15 n in marine plants modified by angela quiros. (montoya 2007) there is lots of variation in the 15...
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15N in marine plants
Modified by Angela Quiros
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: 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 15N of zooplankton
• 15N values lowest with highest abundance of Trichodesmium,
• -1 to -2 ‰
• 15N 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
QuickTime™ and a decompressor
are needed to see this picture.
(Montoya 2007)
Nitrate
All marine autotrophsbesides N2-fixingprokaryotes need combined N: nitrate(NO3
-), nitrite (NO2-),
ammonium (NH4+),
typically 4-5‰.
Global Average 15NO3- ~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
QuickTime™ and a decompressor
are needed to see this picture.
(Montoya 2007)
Different areas of the world are on different parts of this curve…
If N in = N out, then productis lighter than the initial N, but as the pool of N is used,the product (phytoplankton)gets heaver. If all the N isused, the product (phytoplankt)N = nitrate value.
QuickTime™ and a decompressor
are needed to see this picture.
(Montoya 2007)
ParticulateOrganicNitrogen
Nitrogen in the Ocean
• PON- Particulate Organic Nitrogen
a. Rapidly sinking particles (marine snow)
b. Slowly sinking particles
c. 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 14NH4
+ & an enrichment of the 15N 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.
QuickTime™ and a decompressor
are needed to see this picture.
(Montoya 2007)
Using isotopes to trace a phytoplankton bloom…isotopic transients
• 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 ofresidual NO3
- pool.• Zooplankton lag behind
Outline
• Broad processes & Inputs
• How much nitrate is used up
• Case Study: Seasonal processes in the Eastern North Pacific
• Nitrogen isotopes in seagrass
The nitrogen isotope biogeochemistry of sinking
particles from the margin of the Eastern N. Pacific (Altabet et al 1999)
QuickTime™ and a decompressor
are needed to see this picture.
-Collected sediment traps,water column samples-Isotopic analysis of NO3
- from seawater-Compared time series sediment traps w/ materialfluxes & compared sediment traps with actualsediments
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 15N values
• 15N of sediment increases with depth, so it’s hard to use sediments to map phytoplankton!
• Isotopically light sinking organic matter lowers the 15N of the subsurface pool below the global deep water average
• Subsurface pool 15N 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 15NO3- (nitrate)
• 15NO3- decreases with depth due
to remineralization of sinking particles
• 15NO3- 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 15N 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.
QuickTime™ and a decompressor
are needed to see this picture.
(Altabet et al 1999)
But time-series data show lotsof variation!
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 15NO3
- to subsurface waters
Bottom Line…
15N 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
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