Nitrogen (N) loss rates were determined in permeable sediments of the Wadden Sea using a combination of stable N isotope incubation experiments and model simulation approaches during three seasons. Three different incubation methods that employed the isotope pairing technique were used: intact core incubations simulating either (1) diffusive or (2) advective transport conditions and (3)
In this article, we describe the dynamics of pH, O 2 and H 2 S in the top 5-10 cm of an intertidal flat consisting of permeable sand. These dynamics were measured at the low water line and higher up the flat and during several seasons. Together with pore water nutrient data, the dynamics confirm that two types of transport act as driving forces for the cycling of elements (Billerbeck et al. 2006b): Fast surface dynamics of pore water chemistry occur only during inundation. Thus, they must be driven by hydraulics (tidal and wave action) and are highly dependent on weather conditions. This was demonstrated clearly by quick variation in oxygen penetration depth: Seeps are active at low tide only, indicating that the pore water flow in them is driven by a pressure head developing at low tide. The seeps are fed by slow transport of pore water over long distances in the deeper sediment. In the seeps, high concentrations of degradation products such as nutrients and sulphide were found, showing them to be the outlets of deep-seated degradation processes. The degradation products appear toxic for bioturbating/ bioirrigating organisms, as a consequence of which, these were absent in the wider seep areas. These two mechanisms driving advection determine oxygen dynamics in these flats, whereas bioirrigation plays a minor role. The deep circulation causes a characteristic distribution of strongly reduced pore water near the low water line and rather more oxidised sediments in the centre of the flats. The two combined transport phenomena determine the fluxes of solutes and gases from the sediment to the surface water and in this way create specific niches for various types of microorganisms.
Sulfidic seeps with methane ebullition were observed at the low-water line of intertidal sand flats at a number of locations in the Wadden Sea. Bioturbating fauna was absent in the seep areas but abundant in the more central areas of the tidal flat. At one site, the vertical methane and sulfate distribution in pore water was determined along transects from the low-water line toward the interior of the sand flat. The resulting two-dimensional distributions showed a plume of methane-rich and sulfate-depleted pore water reaching from a depth below 1.2 m beneath the sand surface up to the sediment surface at the low-water line. The d 13 C of methane released at the seeps was 268.6%, indicating a biological origin. The 14 C signature of methane was clearly elevated by anthropogenic radiocarbon, which shows that the methane was formed less than 50 yr ago. The observations indicate an internal circulation, where water enters the sand flats in the central area and exits at the low-water line. Pore-water flow patterns in the sand flat during the tidal cycle were calculated from the surface topography and from the pressure distribution at the flat surface across the tidal cycle. The calculated flow patterns explain the measured methane and sulfate distributions and predict a residence time of the seepage water of about 30 yr. Intertidal sand flats act as one-way valves, passing water from the central surface through the interior of the flat to an outflow zone at and below the low-water line with a velocity of millimeters to centimeters per day. The flow causes permeable tidal flats to emit methane to the surface water and atmosphere in substantial amounts.The Wadden Sea is one of the largest coherent tidal flat systems in the world. It is separated from the North Sea by a chain of barrier islands across 500 km of coastline from Den Helder in the Netherlands, across the German coast to Blå vands Huk in Denmark (Fig. 1). The tidal range is between 3.5 m in the central part and 1.2 m in the northernmost and southernmost areas (van Beusekom 2005). The original ecosystems of the Wadden Sea were salt marshes, mudflats, and sea grass beds that developed after the last ice age. The salt marshes disappeared because of peat harvesting, and the floodplain areas decreased, primarily because of land reclamation. Thus, the sediments of the Wadden Sea are changing via a gradual displacement of silt with sands, especially toward the barrier islands and the large tidal channels where intertidal sand flats now dominate (Lotze 2005).The incentive for this study was methane ebullition and seepage of sulfidic pore water observed at the low-water line of the intertidal sand flat Janssand. A former study (Billerbeck et al. 2006a) concentrated on nutrient fluxes carried by relatively surficial flow. This study aims to identify the source and pathway of the methane-rich water that seeps out near the low-water line, i.e., the deeper part of what was defined as body flow (Billerbeck et al. 2006b). Methane is globally one of the largest rese...
In the analysis of metal biouptake from complexing environments, both chemical speciation and biological uptake characteristics have to be taken into account. The commonly used free ion activity model is based on equilibrium speciation and implies that diffusion of the bioactive free metal toward the organism is not rate-limiting. In the presence of complexes, however, sufficiently labile species might contribute to the biouptake via preceding dissociation. Coupling of the ensuing diffusional mass transfer flux of metal with the biouptake flux of free metal, the supposedly bioactive species, shows under which conditions labile metal complexes can contribute to the uptake. The goal of the present paper is to apply this type of analysis to experimental data on metal uptake by mussel (Mytilus edulis) and carp (Cyprinus carpio) in complexing environments. These biosystems have fairly well-characterized uptake parameters, but the uptake fluxes cannot be fully explained by considering equilibrium speciation only. For Zn(II) uptake by mussel, evidence was found for diffusional limitation at low concentrations, whereas for Cd(III) uptake by carp, diffusion is not limiting at all. The analysis provides an example of how a more comprehensive treatment of complex systems can be applied to real experimental data.
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