Nitrogen uptake conditions at VERTIGO study sites
M Elskens, N Brion, F Dehairs, W Baeyens & N Savoye
Pleinlaan 2, B-1050 Brussel , Belgium ([email protected])
Introduction Nitrogen uptake conditions were surveyed in the photic zone at station Aloha 22°45N-158°W during 2004 VERTIGO Hawaii cruise and at station K2 47°N-161°E during 2005 VERTIGO Northwest Pacific cruise using
stable isotope techniques. We aimed at determining new production rates, and therefore 15N and 13C isotopically labelled compounds were used in a series of tracer experiments conducted over a 3 week period at the VERTIGO sites.
Experimental design
1.At station Aloha, combined 15N (NO3, NO2, NH4, N2) and 13C tracer experiments were conducted over periods of 12
hours under constant illumination and neutral density screens to mimic in situ light attenuation. Sampling depths
were 5, 25, 50 & 125m.
2.At station K2, 15N (NO3, NH4) and 13C tracer experiments were conducted over periods of 2, 4 and 6 hours under
constant illumination and neutral density screens to mimic in situ light attenuation. Sampling depths were 5, 25, 40
& 50m.
Data analysis: A model comparison
approach
New production assessment
1.Primary production PP (13C)
Field measurements
Mass balance differential equations
2.f-ratio (15N):
Numerical integration
Optimization
UptkNO3  UptkN2

f  ratio 
 UptkDIN


 UptkDIN  Uptk NH4  NO3  NO2  N2 
Statistics
3.New production: PNEW = f-ratio x PP
Uncertainty assessment
4.Integration of PP and PNEW over the euphotic layer
Parameter estimates: C and N flux rates
N uptake conditions at the VERTIGO sampling sites
K2
Aloha
100
n = 32
90
80
% of DIN uptake rate
100
90
82%
80%
Stations
80
Specific NO3
uptake rate
d-1 VNO3
f-ratio
%
0.0020.006
720
70
70
60
60
50
50
40
40
Ratio to POC flux at 150m (NBST)
30
30
K2
20
20
18%
10
0.020.06
10
9%
6%
4%
0
NH4
Aloha
NO3
NO2
N2
PP
PNEW
mg C m-2 d-1 mg C m-2 d-1
1024
Ratio to POC flux at 150m (NBST)
0
143240
1242
10-12
0.8-1
348585
50171
10-17
2-3
NH4 NO3
Acknowledgments
Primary and new production variability over the sampling period
320
13
280
700
C-PP
13
t-test p < 0.001
95% CI
600
Mean PP
500
C-PP
160
95% CI
120
80
60
PNEW
t-test p < 0.001
40
400
300
200
PNEW
t-test p = 0.005
150
100
50
20
0
170
-2
d
-1
200
Production mg C m
Production mg C m
-2
d
-1
240
172
174
176
178
180
Julian days
Conclusions
182
184
186
188
0
208
210
212
214
216
218
220
Julian days
222
224
226
228
This research was
conducted under
grant GOA22/DSCH
WER4 as part of the
project Geconcerteerde Onderzoekacties supported by
the Vrije Universiteit
Brussel and under
grant G.0021.04 as
part of a Research
Foundation Flanders
project.
The authors would
like to thank Ken
Buesseler for the
scientific coordination of the VERTIGO
project
Two regimes of new production have been identified; K2 shows relatively high nitrate specific
uptake rates (VNO3) but rather low new production rates and is characteristic of HNLC regions. Aloha has low VNO3,
low new production and behaves typically as an oligotrophic area. At both sites there is an agreement between the
estimates of new production and the POC flux samples by neutrally buoyant traps at 150m with an export ratio ranging
from 1 (Aloha) to maximally 3 at K2.