, with values distinctly below the common summer level (by a factor of five). A direct relation between the loss of Mo and scavenging by freshly formed MnO x phases could not be inferred from our data because both metals revealed inverse patterns. Parallel to decreasing dissolved Mo concentrations dissolved Mn showed an increasing trend while particulate Mn decreased. Such finding is compatible with the formation of oxygen-depleted zones in aggregates, which provide suitable conditions for the rapid fixation of Mo and parallel release of Mn by chemically and/or microbially mediated processes. Our assumption is supported by biological (e.g. number of aggregate-associated bacteria) and sedimentological (e.g. aggregate abundance and size) parameters. The production of organic components (e.g. TEP) during breakdown of an algae bloom in July 2005 led to the formation of larger Mo-enriched aggregates, thus depleting the water column in dissolved Mo. After deposition on and incorporation into sandy tidal flats these aggregates are rapidly decomposed by microbial activity. Pore water profiles document that during microbial decomposition of these aggregates, substantial amounts of Mo are released and may replenish and even enrich Mo in the open water column. We postulate a conceptual model for the observed non-conservative behaviour of Mo in coastal waters, which is based on the tight coupling of geochemical, biological, and sedimentological processes.

In this study, we present a multilevel in situ pore water sampler that allows pore water sampling down to 5 m sediment depth. The sampler forms a crucial tool to study biogeochemical processes on different time scales in advective pore water systems. After insertion into the sediment, the sampler stays on site, allowing repetitive sampling at identical locations and depth intervals. The sampler has been successfully tested for 1 year in sandy sediments in the backbarrier tidal flats of Spiekeroog Island at the German North Sea coast. Depth profiles of redox-sensitive elements show a high depth resolution and are not affected by oxidation artifacts during extraction. Seasonal variations because of advection and changing microbial activity are apparent for some element species even at sediment depths of 5 m.

The barrier islands of the southern North Sea were formed during the Holocene sea-level rise. These islands form part of a highly dynamic environment whose evolution continues today. Subjected to sea-level changes, tides and storm events, the sedimentary record reflects processes occurring under varying energy conditions. This article presents geochemical, mineralogical and diatom investigations carried out in the salt marsh of the East Frisian barrier island of Langeoog, which is re-exposed to a rising sea-level due to de-embankment. The major aim of this study is to improve the knowledge of the sedimentological and geochemical development of these deposits under the influence of sea-level rise, with a special focus on the geochemistry and distribution of heavy mineral-associated elements. Correlation diagrams between FeO, TiO 2 and MnO, as well as ternary plots (Al 2 O 3 -SiO 2 -Zr or TiO 2 ), clearly indicate the variable appearance of heavy minerals in different lithological facies, comprising marsh soil, mixed and sand flat, and relocated beach sands. A dominating abundance of ilmenite followed by zircon, garnets and some other heavy minerals is evidenced by Scanning Electron Microscope-Energy Dispersive X-ray measurements. The data presented here suggests that these geochemical proxies are useful tools for characterizing depositional energy conditions. Increasing depositional energy is evident for the lithological units in the following order: marsh soil, mixed flat, sand flat and relocated beach sand. The energetic conditions during sediment deposition, as well as the sedimentary history, are confirmed by diatom analyses as an additional independent indicator. Depending on source rock composition, the geochemical parameters used in this study may also help to investigate depositional energy regimes of other siliciclastic sedimentary systems.

Salt marsh succession after de-embankment was monitored on the East Frisian barrier island Langeoog by investigating permanent plots. Seventy years after embankment salt marsh plants were once again influenced mainly by the tidal regime. From 2002 to 2004 the former high marsh and glycophytic vegetation died out and was replaced by species of lower salt marsh zones. Nitrophytic halophytes like Suaeda maritima, Atriplex prostrata and Artemisia maritima established first because of the high nutrient content in the soil, a direct result of former vegetation decay. With decreasing nitrogen afterwards other species became more competitive. Until 2007 Atriplex portulacoides became more dominant in the lower marsh and Elymus athericus reached dominance in areas where grazing has been abandoned in the high marsh. The dynamics in the study area is much lower than in natural marshes due to the still existing drainage system. Therefore vegetation units with low species diversity are widespread.

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