidified filters to capture NH3 efflux:
Filter impregnation (Quinn 1990)
Extracted captured NH4+ from filters – sonication
Collected and filtered Sargasso seawater using 8L Niskin bottles
Orthophthalaldehde (OPA) fluorescence techniques for measuring nanomolar concentrations of NH4+ (Holmes et al 1999)
Irradiated systems in ambient sunlight with dark control
OPA FluorescenceMeasures nM
concentrations of ammonium
Very accurateExample: Standard
curve with concentrations 0, 10, 25, 100, 250, and 1000 nM
y = 202.1x + 9279.2R² = 0.9998
0
50000
100000
150000
200000
250000
0 500 1000 1500Ra
w fl
uo
res
ce
nce
un
its
(RF
U)
Concentration (nM)
6/7/11 Standard Curve
Testing the Closed SystemFilters were exposed to air on a lab bench for
10, 20, 30, and 40 minutes to investigate contamination
Filters exposed to air for 10 minutes were stored in Ziploc bags for 10, 20, 30, and 40 minutes, 24 and 48 hours to assess contamination associated with storage
One filter was left in the closed system and another filter was left in a fume hood overnight to confirm that the system was air tight
The experiment was run with two bottles: one filled with 10mM NH4+ and the other with HPW
ResultsAir exposure
contaminated filters
Storing in ziploc did not contaminate filters
The closed system is definitely airtight: The filter left in the fume hood had several orders of magnitude more NH4+ than that stored in the closed system
0
50
100
150
200
250
300
350
400
450
0 5 10 15 20 25 30 35 40 45
[NH
4+
] (n
M)
Time (minutes)
Exposure
Times
10min
Linear (Times)
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25 30 35
[NH
4+
]
Time in bag
Ziploc
Time(minutes)
Time(minutes)
[NH
4+]
[NH
4+]
Exposure
Ziploc
The Open SystemVacuum pump
Filter packs (removes ammonia from air)
Ring Stand and clamps
Reflux
Condenser
Filters (capture ammonium)
Testing the Open SystemPhysically construct the system
Develop and test the standard operating procedure in order to optimize the performance of the open system
Incubated filtered Sargasso Sea water collected during scientific cruise under sunlight and dark conditions
Seawater collectionSeawater was collected aboard the R/V Stommel using Niskin bottles and transferred to 4L polycarbonate bottles. The water was filtered through a 0.2 μm polycarbonate filter using vacuum filtration.
Seawater was collected aboard the R/V Atlantic Explorer by inline filtration directly from the ship’s underway system.
Lessons LearnedThe temperature controlled experiments were
supposed to be run on the Atlantic Explorer 2 day cruise. However, it rained continuously and irradiation experiments were not possible.
Ammonium samples could not be analyzed at BIOS. The samples were frozen and brought back to Princeton.
The fluorometer was stolen, and the samples could not be analyzed until it was replaced.
Results from the open system were inconclusive: contamination and/or detection limit problems (see above)
SummaryClosed system: airtight limiting contamination
from ambient air. Could be useful for concept testing if modified to increase air/water surface interaction.
Open system: more representative , can control temperature, enhance air/water surface interaction
Future workContinue experimenting with open systemAmend with higher ammonium concentrationAnalyze samples same day to minimize storage
and/or contamination issues
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