15N
in marine plants
Modified by Angela Quiros
There is lots
of variation
in the 15N
values in the
world’s
oceans.
(Montoya 2007)
Outline
• Broad processes & Inputs
• How much nitrate is used up
• Case Study: Seasonal processes in the
Eastern North Pacific
• Nitrogen isotopes in seagrass
Major Inputs of Nitrogen in the
Ocean
1.
Deep Water: waters ~4.5 ‰
•
Upwelled nitrate
2. Atmospheric deposition: waters ~0 ‰
•
Largest in areas near continental land masses
3. Nearshore and continental shelf waters
•
Terrigenous runoff may be a large source
•
Heavy if fecal material, light if agricultural input, soil signature
if relatively pristine
4. N-fixation from the atmosphere: waters ~0 ‰
Major Processes
• N2 fixation
• Nitrification
• Denitrification
Major Processes
• N2 fixation: 15N ~0‰ for phytoplankton; waters ~0 ‰
- Inert N2 from the atmosphere converted to ammonia NH3.
Diazotrophs fix N, symbiotic w/ diatoms occur in dense blooms,
impt part of phytoplankton, contribute a lot to local N budget
N2 + 6H+ = 6e -> 2NH3 -> NH4+ (ammonium cation)
- Produces organic matter depleted in 15N relative to deepwater
NO3-, so it lowers the 15N, while adding to the pool of combined
N
- BUT…low value could also be an indication of recycled NH4+
being used in oligotrophic waters
- In Bermuda, 15NO3- (nitrate) is 2.8‰ lower than oceanic average
because of N2 fixation.
The importance of N-fixation in oligotrophic waters:
Trichodesmium abundance and d15N of zooplankton
• d15N values lowest
with highest abundance
of Trichodesmium,
• -1 to -2 ‰
• d15N values highest in
areas with low
abundance of
Trichodesmium
• waters ~0 ‰
Major Processes
• Nitrification: N available through upwelling & convection
• Biological oxidation of ammonia w/ oxygen into nitrite
then nitrate, significant isotopic fractionation, a source of
depleted N in water column.
• Mineralization is the complete decomposition of organic
material, release of available N, replenishing N cycle
NH3 + O2 -> NO2 + H2O -> NO3- 15N depends on regional processes
- 15N >0
- NH4+ : available from urea
- typically lighter than the global ocean average;
15N is low
Major Processes
• Denitrification – waters isotopically heavy: waters
~8 ‰
• Microbially facilitated, reduce nitrate to produce N2
NO3- -> NO2 -> NO -> N20 -> N2 (gas)
• Shows fractionation, lighter isotopes of N preferred, leaving
heavier N istopes in residual matter
• Discrimates strongly against 15N; negative delta values -40 ‰
• In oxygen minimum zones, denitrifying bacteria use NO3- as
an electron acceptor to support heterotrophic growth,
reducing it to N2.
• In major pelagic oxygen minimum zones, denitrification
consumes only a part of available NO3-, so there is a
significant enrichment of residual NO3- (15-18 ‰)
Outline
• Broad processes & Inputs
• How much nitrate is used up
• Case Study: Seasonal processes in the
Eastern North Pacific
• Nitrogen isotopes in seagrass
Where in the world is the Nitrogen?
Natural abundance of N stable isotopes vary with marine ecosystem
Nitrate
QuickTime™ and a
decompressor
are needed to see this picture.
All marine autotrophs
besides N2-fixing
prokaryotes need
combined N: nitrate
(NO3-), nitrite (NO2-),
ammonium (NH4+),
typically 4-5‰.
(Montoya 2007)
Global Average d15NO3- ~4‰ - 5‰
global 15N values of deep water
Deepwater NO3- is the largest pool of combined N in the ocean.
N2-fixation adds to it, while denitrification removes N from it
Different areas of the world are on different parts of this curve…
QuickTime™ and a
decompressor
are needed to see this picture.
If N in = N out, then product
is lighter than the initial N,
but as the pool of N is used,
the product (phytoplankton)
gets heaver. If all the N is
used, the product (phytoplankt)
N = nitrate value.
(Montoya 2007)
QuickTime™ and a
decompressor
are needed to see this picture.
Particulate
Organic
Nitrogen
(Montoya 2007)
Nitrogen in the Ocean
•
a.
b.
c.
PON- Particulate Organic Nitrogen
Rapidly sinking particles (marine snow)
Slowly sinking particles
Upwelled PON from below the euphotic
zone
(Michener & Kaufman 2007)
PON plays a role in vertical transport of
material out of the euphotic zone
•
•
•
15N
of PON will determine
the 15N of phytoplankton
Zooplankton are
ammonotelic, so
deamination rxns produce
NH4+ depleted in 15N, there
is a preferential loss of
14NH + & an enrichment of
4
15
the N in the body. 14N is
retained in the upper water
column through tight
recycling.
Rapidly sinking particles
transport 15N into the deep
ocean
QuickTime™ and a
decompressor
are needed to see this picture.
Using isotopes to trace a phytoplankton bloom…isotopic transients
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• Phytoplankton fractionate
15N during assimilation of
nitrate, so preferential
uptake of 14NO3 by
phytoplankton.
At the start of a bloom,
production of organic
matter is depleted in 15N,
relative to available NO3-.
As bloom progresses,
preferential removal of
14NO3- increases the 15N of
residual NO3- pool.
• Zooplankton lag behind
(Montoya 2007)
Outline
• Broad processes & Inputs
• How much nitrate is used up
• Case Study: Seasonal processes in the
Eastern North Pacific
• Nitrogen isotopes in seagrass
Upwelling
equatorward winds
surface waters
nutrient rich water from depth
The nitrogen isotope biogeochemistry of sinking
particles from the margin of the Eastern N. Pacific
(Altabet et al 1999)
-Collected sediment traps,
water column samples
-Isotopic analysis of NO3from seawater
-Compared time series
sediment traps w/ material
QuickTime™ and a
decompressor
fluxes & compared
are needed to see this picture.
sediment traps with actual
sediments
N isotopes in sinking particles in
the Eastern N. Pacific
• Upwelling- filament of cold, nutrient rich water
brought to the surface from 1 section of the coast,
advection at the surface; direction south or
offshore
• Episodes of high productivity & particle flux
• El Nino results in sharp reduction of nutrients
because no persistent upwelling & nutrient rich
water is deeper
• CA Undercurrent- coast to 100km offshore,
10/20m-600m, core @ 150m, source of upwelling
Nitrogen Fixation
• Lower than average d15N values
• d15N of sediment increases with
depth, so it’s hard to use sediments
to map phytoplankton!
• Isotopically light sinking organic
matter lowers the d15N of the
subsurface pool below the global
deep water average
• Subsurface pool d15N is a mix
between particle flux from the
surface and vertical mixing of deep
water
(Altabet et al 1999)
Nutrient Profiles – Monterey Bay
• Inverse relationship between
[NO3-] and d15NO3- (nitrate)
• d15NO3- decreases with depth due
to remineralization of sinking
particles
• d15NO3- mean is 8‰, which is
higher than oceanic values (4‰ 5‰) – probably due to infusion of
California undercurrent waters and
denitrification
(Altabet et al 1999)
Nutrient Profiles – Gulf of California
• Surface waters are enriched
compared to Monterey profile
• Increase in d15N at the surface
is most likely due to uptake by
phytoplankton
•Nitrate drawdown (by
denitrifying bacteria) within
OMZ corresponds with increase
in d15N, mean 10-12‰ because
there is more denitrification in
the south. Denitrification
makes N heavy.
(Altabet et al 1999)
Particles get heavier as you go deeper
QuickTime™ and a
decompressor
are needed to see this picture.
But time-series data show lots
of variation!
(Altabet et al 1999)
QuickTime™ and a
decompressor
are needed to see this picture.
N isotopes in sinking particles in
the Eastern N. Pacific
• New N or other NO3- not significant contributors
• Sediment traps are good paleoceanographic
records for 15NO3• Denitrification is the principle modifier for
subsurface NO3- responsible for >8‰ vs open
ocean is 4.6‰
• 15NO3- high in Monterey & San Pedro even
though they are not zones of active water column
denitrification because the ETNP supplies 15NO3to subsurface waters
Bottom Line…
d15N of phytoplankton depends on:
• denitrification
• nitrogen fixation
• upwelling and currents
Outline
• Broad processes & Inputs
• How much nitrate is used up
• Case Study: Seasonal processes in the
Eastern North Pacific
• Nitrogen isotopes in seagrass
QuickTime™ and a
decompressor
are needed to see this picture.
Food web integrators & environmental tracers
Nitrogen isotopes in seagrass
•
15N
of seagrass (S.G) varies from -2‰ to 12.3‰, with
most frequent values 0 to 8‰
• Variations in isotopic ratio are due to inorganic N
incorporation from the water column and sediment
• 15N close to 0‰ are due to N2 fixation by associated
S.G. organisms
Nitrogen Isotopes in Seagrass
• Food webs
•
•
• 15N used to assess
food webs because of
the 15N enrichment
with increasing trophic
position
Environment
Isotopic signatures from
nitrate in wastewater,
fertiliser, atmospheric
deposition. Wastewater has
higher 15N because of
human sources & isotopic
discrimination during
remineralisation
• Used to map sewage
because longer turnover
time