Download Ammonia concentrations in nutrient deplete oceanic waters

Ammonia concentrations and processes
in Oligotrophic Oceanic Waters
Malcolm Woodward
Plymouth Marine Laboratory, Prospect Place, West Hoe,
Plymouth, PL1 3DH, United Kingdom
email: [email protected]
Presentation outline:
‰ Background, N cycle
‰ Nutrient analytical techniques and developments
‰ Oceanic sampling areas and the AMT
‰ Sampling systems, and limitations
‰ Ammonium concentrations from oligotrophic waters
‰ Water column concentration gradients
‰ Ammonium photoproduction in offshore waters
‰ Near surface ammonium gradients
‰ Nutrient uptake, 15N
Why study the Oceans ?:
About 70% of the Earth's surface consists of water.
The oceans are key for the existence of life on Earth.
The atmosphere we breathe, and which controls the weather and climate,
is intimately connected to the oceans - half of the oxygen produced by
plants is produced in the ocean, and the oceans are also responsible for
absorbing about 50% of the carbon dioxide humans have released into
the atmosphere by burning fossil fuels for energy.
But, our understanding of the oceans and the processes involved is poor
to say the least !
Background
‰ In wide areas of the world’s oceans phytoplankton productivity is
limited by the availability of nitrogen. The surface waters and the surface
mixed layers of many temperate oceans, eg the Atlantic, typically are
greatly depleted of dissolved inorganic nitrogen species (ammonia, nitrate
and nitrite), with concentrations often below 5 nmol L-1 during the months
of summer stratification, and the permanent thermocline.
‰ Studies of oligotrophic waters have for many years reported
undectectable concentrations of the major nutrients. Therefore,
quantification of biogeochemical processes would not be practicable for
these waters when there are no reliable data.
‰ Results from water column samples in the upper 200m will be presented
from a research transect cruise from the UK, through both the Atlantic
gyres, down to South Africa; also results for cruises to the Tasman Sea
and the eastern Mediterranean.
‰ Reliable ambient concentrations of ammonia, often less than 5
nanomoles l-1 are seen in the water column. This often showing a
biologically derived maximum found at the top of the deep
chlorophyll/nitrite maximum zone, and this is a characteristic of nutrient
deplete oceanic waters.
‰ Results from detailed surface sampling in the upper 10 metres of a
surface mixed layer oceanic water column, and from experiments
investigating ammonium photo-production in open ocean waters.
Nutrient Cycling in surface mixed layer
fate of phytoplankton production
Phytoplankton
key nitrogen fluxes
Bacteria
Ammonification
Organic
Nitrogen
Nitrification
Uptake
N2
De
(-3)
Assimilation
N.
Fixation
2
(0)
NH4+
N2O
FeFe
NP P
Si…
Zooplankton
( (1)
ni
t ri
fic
a
tio
n
NO
NO
(2)
)
NO2
+
(3)
NO3(5)
Thermocline
Exchange
Macro
macro-and
-and micromicro-nutrient
limitation
Method developments:
• This has been and still is a major challenge in marine nutrient chemistry
to develop robust and reliable sea-going analytical techniques for often
extreme oceanic environments.
• A fluorescence detection system following gas diffusion across a
Goretex teflon membrane has given us the required sensitivity for
ammonia, 2-3 nanomoles/litre.
• Most methods measure the sum of NH4+ + NH3. These exist in equilibrium
within the water column with ammonium being the dominant form.
• The fluorescence technique here is based on the conversion of the NH4+
to NH3 by pH and the subsequent diffusion across a membrane.
• Developments in the technology of long path-length Liquid Waveguide
Capillary Cells (LWCC), up to 2 metres in length, and used in conjunction
with sensitive segmented-flow colorimetric analysis systems, allow us
now to achieve these nanomolar detection limits also for nitrate, nitrite
and phosphate.
Analytical Methods:
Manual analysis techniques, slow, not suited to ships.
Micromolar Techniques: Segmented flow autoanalyser
NO3-:
NO2-:
> 25 nmol l-1
> 20 nmol l-1
NH4+:
>80 nmol l-1
PO43-:
> 20 nmol l-1
Woodward (1994)
Woodward, E.M.S.
1994.
Plymouth Marine Laboratory
Mantoura and Woodward
(1983)
Segmented flow colorimeter
Kirkwood (1989)
Woodward, E.M.S., Rees, A.P.
2001.
Deep-Sea Research II 48(4/5),
Liquid Waveguide Capillary
Cells, following segmented
flow analytical techniques.
Woodward, E.M.S, Kitidis, V
(2003). In prep.
Gas Diffusion
and Fluorescence detection
from Jones (1991)
W oodward, E.M.S, Rees, A.P
(2001).
Deep Sea Research II, 48,
Nos. 4-5, 775 – 794.
775-793.
Nanomolar Techniques:
NO3-:
PO43-:
NO2-:
> 1 nmol l-1
NH4+:
> 2-3 nmol l-1
Other techniques for Ammonia: , FIA, selective electrodes, cathodic stripping
voltametry, Chromatography, solvent extraction.
Ammonia Chemistry: Colorimetric
Reagent 1
Tri-sodium citrate, DTT
(Cl- donor), NaOH
Reagent 2
Heater 55°C
Phenol, Na-Prusside,
Ethanol
Sample
Air
Cooling
Mixing
Coil
Mixing coil
Colorimeter
waste/return
Digital colorimeter
λ=630 nm
Waste
Debubbler
Computer
• Segmented flow Colorimetric analysis
• Developed from manual,
• Produces Indophenol Blue, 630nm
• Hazardous – Phenol
• Lacks sensitivity: salt effects, precipitation
• Mostly “less than” for offshore
Nanomolar Ammonia Chemistry Manifold
Debubbler
Waste
Sample/Standard
Sampling
Unit
Carrier Solution
NaOH/Citrate
OPA
10% H2SO4
Teflon
5 µm
tubing
25ml
reaction
coil
Fluorescence Detector
Excitation: 335 nm
Emission: 470 nm
75ml
reaction
coil
Waste
5 µm
Teflonmembrane
diffusion
cell
Waste
Clean reagents
Constant
temperature
chamber
35oC
Analysis at sea:
Needs to be robust and reliable,
and tied down securely.
So does the operator !
Analysis at sea:
CTD-Rosette System
24 x 20 litre bottles
Target the depths
Clean Sampling and analysis
•Gloves
•Clean ‘aged’ sampling bottles
•Position in sampling system
as early as possible
•Analyse asap within 2 hours
•Store cool and in dark
•No filtering
•Definitely no freezing or
poisoning the samples
The Perceived View
Calm Seas
Beautiful sunsets
G and T’s on the foredeck…………………… dream !
Often the Reality
The Atlantic Meridional Transect Programme - AMT
17 Atlantic cruises, 1995-2005
First NERC Consortium project
Website at
www.amt-uk.org
Northern Gyre
AMT-17
AMT-16
AMT-15
Southern Gyre
Composite Atlantic Chlorophyll
A multidisciplinary programme
to determine:.
1. -how the structure,
functional properties and
trophic status of the major
planktonic ecosystems vary
in space and time.
2. - the role of physical
processes in controlling the
rates of nutrient supply,
including DOM, to the
planktonic ecosystem
3. - the role of atmosphereocean exchange and photodegradation in the formation
and fate of organic matter.
Atlantic Ocean Macro-Nutrient Structure
Twice Daily CTD profile
7 weeks at sea
Nitrate (uM)
Phosphate (uM)
AMT-17, Nov, 2005
Nanomolar
Ammonium
Nanomolar
Nitrite
Chlorophyll-a
(mg m-3)
1
7
11
Northern gyre
18
Cruise Track, AMT-17,
Positions of CTD Sections
24
34
Southern Gyre
44
55
CTD Profiles: Off Shelf, Intermediate and Northern Gyre
0.0
nm
0.0
50.0
100.0
150.0
200.0
250.0
0
Depth (m)
100
150
200
250
CTD 1
300
350
0
0
50
100
150
200
250
300
350
0.1
0.2
0.3
0.4
CTD 7
0
0.5
0.1
0.2
chlorophyll mg L-1
0.0
0.0
50.0
100.0
D e p th (m )
D ep th (m )
0
50
100
150
200
250
300
350
CTD 11
0.05
0.1
0.15
0.2
0.25
0.3
0.4
0.5
chlorophyll mg -L
nm
0
200.0
D e p th (m )
50
nm
100.0
0.3
0.35
0.4
chlorophyll mg-L
DB=NH4 ; Gr=No3; Mau=NO2; LG=Chl
50.0
nm
100.0
0
50
100
150
200
250
300
350
150.0
CTD 18
0
0.05
0.1
0.15
0.2
chlorophyll mg-L
0.25
0.3
0.35
1
7
11
Northern gyre
18
Cruise Track, AMT-17,
Positions of CTD Sections
24
34
Southern Gyre
44
55
CTD Profiles: Equatorial Upwelling, Southern Gyre and Gyre Edge
100.0
200.0
nm
300.0
0
50
100
150
200
250
300
350
0.0
400.0
CTD 24
0
0.1
0.2
0.3
0.4
50.0
50.0
nm
nm
50
100
150
200
250
300
350
150.0
200.0
250.0
300.0
CTD 34
0
0.5
0.1
0.2
0.3
0.4
0.5
400.0
500.0
chlorophyll mg-L
chlorophyll mg -L
0.0
100.0
0
D e p th (m )
D ep th (m )
0.0
100.0
150.0
0
50
0.0
100.0
200.0
nm
300.0
0
50
100
D e p th (m )
D e p th (m )
100
150
200
150
200
250
300
350
250
CTD 44
300
350
0
0.05
0.1
0.15
0.2
0.25
0.3
chlorophyll mg-L
DB=NH4 ; Gr=No3; Mau=NO2; LG=Chl
0.35
CTD 55
0
0.1
0.2
0.3
Chlorophyll mg-L
0.4
0.5
Water column maxima:
•
Ammonia Concentrations exhibit a sub-surface maximum at a depth consistently
near to the observed deep nitrite maximum.
•
The nitrite maximum was present along with the Ammonia maximum in the region
of maximum vertical temperature change, effectively at the thermocline, and it was
also associated with the chlorophyll maximum region, where there is increased
biological activity.
•
Nitrite and ammonia maximum have been observed elsewhere in oligotrophic oceans
•
The ammonia maximum may be as a result of the heterotrophic bacterial
decomposition in the water column, or may be as a result of phytoplankton excretion.
•
Generally it is produced by biological processes that are associated
with zooplankton grazing and the bacterial decomposition of PON.
•
The nitrite maximum has 2 suggested maintenance mechanisms:
a) Bacterial nitrification and b) phytoplankton nitrite excretion during algal reduction
of nitrate.
•
However these two maxima are both linked as being at the region of the
chlorophyll maximum and further detailed studies of all the possible processes
need to be carried out simultaneously to confirm these processes.
CDOM experiments (Coloured dissolved organic matter)
Ammonium production within the water column in oligotrophic waters:
observed when studying the photo-production processes involved when exposing CDOM to
UV radiation.
Also nitrite and urea produced by irradiance.
Ammonia production rate
Photoammonification rates (photo-production of ammonium) were significantly
correlated with Chromophoric Dissolved Organic Matter (CDOM) absorbance at 300 nm
normalised to Dissolved Organic Carbon (DOC). i.e. the more coloured DOM is, the
more NH4+ is likely to be photo-produced.
CDOM/Photochemistry technique
•Samples were 0.1 μm filtered
•Removal of >99.5 % of bacteria and picoplankton
•9 irradiation experiments
•Measure NH4+, NO3-, NO2-, CDOM (a300)
Irradiation results, ammonia production.
84
NH4+ (nmol L-1)
NH4+ (nmol L-1)
98
96
94
92
83
82
81
80
79
90
0
500
0
1000 1500 2000 2500
Integrated Irradiance (W m-2 h)
1000 1500 2000 2500
Integrated Irradiance (W m-2 h)
100
NH4+ (nmol L-1)
68
NH4+ (nmol L-1)
500
64
60
56
98
96
94
92
90
52
0
500
1000 1500 2000 2500
Integrated Irradiance (W m-2 h)
0
500
1000 1500 2000 2500
Integrated Irradiance (W m-2 h)
Ammonia produced over time, exposure over solar noon
Low but significant increases
Ammonium Production in the water column
Ammonia is photo-produced in oligotrophic waters
Photoproduction of ammonium via the photodegradation of CDOM can
significantly modify the inorganic nutrient budgets of aquatic ecosystems
(Bushaw et al., 1996). He also estimated photoammonification on the
Southeastern US continental shelf to contribute an additional 20% to riverine
inorganic nitrogen export. Similarly, Morell and Corredor (2001) estimated the
contribution of NH4+ photoproduction to the phytoplankton nitrogen (N) demand
of the eastern Caribbean Sea at about 50 %.
These were results for coastal and near shore waters.
No studies have previously looked at oligotrophic waters – analytical capability !
Combining our experimental data with estimates of annual solar irradiance and
water column light attenuation yields we calculated an annual
photoammonification rate for the Cyprus Gyre of 40 ± 17 mmol m-2 a-1,
equivalent to ~12 ± 5 % of the previously estimated annual nitrogen requirement
of new production and in the same order of magnitude as atmospheric N
deposition in this region.
Near surface studies of ammonia
Ammonia (nm) surface - 113 COST
Ammonia (nm) surface - 25m COST
Conc (nm)
conc. (nm)
10.0
20.0
30.0
40.0
50.0
0.0
60.0
0
20
5
40
Depth (m)
depth (m)
0.0
0
60
80
10.0
20.0
30.0
40.0
50.0
10
15
100
20
120
25
Ammonia (nm) surface - N-CYCLE
Conc (nm)
Oligotrophic Tasman Sea
CTD Bottles – 1 metre !
0.0
0.0
Clean sampling system, teflon tubes
and syringes
Indications of surface maxima
and also at sub-surface region
Depth (cm)
50.0
100.0
150.0
200.0
250.0
300.0
5.0
10.0
15.0
20.0
60.0
Near Surface ammonia profiles
Surface bottles 02.07.05- Core cruise 2005
Implications for a source
(photoproduction) and sink
(atmospheric losses) imbalence.
concentration (nm)
0.00
10.00
20.00
30.00
40.00
50.00
60.00
0
depth (c m )
50
100
150
Ammonia (nm)
200
Nitrite (nm)
250
Nitrate+Nitrite (nm)
Surface bottles 24.06.05 - Core cruise 2005
concentration (nm)
0.00
100.00
200.00
300.00
400.00
0
Indications:
Ammonia is photoproduced at
100-150 cms area
Nearer the surface there is then
gas exchange so consumption
to atmophere exceeds
production
Right at the surface few cms
photoprod becomes dominant
and NH3 increases again.
depth (cm)
50
100
150
200
250
Ammonia (nm)
Nitrite (nm)
Nitrate+Nitrite (nm)
Other aspect is the contribution
of phytoplankton – unknown.
Known that they take up the
ammonia deeper in the water
column.
Nutrient Uptake studies
3 main sources of nitrogen for the phytoplankton – Ammonia, Urea and Nitrate
Studies have shown that ammonium supports up to 95% of the phytoplankton
demand for nitrogen in oligotrophic seas.
Kinetic analysis has showed that ammonium is preferentially utilised over Nitrate
over the full concentration spectrum, nano – micromolar.
Compared to nitrate, phytoplankton generally prefer ammonium because of the
additional energy that is required for them to reduce nitrate to ammonium.
1.40
Urea uptake (µM N d-1)
y = 1.187x - 0.004
r20.819 =
0.12
0.10
0.08
0.06
0.04
y = 0.913x - 0.003
r20.819 =
0.02
0.00
0
0.02
0.04
0.06
0.08
[NH4] (µM)
NO3 uptake (µM N d-1)
-1
NH4 uptake (µM N d )
0.14
0.1
0.12
1.20
y = 0.768x + 0.005
r20.955 =
1.00
0.80
0.60
0.40
0.20
0.00
0.14
0
0.12
0.5
1
1.5
2
[UREA] (µM)
y = 1.363x + 0.021
r20.519 =
0.10
0.08
0.06
Uptake proportional
to concentration
0.04
y = 0.983x - 0.001
r20.895 =
0.02
0.00
0
0.01
0.02
0.03
[NO3] (µM)
0.04
0.05
f ratio
By combining these uptake ratios we can produce the f ratio to indicate what
nitrogen species is dominating phytoplankton production.
Less than 0.5 indicates that NH4 is dominating the production
In the Atlantic gyres and other oligotrophic oceans ammonium is supporting
the primary production.
1
y = 0.0907x + 0.502
2
r = 0.92
f'-ratio
0.8
0.6
0.4
0.2
b)
0
-8
-6
-4
-2
-
0
2
-1
Ln NO3 (µmol l )
Concs: 0=1uM NO3
-2=14nM NO3
4
Conclusions
Ammonium concentrations less than 10nM in the surface oceanic waters
But more studies need to be done to increase the database for the oceans
The results could have wider implications for global nutrient budget
calculations, also nutrient limitation studies
Ammonium increases due to photoproduction over solar noon, again
important in budget calculations
Maxima observed sub surface (100-150cms) in fine scale studies
Also surface maxima observed
Links between air-sea exchange and the fluxes need to be studies
Ammonium is the preferred species for phytoplankton and for driving the
primary production in the nutrient deplete oceanic waters.
Most important is need for many more oceanic measurements !
Thanks to my colleagues at PML:
Vas Kitidis and Katie Chamberlain
THANK YOU for Listening