Ocean & Coastal Management 68 (2012) 102e113 Contents lists available at SciVerse ScienceDirect Ocean & Coastal Management journal homepage: www.elsevier.com/locate/ocecoaman Biogeochemical cycles in sediment and water column of the Wadden Sea: The example Spiekeroog Island in a regional context Melanie Beck*, Hans-Jürgen Brumsack Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University, Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany article info abstract Article history: Tidal flats like the Wadden Sea are areas of high primary production and organic matter remineralization Available online 17 June 2012 rates. This paper provides an overview of benthic remineralization pathways and the recycling of various metabolic products, exemplified by interdisciplinary studies around Spiekeroog Island (Germany). Organic matter produced in the Wadden Sea area as well as material imported from the North Sea is remineralized in tidal flat sediments. Wadden Sea sediments may thus be regarded as biogeochemical reactors promoting or accelerating organic matter remineralization. Due to advective flow, which is of special importance in permeable sandy sediments, pore waters enriched in remineralized nutrients and methane are actively released from sediments into the overlying water column. This biogeochemical recycling forms the prerequisite for continuously high primary production in the Wadden Sea, and proves a tight coupling between benthic and pelagic dynamics. Additionally, the export of excess nutrients from the Wadden Sea further offshore may trigger biological activity in coastal waters of the North Sea. In this contribution, we will also summarize open questions which need to be answered for a thorough understanding, management and protection of the unique Wadden Sea ecosystem. In particular, the currently understudied, but potentially significant effects of climate change (e.g., rising sea level and increase in storm surge extremes) on biogeochemical cycles in sediments and open waters of the Wadden Sea are discussed. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction exhibits high rates of primary production (Cadée and Hegeman, 2002; Loebl et al., 2007; Poremba et al., 1999). These high rates The tidal flats of the Wadden Sea form the largest continuous are not only supported by the presence of well-adapted phyto- area of sand- and mudflats worldwide, accounting for 60% of all plankton and microorganism consortia, but also by enhanced tidal areas in Europe and North Africa (Marencic, 2009). The region nutrient availability due to rapid organic matter (OM) reminerali- sustains a rich and diverse flora and fauna and is of outstanding zation. In intertidal areas, aerobic and anaerobic OM degradation international importance as staging and wintering area for migra- processes are fuelled by filtration of suspended particles and dis- tory birds (Marencic, 2009). Due to its unique nature, the Wadden solved OM from the water column within permeable sediments. Sea has been inscribed on the World Heritage List in 2009 and often Continuous supply of organic substrate supports enhanced micro- serves as a worldwide reference for comparisons with other tidal bial activity, and ultimately the release of metabolic products such flat systems. This highlights the importance to attain the best as nutrients and methane (CH4) to the pore waters. Tidal pumping possible understanding of biological, chemical and physical induces advective flushing of permeable sediments and the trans- processes sustaining this ecosystem. The intense biogeochemical port of remineralization products to the open water column, where cycling of carbon and nutrients has been identified as crucial for they can once again support primary production. Furthermore, the controlling life and ecosystem dynamics in the Wadden Sea. Wadden Sea is an open system where water exchange with the Organisms living in tidal flat ecosystems have to tolerate North Sea occurs through tidal inlets. Thus, the quality of water, extreme environmental gradients in salinity, incident light, oxygen sediment and marine habitats is to a large degree influenced by availability and temperature. Nevertheless, this type of landscape processes occurring in the North Sea and vice versa. In this paper we will summarize benthic remineralization pathways, with a focus on metabolic products including nutrients * Corresponding author. Tel.: þ49 441 7983627; fax: þ49 441 7983404. and CH4. Benthic OM remineralization and the subsequent nutrient E-mail address: [email protected] (M. Beck). recycling are essential mechanisms providing important building 0964-5691/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ocecoaman.2012.05.026 M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113 103 blocks of life. The trace gas CH4, which is produced as metabolic acceptor in the cascade will occur when the pool of an electron end-member in deeper sediments, may contribute to global acceptor with a higher energy yield is depleted. Nevertheless, warming when released to the atmosphere. However, the assess- microorganisms may create their niche by using substrates not ment of CH4 sources and sinks is still not fully explored in intertidal consumed by the community living in the adjacent zone (van der areas such as the Wadden Sea. Most of the biogeochemical cycles Maarel and Hansen, 1997; Wilms et al., 2006). Furthermore, pore are exemplarily presented using interdisciplinary studies around water exchange may blur the gradients by a continuous resupply of Spiekeroog Island (Germany). Our paper further addresses open electron acceptors (Beck et al., 2008c; Jansen et al., 2009). There- scientific questions and future challenges for the protection of the fore, Fig. 1 does not exhibit the conventional redox sequence in Wadden Sea. A thorough understanding of biogeochemical mineralization pathways, but shows that in the highly dynamic processes in the Wadden Sea will be a requirement for its intertidal sediments, the classical redox cascade may be disturbed sustainable future management, as biogeochemical cycles form an by pore water flow, especially in the strongly irrigated top layer. essential mechanism controlling chemical reactions and biological Finally, processes of OM remineralization lead to the accumulation growth in this ecosystem. Gaps in knowledge, e.g. regarding the of dissolved organic carbon (DOC), alkalinity, and nutrients with effects of climate change on the Wadden Sea, need to be recognized increasing depth in pore waters (Fig. 1). and addressed by future research plans. 2.1. Aerobic oxidation, reduction of nitrate and Mn/Fe oxides 2. OM remineralization in Wadden Sea sediments In the uppermost sediment layer, aerobic respiration dominates Bioturbation, bioirrigation, and/or diffusive and advective OM degradation. The depth where oxygen is still available for processes introduce OM into surface sediments of the Wadden Sea carbon (C) mineralization depends on sediment permeability. The (Meysman et al., 2005, 2007; Rusch et al., 2001; Volkenborn et al., high permeability of sand facilitates advective pore water trans- 2007; see chapter 3) where it is degraded by a cascade of redox port, in contrast to diffusion-controlled muddy sediments. processes. Aerobic respiration is followed by nitrate reduction, Furthermore, pore water flow is enhanced in surface sediments reduction of manganese (Mn) and iron (Fe) oxides, sulfate reduc- with ripple structures due to pressure gradients generated by the tion and finally methanogenesis (Froelich et al., 1979; Jørgensen, interaction of bottom currents with sediment topography (Huettel 2006). Theoretically, the sediments are divided into different and Gust, 1992; Huettel et al., 2003). This pore water flow at the zones, each characterized by a microbial community using sediment surface is an effective mechanism for rapid exchange of a specific electron acceptor. In general, a shift to the next electron oxygen (Precht et al., 2004; Ziebis et al., 1996). Consequently, NO - [µM] Mn [µM] Fe [µM] SO 2- [mM] 3 4 0369120 8 16 24 32 0 4 8 12 16 0 8 16 24 32 0 1 2 3 Depth [m] Depth 4 5 DOC [mM] Alkalinity [mM] NH + [mM] PO 3- [µM] 4 4 Si(OH)4 [µM] 012340 122436480369120 300 600 900 1200 0 300 600 900 1200 0 1 2 3 Depth [m] Depth 4 5 Fig. 1. The redox cascade in Wadden Sea sediments: pore water profiles of nitrate, manganese (Mn), iron (Fe), and sulfate. The compounds are either used as electron acceptors or þ 3À produced during OM remineralization. Dissolved organic carbon (DOC), alkalinity, NH4 ,PO4 , and Si(OH)4 accumulate with depth as they are released when OM or diatom frustules are degraded. The sampling site is located in an intertidal sand flat close to a creek bank in the backbarrier area of Spiekeroog Island. Permanently installed samplers were used to extract pore waters from 20 different depths (Beck et al., 2007). Data are derived from a sampling campaign in April 2006. The figure was modified after Beck et al. (2008a,b). 104 M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113 oxygen is depleted within the uppermost millimeters in clay-rich Sulfate [mM] dsrA targets [g-1 sediment] sediments (Böttcher et al., 2000), whereas oxic conditions persist 0102030 1x106 2x106 down to some centimeters depth in permeable sediments (Jansen 0 et al., 2009). The oxygen penetration in permeable surface sediments leads to enhanced aerobic mineralization. Highest areal oxygen consump-