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 INTRODUCTION ABSTRACT

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.

percolation ranged from 1 to 21  mol N m -2 d -1  mol N m -2 d -1 Denitrification rates with pore water at the protected Bay site and 70 to 194 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 d -1 under diffusive conditions to 870  mol N m -2 d -1 with pore water advection.

 mol N m -2

BACKGROUND

• 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.

Nitrogen cycling in estuaries

© Information Services Branch, Geoscience Australia N 2 microbes respire oxygen while consuming dead plants & animals

RESULTS Oxygen consumption in Gulf of Mexico SGI chambers nitrate + organics

12 10 8 6

N

2

+ CO

2 4 more water circulation leads to more detritus decomposition 2 0 10 20 RPM 40 increasing porewater circulation n=5 cores per speed; CV < 6% 60

N

2

production in Gulf of Mexico SGI chambers

1 0.8

0.6

0.4

RPM N2/O2

60 40 7.6% 5.2% 20 10 4.0% 2.1%

more water circulation leads to more production of N

2

gas

0.2

0 10 20 RPM 40 increasing porewater circulation n=5 cores per speed; CV < 20% 60

Temperature responses of N

2

production Seasonal N

2

production (denitrification)

nitrate, ammonium ammonium nitrate gas Bacteria play a critical, and in certain cases exclusive, role in all of the major steps in nitrogen cycling

OBJECTIVES

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. Determine the pathways and controls of microbial nitrogen cycling in coastal, permeable sediments.

2. Obtain direct rate measurements of N 2 near

in situ

conditions.

production under

METHODS A combination of three approaches was used:

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).

Field Sites:

X X Field sites St. George Island, Gulf of Mexico site

the denitrifying bacteria thrive within a narrow temperature range

73 o 90 o F

N

2

production vs.

seawater nitrate concentration N

2

production may be directly linked with water column nitrate availability Gulf Bay

250 200 150 100 50 25 20 15 10 5 0 0 0.5

1 1.5

 mol NO 3 L -1 2 2.5

0 0 1 2 3  mol NO 3 L -1 4 increasing nitrate in the overlying seawater 5

denitrification is highest in spring (and fall)

250 200

percolation increases denitrification rates

150 100 50 0

Gulf

25 20 15 10 5 0

Bay

lighter color = non-percolated; darker color = percolated SGI-Gulf seawater [NO 3 ] perc.

D (14) D n non-perc.

D (14) D n

SGI

15

NO

3 -

+ Percolation Core Incubations

winter denitrification Dec 07, 18 o C 1.1  M

104

93 %

7

94 % spring denitrification summer denitrification  mol N m -2 d -1 Apr 08, 21 o C 2.0  M Jul 08, 30 o C 0.4  M

194

93 %

69

94 %

83

87 %

29

93 % fall denitrification Oct 08, 23 o C 1.8  M

199

86 %

11

95 % SGI-Bay seawater [NO 3 ] perc.

D (14) D n Jan 08, 16 o C 0.6  M

b.d.

Apr 08, 23 o C 4.8  M

21

30 % Jul 08, 28.5 o C 0.7  M

10

52 % Oct 08, 22 o C 0.9  M

2

5 % non-perc.

D (14) D n

1

86 %

16

24 %

6

45 %

1

13 % D (14) = total 14 N 2 production; D n = coupled nitrification-denitrification St. George Island, Apalachicola Bay site Diagram of benthic chambers Percolated vs. non-percolated core incubations 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.

Cook et al. 2006 Analytical techniques- Rates of denitrification in cores and slurries were calculated from the production of excess 29 N 2 15 N-NO 3 and (Nielsen 1992, Risgaard-Peterson et al. 2003).

30 N 2 from 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.

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.

CONCLUSIONS

• Denitrification rates with pore water percolation ranged from 1 to 21  mol N m -2 site and 70 to 194  mol N m -2 d -1 at the site open to the Gulf of Mexico.

d -1 at the protected Bay • 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 m -2 d -1 efflux were directly correlated with the extent of pore water flow increasing from 125  mol N 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.