Nitrate Dynamics at ALOHA and K2

Karen CasciottiWoods Hole Oceanographic Institution

Department of Marine Chemistry and Geochemistry

VERTIGO Experiments

ALOHA 2004• Nitrate isotopic profiles

Coupled 15N and 18O analysis• Rates of microbial activity

Nitrification Rates Whole water samples and MULVFS particles

• Microbial community analysis: free-living vs. particle attached Nitrifying and denitrifying bacteria Suspended particles (MULVFS) Sinking particles (CLAP traps Hg vs. Formalin preservation) Hand-picked squiggles, tufts, and puffs

K2 2005• Nitrate isotopic profiles

Coupled 15N and 18O analysis• Microbial community analysis: Nitrifying and denitrifying

bacteria Free-living vs. particle attached

What we can learn from NO3

- isotopes?

• Supply of N to the surface: NO3-, N2?

• N export and remineralization• N cycling/turnover

Nitrification

Norg

N

N

NH4+

Assimilation

Nitrogen fixation

Denitrification

O

ONO3-

NO

Map of Surface Nitrate

Subarctic Front

Comparison of Basic Chemical and Physical

Parameters

T

S

O2

NO3-

Si

ALOHA = BlueK2 = Red

Nitrate Isotopic Profiles

• 10 profiles from ALOHA

• 6 profiles from K2

• Analyzed for 15N and 18O using “denitrifier method” Allows isotopic analysis of previously

inaccessible pools of NO3-

Pseudomonas aureofaciens

Pseudomonas chlororaphisor

Sigman et al., 2001. Analytical Chemistry: 15N-NO3-

Casciotti et al., 2002. Analytical Chemistry: 18O-NO3-

Inject NO3- or NO2

- sample

N2O (g) Analyze N2O isotopes

NO3- NO2

- NO N2O N2Denitrification takes place

The denitrifier method

K2: T, S, NO3-, and PO4

-

1000

900

800

700

600

500

400

300

200

100

0

Dep

th (

m)

50403020100

Nitrate (M)

43210Phosphate (M)

Nitrate, cast 24 Nitrate, cast 73 Nitrate, cast 27 Nitrate, cast 50 Nitrate, cast 72

Phoshpate, cast 24 Phosphate, cast 73

1000

900

800

700

600

500

400

300

200

100

0

Dep

th (

m)

121086420

Temperature (¼C)

35.034.534.033.533.032.5 Salinity (PSU)

28.027.026.025.0 Sigma Theta

15N and 18O-NO3- at K2

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

12108642018O-NO3

-

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

12108642015N-NO3

-

Temperature vs. d15N and d18O

K2 Station 24

0.000

2.000

4.000

6.000

8.000

10.000

12.000

0.000 2.000 4.000 6.000 8.000 10.000 12.000

d15N

Tem

p

d15N vs. T d18O vs. T

d15N and d18O vs. density

25.200

25.400

25.600

25.800

26.000

26.200

26.400

26.600

26.800

27.000

27.200

27.400

0.000 2.000 4.000 6.000 8.000 10.000 12.000

de

nsit

y

d15N

d18O

Thorium excess

15N and 18O-NO3- at K2

1000

900

800

700

600

500

400

300

200

100

0

Dep

th (

m)

121086420

Temperature (¼C)

35.034.534.033.533.032.5 Salinity (PSU)

28.027.026.025.0 Sigma Theta

1000

800

600

400

200

0

Dep

th (

m)

121086415N-NO3

-

wave5 wave11 wave17 wave23 wave29 wave36

15N-NO3- at K2

T 15NNO3[NO3-]

MLD

EZD

PN

Fluxes of N at Station K2

300 m trap

150 m trap

500 m trap

Export

Remineralization flux

Remineralization flux

PN Flux, 15N

PN Flux, 15N

PN Flux, 15N

NO3-

NO3- NH4

+

Remineralization flux

15N vs. 18O

3.0

2.5

2.0

1.5

1.0

18O

-NO

3-

7.06.56.05.55.015N-NO3

-

15N vs. 18

O, slope =1.11 ± 0.02

12

10

8

6

4

2

0

18O

-NO

3-

12108642015N-NO3

-

15N vs. 18O, slope =1.11 ± 0.02

All data Close-up, station 73

Assimilation

15N-NO3-

18O

-NO

3-

Initial NO3-

N and O isotope dynamics in nitrate

Assimilation

15N-NO3-

18O

-NO

3-

Partially consumed NO3-

Produced PN

N and O isotope dynamics in nitrate

Assimilation

15N-NO3-

18O

-NO

3-

Partially consumed NO3-

Produced PN

N and O isotope dynamics in nitrate

15N vs. 18O

3.0

2.5

2.0

1.5

1.0

18O

-NO

3-

7.06.56.05.55.015N-NO3

-

15N vs. 18

O, slope =1.11 ± 0.02

12

10

8

6

4

2

0

18O

-NO

3-

12108642015N-NO3

-

15N vs. 18O, slope =1.11 ± 0.02

All data Close-up, station 73

ALOHA T, S, NO3-, and PO4

-

3200

2800

2400

2000

1600

1200

800

400

0

Dep

th (

m)

50403020100

Nitrate (M)

3.02.01.00.0Phosphate (M)

3200

2800

2400

2000

1600

1200

800

400

0

Dep

th (

m)

20100

Temperature (C)

35.034.0

Salinity (PSU)

27.525.0

Sigma Theta (kg m-3

)

15N and 18O-NO3- at ALOHA

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321015N-NO3

-

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321018O-NO3

-

Fluxes of N at Station ALOHA

300 m trap

150 m trap

500 m trap

Export

Remineralization flux

Remineralization flux

PN Flux, 15N

PN Flux, 15N

PN Flux, 15N

NO3- NH4

+

Remineralization flux

N2

ALOHA 15N vs. 18O

Assimilation

15N-NO3-

18O

-NO

3-

Fully consumed NO3-

Produced PN

N and O isotope dynamics in nitrate

Assimilation

15N-NO3-

18O

-NO

3-

Fully consumed NO3-

PN

N and O isotope dynamics in nitrate

N2 fixation

ALOHA 15N vs. 18O

15N and 18O-NO3- at ALOHA

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321015N-NO3

-

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321018O-NO3

-

15N and 18O-NO3- at ALOHA

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321015N-NO3

-

3000

2500

2000

1500

1000

500

0

Dep

th (

m)

87654321018O-NO3

-

15N and 18O-NO3- at K2

15NNO3

MLD

EZD

PN

0

50

100

150

200

250

300

350

400

450

-4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0

predicted d15N measured d15N difference

Real data from K2 cast 24