Journal of Coastal Research SI 63 x-xx Coconut Creek, Florida 2013 DOI: 10.2112/SI63-001.1 received (Day) (Month) 2012; accepted (Day) (Month) 2012. © Coastal Education & Research Foundation 2013 www.JCRonline.org www.cerf-jcr.org Seasonal summer stratiication and enhanced nutrient loading of the Louisiana continental shelf (USA) west of the Mississippi River create hypoxic regions that affect large areas of the benthos. Total sediment oxygen uptake and nutrient recycling were measured during different, (e.g. pre-, early, late and post-) hypoxic regimes using shipboard Batch Micro-Incubation Chambers (BMICs) in 2004 to 2005 and again in 2007 to 2009. Sediment community oxygen consumption during oxic regimes (dissolved oxygen > 63 µmol L -1 ) was -9.5 ± 0.7 mmol O 2 m -2 d -1 (mean ± SE), almost twice that measured (-5.8 ± 0.6 mmol O 2 m -2 d -1 ) during suboxic conditions. During the summer when hypoxia occurred, the benthos consumed nitrate and nitrite (-0.14 ± 0.04 and -0.10 ± 0.02 mmol N m -2 d -1 respectively) and produced ammonium (1.6 ± 0.39 mmol N m -2 d -1 ). Elevated sediment community oxygen consumption and nutrient remineralization occurred near terrestrial river inputs associated with the Mississippi and Atchafalaya Rivers. Net release of dissolved inorganic nitrogen, in the form of ammonium, peaked during late summer. Released ammonium may be a source of nutrients for primary production in bottom waters, and can also provide reduced nitrogen for nitriication and microbial respiration, both of which may reinforce the intensity and duration of hypoxia. Based on chamber results, sediments actively scavenged phosphate from the bottom waters (-98.4 ± 21.3 µmol P m -2 d -1 ) and released silicate (2.62 ± 0.31 mmol Si m -2 d -1 ). The addition of reactive nitrogen and removal of phosphorous due to benthic community metabolism could potentially be accentuating phosphorous limitation on the continental shelf. ADDITIONAL INDEX WORDS: Louisiana continental shelf, Mississippi-Atchafalaya river system, sediment biogeochemistry, remineralization, hypoxia, sediment oxygen demand, positive feedback. †Department of Oceanography Texas A&M University 3146 TAMU College Station, Texas 77843-3146 ‡Department of Marine Biology Texas A&M University at Galveston 200 Seawolf Parkway Galveston, Texas 77553 §University of Hawaii at Manoa, Department of Oceanography 1000 Pope Road Honolulu, HI 96822 cnunn@hawaii.edu ABSTRACT Nunnally, C.C.; Rowe, G.T..; Thornton, D.C.O., and Quigg, A.S., 2012. Sedimentary oxygen consumption and nutrient regeneration in the Gulf of Mexico hypoxic zone. In: Brock, J.C., Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystem of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue, No. 63, pp. x-xx. Coconut Creek (Florida), ISSN 0749-0208. Sedimentary Oxygen Consumption and Nutrient Regeneration in the Northern Gulf of Mexico Hypoxic Zone Clifton C. Nunnally†§, Gilbert T. Rowe†‡, Daniel C. O. Thornton†, and Antonietta Quigg†‡ INTRODUCTION Under the Mississippi-Atchafalaya River plume, along the Louisiana continental shelf (Figure 1), expansive low oxygen areas form during summer months due to strong thermohaline stratiication (Bianchi et al., 2010; Hetland and DiMarco, 2008; Rabalais, Turner, and Wiseman, 1999; Turner et al., 2005; Wiseman et al., 1997) and elevated riverine nutrient inputs (Dortch et al., 1994; Lehrter, Murrell and Kurtz, 2009; Quigg et al., 2011) and sediments (Rowe 2001, Rowe et al., 2002) beneath the pycnocline. Over time and under strong seasonal stratiication, this leads to large zones of hypoxia in bottom waters that persist until oxygen can be physically mixed from the surface or offshore. These processes of eutrophication, stratiication, and respiration combine to create the northern Gulf of Mexico Hypoxic Zone (GOMHZ). All of these processes vary in time and space and thus variations in sedimentary biogeochemical processes (Morse and Rowe, 1999; Rowe et al., 2002) need to be understood to better predict consequences of river runoff (Dagg and Breed, 2003; Quigg et al., 2011; Rabalais, Turner, and Wiseman, 1999), wetland particulate and dissolved organic carbon (POC and DOC) contributions (Bianchi et al., 2009, 2010, 2011; Rosenbauer et al., 2009; Swarzenski et al., 2008), and physical forcing (Cochrane and Kelly, 1986; DiMarco et al., 2009; Wiseman et al., 1997). River plume dynamics and coastal boundary currents create unique zones with differing hypoxic potential within the GOMHZ (Rowe and Chapman, 2002). Near the river mouth, a ‘brown’ zone of heavy sediment loading impedes light penetration and limits primary productivity within the plume. Further away from the river plume, a ‘green’ zone is deined by elevated nutrients and high primary productivity, which ostensibly deposits large loads of marine-derived organics to the benthos (Turner and Rabalais, 1994). West of Terrebonne Bay (Figure 1), a ‘blue’ zone is dominated by intense seasonal stratiication and a strong pycnocline; nutrients are limiting at this distance from river input and most primary production is fueled by recycled nutrients (Dortch and Whitledge, 1992; Quigg et al., 2011). Transition areas associated with coastal bays (Timbalier, Terrebonne and Atchafalaya Bays) between these zones provide inputs of dissolved organic matter independent of the main Mississippi River outlow (Bianchi et al., 2010).