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 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. 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 and OCE ) and Florida State University (PEG ). 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. N2N2 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 siteSt. 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 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 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 decomposition 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 denitrifying bacteria thrive within a narrow temperature range 73 o 90 o F Seasonal N 2 production (denitrification) denitrification is highest in spring (and fall) percolation increases denitrification rates Dec07 Apr08 Jul08 Oct08 Jan08 Apr08 Jul08 Oct08 Bay Gulf  mol N m -2 d -1 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 availability Bay Gulf increasing nitrate in the overlying seawater  mol N m -2 d -1  mol NO 3 - L -1 SGI 15 NO 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% ABSTRACT