Introduction

1. Nitrogen fluxes and rates of microbial processes in the water column and sediment water interface of the Chukchi and Beaufort Seas Souza, A.C1., Dunton, K.H1., and Gardner, W.S1., 1. The University of Texas at Austin, Marine Science Institute, Port Aransas, TX. • Results • Station 103 • In control cores, N2 flux increased from 71 mmoles m-2h-1 in the light to 96 mmoles/m-2h-1 in dark incubation • -Addition of nitrate increased N2 efflux by 53% • -Benthic O2 consumption was similar in both light and dark incubations • -NO3- and PO43- fluxes were highest of all stations • -Nutrient flux increased under dark conditions • -Concentration of NO3- increased with depth. • Station 1015 • - In control cores, N2 flux increased from 2 mmoles m-2h-1 in the light to 15 mmoles m-2h-1 in the dark incubation • -Addition of nitrate increased N2 efflux by 140% • -Benthic O2 consumption in controls was similar in light and dark • -NO3- and PO43- fluxes were about ½ that of station 103 • -Nutrient flux did not change under dark conditions • -Concentration of NO3- increased below 30m. • Station 9 • In control cores, N2 flux increased from 235 mmoles m-2h-1 in the light to 285 mmoles m-2h-1 in the dark incubation • Benthic O2 consumption in control cores was similar in light and dark conditions • -N2 flux changed from efflux to influx after addition of NO3- • -Nutrient flux was similar under light and dark conditions • -Concentration of NO3- was about 12 mM at the bottom. • Station 48 • - In control cores, N2 flux decreased from 34 mmoles m-2h-1 in the light to 14 mmoles m-2h-1 in dark incubations • Benthic O2 consumption in control cores was similar in light and dark conditions • -N2 efflux increased by 44% after addition of NO3- • -Nutrient flux was similar under light and dark conditions • -Concentration of NO3- increased below 30m. • Beaufort Station BR-3 • - In control cores, N2 flux was into the sediment at about 1800 mmoles m-2h-1 in both light and dark incubations • Benthic O2 consumption in control cores was similar in light and dark incubations, and close to that of station 103 • -N2 flux changed from influx to efflux after addition of NO3- • -Nutrient flux was similar under light and dark conditions. • Conclusions • Benthic microalgae did not affect N2, O2, and nutrient fluxes • Addition of NO3- increased N2 fluxes suggesting that denitrification is N-limited, especially in the Beaufort Sea • Bottom water at stations 103, 1015, and 9 are nutrient rich in contrast to surface waters • Introduction • Nitrogen dynamics were studied in the Chukchi and Beaufort Seas during the linked 2010 COMIDA/CAB and Shell Alaska cruise. Results elucidate the effect of benthic algae on the N-cycle and quantify specific rates of key microbial processes, information fundamental to the development of models to describe and predict the effects of changes in N inputs into the region. Primary productivity rates in the Chukchi Sea are high (Sambrotto et al. 1984) and support a diverse and dense population of benthic invertebrates and extensive aquatic food web (Jewett et al. 1981; Grebmeier and Dunton 2000). In spite of the high fraction of water column production recycled by the bacterioplankton (Hanson and Robertson 1992) and grazed by zooplankton, there is still strong benthic-pelagic coupling in the region (Dunton et al. 1989). The presence of a dense mat of filamentous diatoms in the sediment during the summer (Matheke and Horner 1974) with enriched 13-C isotopic values relative to pelagic phytoplankton, suggests that microalgae are a significant source of food to the benthic community (Dunton et al. 1989). • Objectives • This study addressed, in light and dark incubations, (1) fluxes of N2, O2, and nutrients at the sediment-water interface (SWI), (2) denitrification and oxygen consumption rates, and (3) water column nutrient concentrations with depth. • Methods • Water column samples were obtained using pumps (John Trefry) and sediment OLW with a HYPOX corer (Gardner et al. 2009) • Water samples were unspiked or amended with 15-NO3- and 15-NH4+ and fed continuously over the surface of intact cores (Gardner & McCarthy, 2009) • Inflow and core water samples were filtered and stored frozen and later analyzed for N2, O2, nitrate (NO3-), nitrite (NO2-) and phosphate (PO43-). • References • . Dunton, K.H., S.M. Saupe, A.N. Golikov, D.M. Schell, and S.V. Schonberg. (1989). MEPS 56: 89-97 • . Grebmeier, J.M. and Dunton, K.H. (2000). Report of the Marine Mammal Commission • . Gardner and McCarthy (2009). Biogeochemistry 95:185-198 • . Gardner,W.S., McCarthy, M.J., Carini, S.A., Souza, A.C., Lijun, H., McNeal, K.S., Puckett, M.K., Pennington, J. (2009). Continental Shelf Research 29:2207–2213 • . Hanson RB, Robertson CY (1992). US Fish and MIildlife Service, p 60-74 • . Matheke, GEM and Horner, R (1974). Journal Fisheries Research Board of Canada.,31(11): 1779-1786 • . Sambrotto et al. (1984). Science 225:1147-1150 • Acknowledgements • We would like to thank captain John Seville and the crew of the R/V Moana Wave for their invaluable assistance. We thank Nathan McTigue, Erick Hersh, and Dana Sjostrom for their help in the field. We are grateful for Dr. Prentki (BOEMRE) and Dr. Michael Macrander (Shaell Alaska) for their enthusiastic support of this research program. Sampling Sites BR-3 Fig 1: Sampling sites in the Chukchi and Beaufort Seas (Source: COMIDA- 2010 Report ) Results Nutrient Fluxes and Vertical profiles of NO3–, NO2–, and PO43– concentration (NH4+analysis not yet completed) Fig 2: Nitrogen, oxygen, and nutrient fluxes at SWI and at five sites in the Chukchi and Beaufort Seas. Bottom graphs show vertical profiles of nitrate and phosphate in the water column.