Collection of intact sediment cores with overlying water to study nitrogen- and oxygen-dynamics in regions with seasonal hypoxia Wayne S. Gardner a,Ã , Mark J. McCarthy a,1 , Stephen A. Carini a , Afonso C. Souza a , Hou Lijun b , Karen S. McNeal c , Mary Keith Puckett c , Jack Pennington d a The University of Texas Marine Science Institute, Port Aransas, TX 78373, USA b State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China c Department of Geosciences, Mississippi State University, Mississippi State, MS, USA d Louisiana Universities Marine Consortium (LUMCON), Chauvin, LA 70344, USA article info Article history: Received 6 March 2009 Received in revised form 26 August 2009 Accepted 26 August 2009 Available online 1 September 2009 Keywords: Sediment cores Overlying water Nitrogen-dynamics Hypoxia Sediment–water interface Dead zone abstract Settled particles of fresh, labile organic matter may be a significant source of oxygen demand and nutrient regeneration in seasonally-hypoxic regions caused by nutrient inputs into stratified coastal zones. Studying the dynamics of this material requires sediment sampling methods that include flocculent organic materials and overlying water (OLW) at or above the sediment–water interface (SWI). A new coring device (‘‘HYPOX’’ corer) was evaluated for examining nitrogen- (N) and oxygen-dynamics at the SWI and OLW in the northern Gulf of Mexico (NGOMEX). The HYPOX corer consists of a ‘‘Coring Head’’ with a check-valve, a weighted ‘‘Drive Unit,’’ and a ‘‘Lander,’’ constructed from inexpensive components. The corer collected undisturbed sediment cores and OLW from sediments at NGOMEX sampling sites with underlying substrates ranging from sand to dense clay. The HYPOX corer could be deployed in weather conditions similar to those needed for a multi-bottle rosette water-sampling system with 20 L bottles. As an example of corer applicability to NGOMEX issues, NH 4 + cycling rates were examined at hypoxic and control sites by isotope dilution experiments. The objective was to determine if N-dynamics in OLW were different from those in the water column. ‘‘Ammonium demand,’’ as reflected by potential NH 4 + uptake rates, was higher in OLW than in waters collected from a meter or more above the bottom at both sites, but the pattern was more pronounced at the hypoxia site. By contrast, NH 4 + regeneration rates were low in all samples. These preliminary results suggest that heterotrophic activity and oxygen consumption in OLW in the hypoxic region may be regulated by the availability of NH 4 + , or other reduced N compounds, rather than by the lack of sufficient labile organic carbon. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction The detrimental impact of high tributary nitrogen (N) and other nutrients into coastal regions is an emerging global issue (Diaz, 2001; Diaz and Rosenberg, 2008). Seasonal development of hypoxia ( o2 ppm oxygen) is common in shallow ( o30 m depth) coastal regions receiving high nutrient inputs from coastal rivers (Turner and Rabalais, 1994; Rowe, 2001; Turner et al., 2007). These hypoxic regions (often referred to as ‘‘Dead Zones’’) have become more frequent in recent decades and are a source of increasing national and international concern (Rabalais et al., 2007; Turner et al., 2007). Global climate changes may intensify the problem (Justi ´ c et al., 2003, 2005). The exact physical and biogeochemical mechanisms respon- sible for hypoxia are not well defined (e.g., Rowe, 2001; Dagg et al., 2008). Organic particles formed in enriched coastal regions often settle to the bottom (Lohrenz et al., 1994), where they accumulate and decompose at or above the sediment–water interface (Rowe et al., 2002; Dagg et al., 2008). Decomposition of these ‘‘short-term’’ organic particles at the sediment–water interface (SWI) or suspended in the overlying water (OLW) removes oxygen and transforms nutrients (Chin-Leo and Benner, 1992; Amon and Benner, 1998). The decomposition of such labile particles should be included in measurements of oxygen and nutrient dynamics in the sediments and OLW of hypoxic regions, such as the northern Gulf of Mexico (NGOMEX), to define and assess biogeochemical mechanisms responsible for hypoxia development. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2009.08.012 Ã Corresponding author. Tel.: +1361749 6823. E-mail address: wayne.gardner@mail.utexas.edu (W.S. Gardner). 1 Current address: D epartement des sciences biologiques, Universit  e du Qu  ebec  a Montr  eal, QC, Canada. Continental Shelf Research 29 (2009) 2207–2213