BioGeoChemistry of Tidal Flats


Short Descriptions of all Sub-Projects

Sub-Project A : Bioreactor Janssand

Sub-Project B : Processes in the Water Column­

Sub-Project C : Dynamics and Particulate Matter Transport

Sub-Project D : Modelling in Tidal Flats


Sub-Project A: Jürgen Rullkötter, Dirk de Beer

Bioreactor Janssand

The Janssand sandy plate, situated southwest of Spiekeroog island, is a common feature of the backbarrier tidal flats of the East Frisian islands. In the permeable sediments of this bioreactor, we observed high microbial activity, complete consumption of sulphate and release of methane. Our aim is to clarify the following sedimentological, geochemical and microbiological issues: Is the sediment structured homogeneously ? Which influence do heterogeneities have on transport processes and transformation reactions? Are there different reaction zones from a geochemical point of view? Which electron donors and acceptors are available in the zones? Which ways does the pore water take through the plate? How high is the flow velocity and how long is the average residence time? Which influence do tidal pumping and advection have on the processes in the sediment? Which microorganisms are responsible for the initial steps of organic matter transformation? Which metabolic processes (e.g. fermentation) are the most important ones near the surface and in deeper layers? Modelling of transport processes and microbial transformations will be carried out. We will try to determine the influence of the released porewater on the budget of dissolved components in the water masses of the backbarrier tidal flats.

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Sub-Project B : Meinhard Simon, Hans-Jürgen Brumsack

Processes in the Water Column

In order to elucidate the material budget in the tidal flats the hypothesis will be tested that events such as plankton blooms and the corresponding microbial activity have a key influence on the transformation of organic matter, and on the concentrations of manganese (Mn) and molybdenum (Mo) in the water column and in the surface sediments. We postulate that the collapse of phytoplankton blooms will lead to aggregate formation by microorganisms and that the number of bacteria on the aggregates strongly increases during the lower current velocity periods at high and low tide. Anoxic microzones will be formed as a consequence of the intense microbial degradation of organic matter, resulting in a partial reduction of Mn(IV) and release of Mn(II). At the same time, Mo will be bound to the aggregates and withdrawn from the dissolved phase. Part of the aggregates will settle and be deposited on the flats. Subsequent mineralization of the aggregates entails a slow liberation of Mn and Mo and a slow release into the water column. We plan to verify the hypothesis by means of intense in-situ studies lasting several weeks, by rolling tank experiments on sediment cores under controlled conditions in the laboratory, and by modelling.

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Sub-Project C : Rainer Reuter, Jörg-Olaf Wolff

Dynamics and Material Transport

In order to understand the kind and extent of the exchange processes between the near-coastal zone and the open sea in a scientifically satisfactory way, models as well as in-situ measurements and remote sensing data with extremely high resolution are required. The spatial resolution in the model will be obtained by downscaling a large-scale North Sea model to the tidal flats with a grid size of 50 m.

Recent satellite techniques even allow a spatial resolution down to the 10-m level. We will increase the quality of the experimental time series through extended validation methods and improved long-term stability of the sensors. The complex transport, turbulence and sediment structures which we observed must be reproduced by numerical models as quality features. In this way, we will be able to give a reliable estimate of the dynamic off-shore processes and predict their effects. A fundamental understanding based on observations and modelling is the basis for a sustainable development of coastal zones. The overall aim of this subproject is to combine observation strategies and hydrodynamic models in order to achieve a validated representation of movement processes and material transport in the water column of the East Frisian tidal flats.

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Sub-Project D : Ulrike Feudel

Modelling in Tidal Flats

Our study focuses on the interaction between biogeochemical transformation and  hydrodynamic tidal currents, especially on the effects of tidal dynamics on important single processes and on the system as a whole. With regard to the single processes, our primary focus is on the description of bacterial colonization of aggregates with an individuum-based model and the analysis of aggregate dynamics with a distribution-based model. The investigation of these single processes will provide information on how far there is a feedback of biological and physical processes. The significance of the single processes will be studied in the complex materials cycle model EcoTim by deriving parameter-based descriptions from conceptional models. The complex model developed in this way will become the basis for analysing the dynamics and budgets of the backbarrier tidal flats. As one of the key processes that has a significant influence on the whole system, the phytoplankton dynamic in the model will be better adjusted to the conditions in the tidal flats, because this process occurs early on in the flow chain of energy and materials. Furthermore, we will examine the boundary conditions imposed by the North Sea.

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