Rates and controls of benthic nitrogen cycling in sublittoral Gulf of Mexico permeable sediments Tom...
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Rates and controls of benthic nitrogen cycling in sublittoral Gulf of Mexico permeable sediments Tom Gihring , Ashley Riggs, Markus Huettel, and Joel E. Kostka Department of Oceanography, Florida State University RESULTS INTRODUCTION METHODS CONCLUSIONS OBJECTIVES • High rates of marine primary production in continental margins are fueled largely by nutrients regenerated during mineralization of organic matter in sediments. • The majority of continental shelf surficial deposits are sandy sediments which are low in organic carbon content due to relatively frequent sediment resuspension, highly active microbial communities and rapid rates of organic matter mineralization. • Bottom currents interacting with ripples cause water pumping through the upper layers of permeable sediments. It is now well-established that advective exchange of porewater stimulates benthic mineralization. • There are currently very few studies of denitrification in sandy continental shelf sediments. • Direct measurements of denitrification in continental shelf sediments are critical for resolving the global nitrogen balance. We examined nitrogen cycling over a one-year period in sublittoral sandy sediments. Two contrasting sites near a barrier island in the Florida panhandle were studied. The primary objectives were to: 1. Nitrogen stable isotope tracer techniques (Risgaard-Petersen et al. 2003) were used to measure N 2 production rates and pathways in intact sediment cores. To simulate pore water movement which occurs in permeable sands due to interactions between water currents and surface topography, sediment cores were percolated with aerated seawater to a nominal depth of 5 cm (deBeer et al. 2005). • Denitrification rates with pore water percolation ranged from 1 to 21 mol N m -2 d -1 at the protected Bay site and 70 to 194 mol N m -2 d -1 at the site open to the Gulf of Mexico. • Pore water percolation increased denitrification rates up to 2.5-fold and 15- fold for the Bay and Gulf sites, respectively, relative to non-percolated cores. • Seasonal N 2 production rates were highest in spring and fall. • Denitrified nitrate was derived from the water column at the Bay site whereas benthic nitrification was more important at the Gulf site. • Rates of N 2 efflux were directly correlated with the extent of pore water flow increasing from 125 mol N m -2 d -1 under diffusive conditions to 870 mol N m -2 d -1 with pore water advection. • Denitrification rates are controlled largely by seasonal and short-term changes in bottom currents and the availability of nitrate and organic matter. AKNOWLEDGEMENTS This study was supported by grants from the National Science Foundation (OCE-0424967 and OCE-0726754) and Florida State University (PEG 513680014). TMG was supported in part by a fellowship from the NOAA Estuarine Reserves Division. We thank Dave Oliff, Jon Delgardio, Andy Canion, Dilo Senanayake, Jeff Cornwell, Mike Owens, and Todd Kana, for their assistance. Continental shelf sediments are important sites of organic matter mineralization and denitrification. Although the majority of shelf surficial deposits are sands, direct measurements of denitrification in sandy sediments are rare. We examined nitrogen cycling over a one-year period in sublittoral sandy sediments from two contrasting sites near a barrier island in the Florida panhandle. Nitrogen stable isotope tracer techniques were used to measure N 2 production rates and pathways in sediment cores and slurries. To simulate pore water movement which occurs in permeable sands due to interactions between water currents and surface topography, sediment cores were percolated with aerated seawater to a nominal depth of 5 cm. Denitrification rates with pore water percolation ranged from 1 to 21 mol N m -2 d -1 at the protected Bay site and 70 to 194 mol N m -2 d -1 at the site open to the Gulf of Mexico. Pore water percolation increased denitrification rates up to 2.5-fold and 15-fold for the Bay and Gulf sites, respectively, relative to non-percolated cores. Seasonal N 2 production rates were highest in spring and fall for both sites. Denitrified nitrate was derived from the water column at the Bay site whereas benthic nitrification was more important at the Gulf site. Benthic chambers were used to determine oxygen, N 2 , nitrate, and ammonium fluxes at the sediment-water interface during varied degrees of continuous pore water exchange. Rates of N 2 efflux were directly correlated with the extent of pore water flow increasing from 125 mol N m -2 d -1 under diffusive conditions to 870 mol N m -2 d -1 with pore water advection. BACKGROUND 1. Determine the pathways and controls of microbial nitrogen cycling in coastal, permeable sediments. 2. Obtain direct rate measurements of N 2 production under near in situ conditions. N 2 nitrate, ammonium ammonium nitrate gas Bacteria play a critical, and in certain cases exclusive, role in all of the major steps in nitrogen cycling © Information Services Branch, Geoscience Australia Nitrogen cycling in estuaries X X St. George Island, Apalachicola Bay site St. George Island, Gulf of Mexico site Field sites A combination of three approaches was used: 2. Benthic chambers were used to determine O 2 , N 2 , nitrate, and ammonium fluxes at the sediment- water interface during varied degrees of continuous pore water exchange. 3. Sediment slurries, in combination with nitrogen stable isotope tracers, were used to measure anammox rates and temperature responses of denitrification. Field Sites: Cook et al. 2006 Analytical techniques- Rates of denitrification in cores and slurries were calculated from the production of excess 29 N 2 and 30 N 2 from 15 N-NO 3 - (Nielsen 1992, Risgaard-Peterson et al. 2003). Net N 2 and O 2 fluxes in benthic chambers were calculated from changes in O 2 :Ar and N 2 :Ar measured using a membrane inlet mass spectrometer (Kana et al. 1994). Dissolved nitrate and ammonium were measured by chemiluninesce after vanadim reduction to NO (Braman and Hendrix 1989) and colorimeteric assays (Bower and Holm-Hansen 1980), respectively. Diagram of benthic chambers Percolated vs. non-percolated core incubations Oxygen consumption in Gulf of Mexico SGI chambers 0 2 4 6 8 10 12 60 40 20 10 RPM mmol O 2 m -2 d -1 increasing porewater circulation increasing oxygen consumption microbes respire oxygen while consuming dead plants & animals more water circulation leads to more detritus decompositio n 0 0.2 0.4 0.6 0.8 1 60 40 20 10 N 2 production in Gulf of Mexico SGI chambers RPM mmol N m -2 d -1 increasing porewater circulation increasing denitrification more water circulation leads to more production of N 2 gas nitrate + organics N 2 + CO 2 Temperature responses of N 2 production the denitrifyi ng bacteria thrive within a narrow temperatur e range 73 o 90 o F 0 50 100 150 200 250 Seasonal N 2 production (denitrification) denitrific ation is highest in spring (and fall) percolatio n increases denitrific ation rates Dec 0 7 Apr 0 8 Jul 0 8 Oct 0 8 Jan0 8 Apr 0 8 Jul 0 8 Oct 0 8 Bay Gulf mol N m -2 d -1 0 5 10 15 20 25 lighter color = non-percolated; darker color = percolated N 2 production vs. seawater nitrate concentration N 2 production may be directly linked with water column nitrate availabili ty Bay Gulf increasing nitrate in the overlying seawater 0 1 2 3 4 5 0 5 10 15 20 25 0 0.5 1 1.5 2 2.5 0 50 100 150 200 250 mol N m -2 d -1 mol NO 3 - L -1 mol NO 3 - L -1 13 % 1 5 % 2 0.9 M Oct 08, 22 o C 95 % 11 86 % 199 1.8 M Oct 08, 23 o C fall denitrification 0.7 M 4.8 M 0.6 M seawater [NO 3 - ] 0.4 M 2.0 M 1.1 M seawater [NO 3 - ] 45 % 6 52 % 10 Jul 08, 28.5 o C 93 % 29 94 % 69 Jul 08, 30 o C suer denitrification Apr 08, 23 o C Jan 08, 16 o C SGI-Bay 21 b.d. D (14) perc. 30 % - D n 16 1 D (14) non-perc. 24 % 86 % D n 83 7 D (14) non-perc. 87 % 94 % D n Apr 08, 21 o C Dec 07, 18 o C SGI-Gulf 93 % 104 ol N -2 d -1 winter denitrification D n D (14) 93 % 194 spring denitrification perc. SGI 15 NO 3 - + Percolation Core Incubations D (14) = total 14 N 2 production; D n = coupled nitrification-denitrification n=5 cores per speed; CV < 20% n=5 cores per speed; CV < 6% RPM N2/O2 60 7.6% 40 5.2% 20 4.0% 10 2.1% ABSTRACT
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Transcript of Rates and controls of benthic nitrogen cycling in sublittoral Gulf of Mexico permeable sediments Tom...
Rates and controls of benthic nitrogen cycling in sublittoral Gulf of Mexico permeable sediments
Tom Gihring, Ashley Riggs, Markus Huettel, and Joel E. Kostka Department of Oceanography, Florida State University
RESULTSINTRODUCTION
METHODS
CONCLUSIONS
OBJECTIVES
• High rates of marine primary production in continental margins are fueled largely by nutrients regenerated during mineralization of organic matter in sediments.
• The majority of continental shelf surficial deposits are sandy sediments which are low in organic carbon content due to relatively frequent sediment resuspension, highly active microbial communities and rapid rates of organic matter mineralization.
• Bottom currents interacting with ripples cause water pumping through the upper layers of permeable sediments. It is now well-established that advective exchange of porewater stimulates benthic mineralization.
• There are currently very few studies of denitrification in sandy continental shelf sediments.
• Direct measurements of denitrification in continental shelf sediments are critical for resolving the global nitrogen balance.
We examined nitrogen cycling over a one-year period in sublittoral sandy sediments.
Two contrasting sites near a barrier island in the Florida panhandle were studied.
The primary objectives were to:
1. Nitrogen stable isotope tracer techniques (Risgaard-Petersen et al. 2003) were used to measure N2 production rates and pathways in intact sediment cores.
To simulate pore water movement which occurs in permeable sands due to interactions between water currents and surface topography, sediment cores were percolated with aerated seawater to a nominal depth of 5 cm (deBeer et al. 2005).
• Denitrification rates with pore water percolation ranged from 1 to 21 mol N m-2 d-1 at the protected Bay site and 70 to 194 mol N m-2 d-1 at the site open to the Gulf of Mexico.
• Pore water percolation increased denitrification rates up to 2.5-fold and 15-fold for the Bay and Gulf sites, respectively, relative to non-percolated cores.
• Seasonal N2 production rates were highest in spring and fall.
• Denitrified nitrate was derived from the water column at the Bay site whereas benthic nitrification was more important at the Gulf site.
• Rates of N2 efflux were directly correlated with the extent of pore water flow increasing from 125 mol N m-2 d-1 under diffusive conditions to 870 mol N m-2 d-1 with pore water advection.
• Denitrification rates are controlled largely by seasonal and short-term changes in bottom currents and the availability of nitrate and organic matter.
• Competitive uptake of dissolved inorganic nitrogen and inhibition of nitrification are also major controls on nitrogen removal via denitrification.
AKNOWLEDGEMENTS This study was supported by grants from the National Science Foundation (OCE-0424967 and OCE-0726754) and Florida State University
(PEG 513680014). TMG was supported in part by a fellowship from the NOAA Estuarine Reserves Division. We thank Dave Oliff, Jon Delgardio, Andy Canion, Dilo Senanayake, Jeff Cornwell, Mike Owens, and Todd Kana, for their assistance.
Continental shelf sediments are important sites of organic matter mineralization and denitrification. Although the majority of shelf surficial deposits are sands, direct measurements of denitrification in sandy sediments are rare. We examined nitrogen cycling over a one-year period in sublittoral sandy sediments from two contrasting sites near a barrier island in the Florida panhandle. Nitrogen stable isotope tracer techniques were used to measure N2 production rates and pathways in sediment cores and slurries. To simulate pore water movement which occurs in permeable sands due to interactions between water currents and surface topography, sediment cores were percolated with aerated seawater to a nominal depth of 5 cm. Denitrification rates with pore water percolation ranged from 1 to 21 mol N m-2 d-1 at the protected Bay site and 70 to 194 mol N m-2 d-1 at the site open to the Gulf of Mexico. Pore water percolation increased denitrification rates up to 2.5-fold and 15-fold for the Bay and Gulf sites, respectively, relative to non-percolated cores. Seasonal N2 production rates were highest in spring and fall for both sites. Denitrified nitrate was derived from the water column at the Bay site whereas benthic nitrification was more important at the Gulf site. Benthic chambers were used to determine oxygen, N2, nitrate, and ammonium fluxes at the sediment-water interface during varied degrees of continuous pore water exchange. Rates of N2 efflux were directly correlated with the extent of pore water flow increasing from 125 mol N m-2 d-1 under diffusive conditions to 870 mol N m-2 d-1 with pore water advection.
BACKGROUND
1. Determine the pathways and controls of microbial nitrogen cycling in coastal, permeable sediments.
2. Obtain direct rate measurements of N2 production under near in situ conditions.
N2
nitrate, ammonium
ammonium nitrate
gas
Bacteria play a critical, and in certain cases exclusive, role in all of the major steps in nitrogen cycling
© Information Services Branch, Geoscience Australia
Nitrogen cycling in estuaries
XX
St. George Island, Apalachicola Bay site St. George Island, Gulf of Mexico site
Field sites
A combination of three approaches was used:
2. Benthic chambers were used to determine O2, N2, nitrate, and ammonium fluxes at the sediment-water interface during varied degrees of continuous pore water exchange.
3. Sediment slurries, in combination with nitrogen stable isotope tracers, were used to measure anammox rates and temperature responses of denitrification.
Field Sites:
Cook et al. 2006
Analytical techniques- Rates of denitrification in cores and slurries were calculated from the production of excess 29N2 and 30N2 from 15N-NO3
- (Nielsen 1992, Risgaard-Peterson et al. 2003).
Net N2 and O2 fluxes in benthic chambers were calculated from changes in O2:Ar and N2:Ar measured using a membrane inlet mass spectrometer (Kana et al. 1994).
Dissolved nitrate and ammonium were measured by chemiluninesce after vanadim reduction to NO (Braman and Hendrix 1989) and colorimeteric assays (Bower and Holm-Hansen 1980), respectively.
Diagram of benthic chambers
Percolated vs. non-percolated core incubations
Oxygen consumption in Gulf of Mexico SGI chambers
0
2
4
6
8
10
12
60402010RPM
mm
ol O
2 m
-2 d
-1
increasing porewater circulation
incr
eas
ing
oxy
gen
co
nsu
mp
tionmicrobes respire
oxygen while consuming dead plants & animals
more water circulation leads to more detritus decomposition 0
0.2
0.4
0.6
0.8
1
60402010
N2 production in Gulf of Mexico SGI chambers
RPM
mm
ol N
m-2 d
-1
increasing porewater circulation
incr
eas
ing
de
nitr
ifica
tion
more water circulation leads to more production of N2 gas
nitrate+
organics
N2
+CO2
Temperature responses of N2 production
the denitrifying bacteria thrive within a narrow temperature range
73o 90o F
0
50
100
150
200
250
Seasonal N2 production(denitrification)
denitrification is highest in spring (and fall)
percolation increases denitrification rates
Dec
07
Apr
08
Jul0
8
Oct
08
Jan0
8
Apr
08
Jul0
8
Oct
08
BayGulf
mol
N m
-2 d
-1
0
5
10
15
20
25
lighter color = non-percolated; darker color = percolated
N2 productionvs.
seawater nitrate concentration
N2 production may be directly linked with water column nitrate availability
BayGulf
increasing nitrate in the overlying seawater
0 1 2 3 4 50
5
10
15
20
25
0 0.5 1 1.5 2 2.50
50
100
150
200
250
mol
N m
-2 d
-1
mol NO3- L-1 mol NO3
- L-1
13 %
1
5 %
2
0.9 M
Oct 08, 22 oC
95 %
11
86 %
199
1.8 M
Oct 08, 23 oC
falldenitrification
0.7 M4.8 M0.6 M [seawater NO3-]
0.4 M2.0 M1.1 M [seawater NO3-]
45 %
6
52 %
10
08, 28.5 Jul oC
93 %
29
94 %
69
08, 30 Jul oC
summerdenitrification
08, 23 Apr oC 08, 16 Jan oC-SGI Bay
21. .b dD(14) .perc
30 %-Dn
161D(14) - .non perc
24 %86 %Dn
837D(14) - .non perc
87 %94 %Dn
08, 21 Apr oC 07, 18 Dec oC-SGI Gulf
93 %
104
mol N m-2 d-1
winterdenitrification
Dn
D(14)
93 %
194
springdenitrification
.perc
SGI 15NO3- + Percolation Core Incubations
D(14) = total 14N2 production; Dn = coupled nitrification-denitrification
n=5 cores per speed; CV < 20%n=5 cores per speed; CV < 6%
RPM N2/O260 7.6%40 5.2%20 4.0%10 2.1%
ABSTRACT
