This paper gives the physical oceanography background for the Bremerhaven Workshop on Biological Effects of Contaminants in the North Sea and the resulting papers contained in the volume. For the water quality bioassays, the benthic community analysis, and the benthic sediment bioassays the relevant hydrographic conditions are described and the results of model simulations carried out for the workshop are presented as well as an evaluation of the oceanographic observations carried out on board RV 'Valdivia' during the workshop. Two main conclusions must be drawn: (1) The German Bight is an area characterized by strong mesoscale variability in physical properties such as fronts, meanders and eddies. This vanabil~ty is also seen in the mobile superficial sediment and the resulting variability of the suspended matter content. (2) During the workshop the hydrographic situation in the German Bight was marked by highly saline inflows to the German Bight from the English Channel. The stations close to the East-Friesian coast are located within the contaminated Continental Coastal Water; the more offshore stations met more or less undiluted water originating from the English Channel, with a high portion of Atlantic Water.

Knowledge about the distribution of suspended particulate matter (SPM) is an important prerequisite for the description and prognostication of the ecological conditions of the North Sea. SPM concentrations in the water column regulate the penetration depth of light and, therefore, is an important parameter influencing the primary production of plankton. Fine sediment in the bottom acts like a dynamical buffer representing sources and sinks for nutrients simultaneously. Moreover, the determination of the distribution of SPM and fine sediment provides evidence of the disposition of pollutants, adhered to the particles. In cooperation with GKSS Research Center, the transport model for dissolved conservative substances of the Federal Maritime and Hydrographic Agency (Bundesanstalt für Seeschifffahrt und Hydrographie, BSH) was extended by a Suspended Particulate Matter module to take into account the processes of vertical exchange and horizontal distribution of SPM due to currents and waves. The success of such a model depends on an adequate description of the physical processes, as well as a careful preparation of maps of finegrained sediment in the bottom and other sources of SPM (e.g., fluvial input and cliff erosion). In this paper, we examine quantitatively and qualitatively SPM distributions during two periods in 2000 and 2001. The results of a simulation during a storm at the beginning of 2001 are examined in detail to show the role of single exchange processes. Another application of the SPM transport model is presented as the simulation of dispersion of suspended material after a big river flood.

We evaluated the patterns of larval settlement of Carcinus maenas on natural substratum and its relationships with forcing factors related to larval transport mechanisms. Settlement was estimated as colonisation of traps containing defaunated substratum. Traps were deployed at low tide for 24 to 26 h, in the intertidal of the island of Helgoland (North Sea, German Bight), every day dur- rates peaked mainly at, or 1 to 3 d after, spring tide; periods of low colonisation were short, except at the end of the settlement season. According to an existing 2-step model and to local movement of megalopae, colonisation patterns in the intertidal may reflect transport processes: (1) settlement rates on Helgoland are primarily affected by wind-driven currents: SW winds transport larvae out of Helgoland waters, leading to low settlement rates, while other wind regimes transport them towards Helgoland. In the absence of strong SW winds, peaks of settlement at or after spring tide suggest (2) selective tidal stream transport or some form of tidally mediated flow. The absence of SW winds leads to a higher influence of local movement of megalopae, as the number of larvae reaching the benthic habitat is high. In consequence, colonisation of natural substratum may reflect larval transport processes and local movement of larvae in C. maenas and other invertebrates with mobile benthic stages.

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Abstract. For understanding and forecasting of hydrodynamics in coastal regions, numerical models have served as an important tool for many years. In order to assess the model performance, we compared simulations to observational data of water temperature and salinity. Observations were available from FerryBox transects in the southern North Sea and, additionally, from a fixed platform of the MARNET network. More detailed analyses have been made at three different stations, located off the English eastern coast, at the Oyster Ground and in the German Bight.FerryBoxes installed on ships of opportunity (SoO) provide high-frequency surface measurements along selected tracks on a regular basis.The results of two operational hydrodynamic models have been evaluated for two different time periods: BSHcmod v4 (January 2009 to April 2012) and FOAM AMM7 NEMO (April 2011 to April 2012). While they adequately simulate temperature, both models underestimate salinity, especially near the coast in the southern North Sea. Statistical errors differ between the two models and between the measured parameters. The root mean square error (RMSE) of water temperatures amounts to 0.72 • C (BSHcmod v4) and 0.44 • C (AMM7), while for salinity the performance of BSHcmod is slightly better (0.68 compared to 1.1).The study results reveal weaknesses in both models, in terms of variability, absolute levels and limited spatial resolution. Simulation of the transition zone between the coasts and the open sea is still a demanding task for operational modelling. Thus, FerryBox data, combined with other observations with differing temporal and spatial scales, can serve as an invaluable tool not only for model evaluation, but also for model optimization by assimilation of such high-frequency observations.