Regional variation in eelgrass (Zostera marina) morphology, production and stable sulfur isotopic composition along the Baltic Sea and Skagerrak coasts Marianne Holmer a, *, Susanne Baden b , Christoffer Bostro ¨m c , Per-Olav Moksnes d a Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark b Department of Marine Ecology, University of Gothenburg, Box 566, Kristineberg, 45034 Fiskeba ¨ckskil, Sweden c A ˚ bo Akademi University, Environmental and Marine Biology, Artillerigatan 6, 20520 A ˚ bo, Finland d Department of Marine Ecology, University of Gothenburg, Box 461, 40530 Go ¨teborg, Sweden 1. Introduction Sulfur content (TS) and sulfur isotopic composition (d 34 S) have been proposed as indicators of sulfide invasion into seagrasses (Frederiksen et al., 2006), and as sulfide invasion is considered an important factor affecting seagrass decline (Orth et al., 2006), there is a need to understand the natural variability in TS and d 34 S at local as well as regional scales. Seagrasses obtain sulfur by active uptake of sulfate by leaves directly from the water column or by roots from the sediment pore waters (Kylin, 1960; Rennenberg, 1984), or by passive invasion of sedimentary sulfides into the below-ground tissues (Pedersen et al., 2004). In oceanic seawater the stable isotopic signal of sulfate (d 34 S) is approximately +21% (Rees et al., 1978), but due to bacterial fractionation by sulfate reducing bacteria, the sedimentary sulfides are isotopically lighter than sulfate (Fry et al., 1982), typically with d 34 S signals around 25% (Canfield, 2001; Bo ¨ttcher et al., 2004). The sulfide dynamics in sediments are controlled by a number of factors such as the sulfide production during mineralization of organic matter by sulfate reducing bacteria and by sulfide removal through precipitation with iron to ironsulfides (e.g. pyrite) or through reoxidation of sulfides back to sulfate or other oxidation products mediated by iron-oxides or oxygen (Van Capellen and Wang, 1996). Sulfide production is stimulated in seagrass sediments by release of photosynthetic organic compounds from the roots (Long et al., 2008) and enrichment of sediments with organic matter (Marba ` et al., 2006). Sulfide production is, however, besides organic matter availability, controlled by the availability of sulfate as electron acceptor (Pallud and Van Cappellen, 2006), and the low salinity, and thus low sulfate concentrations, in the Baltic Sea could limit sulfate reduction, in particular in the deeper diffusion controlled parts of the sediments. In this case mineralization processes, such as methano- genesis, replace sulfate reduction and potentially lower the sulfide pressure on the plants. Also sulfide reoxidation is highly variable in seagrass sediments, and diurnal oscillations of sulfide concentra- tions in rhizosphere sediments as a result of variation in leakage of oxygen from the plants have been observed (Lee and Dunton, 2000; Pedersen et al., 2004), but also factors such as advection through wind exposure and tides may lead to rapid fluctuations in sulfide pools (Van Capellen and Wang, 1996). Aquatic Botany 91 (2009) 303–310 ARTICLE INFO Article history: Received 15 April 2009 Received in revised form 9 July 2009 Accepted 14 August 2009 Available online 21 August 2009 Keywords: Seagrass Productivity Sulfide Epiphytes Indicator ABSTRACT Morphology, total sulfur content and stable sulfur isotopic composition of Zostera marina were examined in the Baltic Sea–Skagerrak transition zone through surveys. The seagrass meadows were denser and less productive at the low salinities in the Baltic Sea (salinity 6–7 psu), and total sulfur accumulations in plants were lower and d 34 S values were higher compared to the west coast of Sweden (salinity 21– 29 psu). The d 34 S values of the three plant compartments (leaves, rhizomes, roots) indicated lower sulfide invasion at low salinities, which was mainly due to environmental conditions (e.g. low epiphytic biomass, low sediment organic matter and low sulfate concentration) and plant characteristics (productivity, shoot morphology). Between 13% and 63% of the sulfur in the plants was derived from sediment sulfides with highest percentages in the roots (27–63%) and lower in rhizomes (13–50%) and leaves (14–51%). The high sulfide invasion on the west coast of Sweden was coincident with high sediment organic matter, probably increasing sulfide pressure on the plants, and high epiphytic biomass, probably constraining the oxygen dynamics in the plants and enhancing sulfide invasion. Regional and spatial variability in the d 34 S were extensive, emphasizing the need for detailed analysis of local sources when applying stable sulfur isotopes in food web analyses. The observed invasion of sulfides suggests sulfide as a contributing factor to reported declines of Z. marina in the Skagerrak region. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Fax: +45 6550 2787. E-mail address: holmer@biology.sdu.dk (M. Holmer). Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot 0304-3770/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2009.08.004