Th. Pohlmann

Exchange of matter and energy between the Wadden Sea and the coastal waters of the German Bight-Estimations based on numerical simulations and field measurements

Based on numerical simulations and high resolution measurements, budgets have been computed for transports of water and heat as well as dissolved and particulate matter between the coastal waters of the German Bight and the North Frisian Wadden Sea. p]During 3 measuring campaigns in summer, spring and winter, current conditions and water transports differed only slightly. In spite of different meteorological situations, inflow and outflow of North Sea water to and from the Wadden Sea were largely balanced. A net heat export from the North Frisian Wadden Sea was determined both in summer 1994 and spring 1995, i. e. the Wadden Sea represented a heat source for the adjacent North Sea waters.

Combined analysis of field and model data: A case study of the phosphate dynamics in the German Bight in summer 1994

The intention of this paper is to analyse a specific phenomenon observed during the KUSTOS campaigns in order to demonstrate the general capability of the KUSTOS and TRANSWATT approach, i.e. the combination of field and modelling activities in an interdisciplinary framework. The selected phenomenon is the increase in phosphate concentrations off the peninsula of Eiderstedt on the North Frisian coast sampled during four subsequent station grids of the KUSTOS summer campaign in 1994. First of all, a characterisation of the observed summer situation is given. The phosphate increase is described in detail in relation to the dynamics of other nutrients. In a second step, a first-order estimate of the dispersion of phosphate is discussed. The estimate is based on the box model approach and will focus on the effects of the river Elbe and Wadden Sea inputs on phosphate dynamics. Thirdly, a fully three-dimensional model system is presented, which was implemented in order to analyse the phosphate development. The model system is discussed briefly, with emphasis on phosphorus-related processes. The reliability of one of the model components, i.e. the hydrodynamical model, is demonstrated by means of a comparison of model results with observed current data. Thereafter, results of the German Bight seston model are employed to interpret the observed phosphate increase. From this combined analysis, it was possible to conclude that the phosphate increase during the first three surveys was due to internal transformation processes within the phosphorus cycle. On the other hand, the higher phosphate concentrations measured in the last station grid survey were caused by a horizontal transport of phosphate being remobilised in the Wadden Sea.

SPM concentrations in the German Bight: comparison between a model simulation and measurements.

A 3-dimensional numerical model calculates the transport of suspended particulate matter (SPM) in the German Bight. The calculations are based on current and wave data computed by other models. The aim is to test the SPM model’s quality by comparing model predicted SPM concentrations (a) with a time series of measured SPM concentrations and (b) with a horizontal distribution of SPM surface concentrations in the German Bight. The comparison shows that the model is not able to reproduce the typical horizontal SPM distribution in the German Bight. The poor performance of the model is primarily due to the fact that its description of currents and waves is too coarse for the hydrodynamically complicated environment of the German Bight.

Mesoscale structures, fluxes and water mass variability in the German Bight as exemplified in the KUSTOS- experiments and numerical models

In this article, the hydrographic conditions are described which prevailed in the German Bight during the three KUSTOS experiments carried out in summer 1994, spring 1995, and winter 1996. It presents the physical background for the companion articles of this volume and provides insight into the complicated and highly variable hydrodynamics of the German Bight. Typically for the German Bight, the distribution of temperature and salinity in the near-bottom layer was found to relate strongly to the topography in all experiments, and variability in the vertical structure showed the expected seasonal pattern. The thermal and haline stratification were found to coincide in most cases, indicating a clear marking of surface and bottom water masses. Between the three KUSTOS experiments, the intensity of salinity fronts varied strongly, with stronger fronts in spring and weaker gradients in summer and winter, but from the limited number of observations this pattern cannot be generalised. The observed temperature and/or salinity patterns differ strongly from the average long-term situation, as expected in a highly variable area. To a large degree, this variability is caused by mesoscale features which - owing to their transient nature - are not well resolved by in-situ observations during the experiments and, therefore, numerical models are used to complement the observations. Finally, the overall mass transport through the German Bight during the KUSTOS experiments, estimated from a North Sea circulation model and a salt water budget and freshwater balance, ranges between 34 (strong cyclonic circulation) and -3 km3/d (weak anticyclonic circulation). North Sea water advection into the German Bight dominates river runoff by a factor of ∼15 (spring 1995) to ∼49 (winter 1996). However, in situations with a low through-flow in combination with a high ratio of freshwater inflow, residence times of river water can be high, thereby aggravating any harmful effects of riverine contaminants.