15 N in marine plants Modified by Angela Quiros

(Montoya 2007) There is lots of variation in the 15 N values in the world’s oceans.

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 N 2 fixation Nitrification Denitrification

Major Processes N 2 fixation: 15 N ~0‰ for phytoplankton; waters ~0 ‰ - Inert N 2 from the atmosphere converted to ammonia NH 3. Diazotrophs fix N, symbiotic w/ diatoms occur in dense blooms, impt part of phytoplankton, contribute a lot to local N budget N 2 + 6H + = 6e -> 2NH 3 -> NH 4 + (ammonium cation) -Produces organic matter depleted in 15 N relative to deepwater NO 3 -, so it lowers the 15 N, while adding to the pool of combined N -BUT…low value could also be an indication of recycled NH 4 + being used in oligotrophic waters -In Bermuda, 15 NO 3 - (nitrate) is 2.8‰ lower than oceanic average because of N 2 fixation.

The importance of N-fixation in oligotrophic waters: Trichodesmium abundance and  15 N of zooplankton  15 N values lowest with highest abundance of Trichodesmium, -1 to -2 ‰  15 N 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 NH 3 + O 2 -> NO 2 + H 2 O -> NO N depends on regional processes - 15 N >0 - NH 4 + : available from urea - typically lighter than the global ocean average; 15 N is low

Major Processes Denitrification – waters isotopically heavy: waters ~8 ‰ Microbially facilitated, reduce nitrate to produce N 2 NO 3 - -> NO 2 -> NO -> N 2 0 -> N 2 (gas) Shows fractionation, lighter isotopes of N preferred, leaving heavier N istopes in residual matter Discrimates strongly against 15 N; negative delta values -40 ‰ In oxygen minimum zones, denitrifying bacteria use NO 3 - as an electron acceptor to support heterotrophic growth, reducing it to N 2. In major pelagic oxygen minimum zones, denitrification consumes only a part of available NO 3 -, so there is a significant enrichment of residual NO 3 - (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

(Montoya 2007) Nitrate All marine autotrophs besides N 2 -fixing prokaryotes need combined N: nitrate (NO 3 - ), nitrite (NO 2 - ), ammonium (NH 4 + ), typically 4-5‰.

Global Average  15 NO 3 - ~4‰ - 5‰ global 15N values of deep water Deepwater NO 3 - is the largest pool of combined N in the ocean. N 2 -fixation adds to it, while denitrification removes N from it

(Montoya 2007) Different areas of the world are on different parts of this curve… 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) Particulate Organic Nitrogen

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 15 N of PON will determine the 15 N of phytoplankton Zooplankton are ammonotelic, so deamination rxns produce NH 4 + depleted in 15 N, there is a preferential loss of 14 NH 4 + & an enrichment of the 15 N in the body. 14 N is retained in the upper water column through tight recycling. Rapidly sinking particles transport 15 N into the deep ocean

(Montoya 2007) Using isotopes to trace a phytoplankton bloom…isotopic transients Phytoplankton fractionate 15 N during assimilation of nitrate, so preferential uptake of 14 NO 3 by phytoplankton. At the start of a bloom, production of organic matter is depleted in 15N, relative to available NO 3 -. As bloom progresses, preferential removal of 14NO 3 - increases the 15 N of residual NO 3 - 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

i Upwelling nutrient rich water from depth equatorward winds surface waters

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 NO 3 - from seawater -Compared time series sediment traps w/ material fluxes & compared 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, 150m, source of upwelling

Nitrogen Fixation Lower than average  15 N values  15 N of sediment increases with depth, so it’s hard to use sediments to map phytoplankton! Isotopically light sinking organic matter lowers the  15 N of the subsurface pool below the global deep water average Subsurface pool  15 N 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 [NO 3 - ] and  15 NO 3 - (nitrate)  15 NO 3 - decreases with depth due to remineralization of sinking particles  15 NO 3 - 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  15 N at the surface is most likely due to uptake by phytoplankton Nitrate drawdown (by denitrifying bacteria) within OMZ corresponds with increase in d 15 N, 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 (Altabet et al 1999) But time-series data show lots of variation!

N isotopes in sinking particles in the Eastern N. Pacific New N or other NO 3 - not significant contributors Sediment traps are good paleoceanographic records for 15 NO 3 - Denitrification is the principle modifier for subsurface NO 3 - responsible for >8 ‰ vs open ocean is 4.6 ‰ 15 NO 3 - high in Monterey & San Pedro even though they are not zones of active water column denitrification because the ETNP supplies 15 NO 3 - to subsurface waters

Bottom Line…  15 N 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

Food web integrators & environmental tracers

Nitrogen isotopes in seagrass 15 N 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 15 N close to 0‰ are due to N 2 fixation by associated S.G. organisms

Nitrogen Isotopes in Seagrass Food webs 15 N used to assess food webs because of the 15 N enrichment with increasing trophic position Environment Isotopic signatures from nitrate in wastewater, fertiliser, atmospheric deposition. Wastewater has higher 15 N because of human sources & isotopic discrimination during remineralisation Used to map sewage because longer turnover time