Benthic metabolism and sulfur cycling along an inundation gradient in a tidal Spartina anglica salt marsh

Central aspects of carbon and sulfur biogeochemistry were studied along a transect extending from an unvegetated mudflat into a Spartina anglica salt marsh. Conditions along the transect differed with respect to tidal elevation, sediment characteristics, vegetation coverage, and benthic macrofauna abundance. Dark sediment O2 uptake and CO2 emission at the highly bioturbated mudflat were low and relatively unaffected by tidal coverage. Sulfate reduction accounted for 30–60% of the daily CO2 emission from the open mudflat sediment. Sediment O2 uptake within the nonbioturbated and vegetated marsh was up to seven times higher during air exposure than during inundation, whereas the difference in CO2 emissions always was less than a factor of 2. The contribution of sulfate reduction to CO2 production was low (,21%) and decreased progressively with tidal elevation as a result of the oxidizing capacity of S. anglica roots in the vegetated marsh. The boundary between the mudflat and the retreating marsh is a unique environment. High near-surface pore-water concentrations of dissolved organic carbon (DOC) above the marsh cliff and highly elevated total carbon dioxide (TCO2) pore-water concentrations at both sides of the cliff during air exposure coincided with extremely high TCO2 emissions and apparent respiratory quotients (up to 14) only below the marsh cliff during inundation. We propose that substantial seepage of DOC-poor and HCO -rich pore water may have occurred from the elevated marsh to the unvegetated sediment below during low 3 tide followed by massive release of HCO during high tide. Accordingly, sulfate reduction accounted for more than 3 the TCO2 release above the marsh cliff, but only for about 40% below the cliff. Mineralization rates and pathways in salt-marsh sediments vary considerably on small spatial and temporal scales and are dependent on inundation frequency as well as the composition and distribution of flora and fauna. Salt marshes form a transitional zone between the marine and terrestrial environment in sheltered and estuarine areas. They are defined by the presence of distinct vascular plant assemblages, commonly dominated by species of Spartina, with tolerance for periodic seawater inundation. Temperate salt marshes are among the most productive ecosystems in the world and are recognized for their important ecological function and societal values (Boorman 1999). They form a buffer zone that protects coastal areas from flooding during catastrophic storm events and act as a sink for both natural and anthropogenic nutrients and organic matter derived from both land and sea. An understanding of microbial transfer between the organic and inorganic reservoirs is vital for determining the role of salt marshes for the dynamics of organic matter in estuaries and coastal waters. Most of the materials entering salt marshes are converted, assimilated, dissipated, and buried within the marsh system (Alongi 1998). As a consequence of efficient particle trapping, salt-marsh sediments 1 To whom correspondence should be addressed. Present address: Center for Estuarine and Marine Ecology, Netherlands Institute of Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands ([email protected]).