Tidal wave transformations in the German Bight

Mesoscale and submesoscale dynamics associated with tidal wave transformations were addressed in the German Bight using numerical simulations. Tidal gauge and velocity observations in several locations were used to validate the numerical model. A downscaling approach included analysis of simulations with horizontal resolutions of 1, 0.4, and 0.2 km. It was shown that the modified tidal wave lost most of its energy after reflection or refraction over the eastern part of the German Bight. Energy loss resulted in a pronounced change of the wave’s spectral composition and generation of overtides. Tidal oscillations were modified by mesoscale processes associated with bathymetric channels. Semidiurnal and quarterdiurnal tides revealed very different spatial patterns. The former were aligned with the bathymetric channels, while the latter were rather “patchy” and had about half the spatial scales. In numerous areas around the bathymetric channels, the major axis of the M₄ ellipses was normal or at some angle with the major axis of the M₂ ellipses. Thus, higher harmonics developed “orthogonal” patterns that drove secondary circulations. Moreover, the ratio between spring and neap tidal amplitudes was relatively low in the Wadden Sea, showing reduced sensitivity of this very shallow area to fortnightly tidal variations. It was demonstrated that simulated hydrodynamics patterns help explain the physical mechanism shaping the median grain size distribution in the German Bight.     

Wind-driven wave heights in the German Bight

Wind speed, friction velocity and significant wave height data from the FINO1 platform in the southern German Bight 45 km off the coast for the years 2004 to 2006 have been evaluated and related to each other. The data show a clear dependence of the hourly mean wave height to the hourly mean friction velocity and wind speed. Wave heights increase with decreasing stratification and increasing fetch. Synoptic weather patterns for the highest wave heights in the southern German Bight are determined. The analysis is made separately for four wind direction sectors. The two strongest storms in the evaluated period, “Britta” and “Erwin”, are analysed in more detail. Finally, the 50-year extreme significant wave height has been estimated to be about 11 m most probably coming from northerly directions.     

A comparison of fluorescence and GC for the determination of petroleum hydrocarbons in mussels

The suitability of fluorescence spectroscopy for the determination of aromatic concentrations in mussel tissue was investigated. A mixture of aromatic compounds, with a total emission spectrum similar to those of the samples, was found to be the most suitable standard with measurement being made at the emission maximum. Significant correlations were found between aromatic hydrocarbon concentrations obtained by fluorescence spectroscopy and both aliphatic and aromatic hydrocarbon concentrations obtained by gas chromatographic analysis (GC). Results indicate that analysis of the aliphatic fraction by GC and fluorescence analysis of the aromatic fraction provides a suitable estimation of the relative degree of contamination of mussel tissues by petroleum hydrocarbons.     

Determining sea level change in the German Bight

Regional mean sea level changes in the German Bight are considered. Index time series derived from 15 tide gauge records are analysed. Two different methods for constructing the index time series are used. The first method uses arithmetic means based on all available data for each time step. The second method uses empirical orthogonal functions. Both methods produce rather similar results for the time period 1924–2008. For this period, we estimate that regional mean sea level increased at rates between 1.64 and 1.74 mm/year with a 90% confidence range of 0.28 mm/year in each case. Before 1924, only data from a few tide gauges are available with the longest record in Cuxhaven ranging back till 1843. Data from these tide gauges, in particular from Cuxhaven, thus receive increasingly more weight when earlier years are considered. It is therefore analysed to what extent data from Cuxhaven are representative for the regional sea level changes in the German Bight. While this cannot be clarified before 1924, it is found that this is not the case from 1924 onwards when changes in Cuxhaven can be compared to that derived from a larger data set. Furthermore, decadal variability was found to be substantial with relatively high values towards the end of the analysis period. However, these values are not unusual when compared to earlier periods.     

Correcting surface winds by assimilating high-frequency radar surface currents in the German Bight

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account European Centre for Medium-Range Weather Forecasts winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint, and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.     

Uptake and discharge of petroleum hydrocarbons by barnacles

The hydrocarbon content of barnacles living on tarballs has been compared with the hydrocarbon composition of the tarballs. While there is some contamination of the barnacles there is no evidence of gross pollution and the analyses suggest that oil hydrocarbons are assimilated and then discharged, unmetabolized, quite rapidly.     

Comparison between measured and calculated tidal ellipses in the German Bight

Using a three-dimensional non-linear shelf model, the elliptical properties (ellipticity, inclination of the ellipse, major and minor semi-axis and phase) of the M₂ tide in the German Bight were calculated and compared with CODAR measurements. A series of barotropic and baroclinic calculations were carried out to investigate the influence of geometry, stratification and particularly inputs of freshwater on these parameters. The elliptical properties undergo stronger changes in zones of influence of embayments and in the deepening of the old Elbe Valley. Friction effects in the shallow areas are responsible for robust vertical variations of the ellipticity. The island of Helgoland induces wakes on its western and eastern sides. The discharge of freshwater of the rivers Elbe, Weser and Ems induced in general negative ellipticity. Although primarily determined by geography, baroclinic effects significantly modified the inclination of the ellipses. The calculated ellipses pattern of anticlockwise and clockwise tidal current rotation agrees quite well with CODAR measurements. The elliptical properties give a general idea of the interaction of tidal waves with coastal geometries.Responsible Editor: Hans Burchard     

A statistical analysis of climate variability and ecosystem response in the German Bight

We compiled homogeneous long-term time series comprising 39 variables representing the German Bight and for the period 1975–2004. A diverse set of variables was selected to cover multiple trophic levels and different environmental forcing thus to examine long-term changes in this coastal region. Previous studies have hypothesised the presence of regime shifts in observations extending over the entire North Sea. Focusing on a smaller spatial scale, and closer to the coast, we investigated the major modes of variability in the compiled time series using principal component analysis. The results obtained confirm a previously identified regime shift in the North Sea in 1987/1988 and suggest that the German Bight is dominantly characterised by long-term modes of variability. In the German Bight, the shift of 1987/1988 is driven primarily by temperature, Gulf Stream index, frost days and Secchi depth. Changes in some of the ecosystem variables (plankton and fish) appear to be related to changes in these driving variables. In particular, we documented strong positive correlations between the long-term trend showed by the first principal component and herring, Noctiluca scintillans, and, to a lesser extent, Pleurobrachia pileus. Two gadoids, namely cod and saithe, showed negative correlations with the observed long-term mode of variability. Changes in the sum of five small calanoid copepods were, however, less marked. Phosphate and ammonium exhibited a decreasing trend over the last 30 years. Diatoms and Calanus helgolandicus did not show evidence of changes in concert to this trend. Specific analyses of the data divided into three different subsets (biological, climatic and chemical) characterise the climate of the German Bight as highly dynamic also on short timescales (a few years) as compared to much smoother biological and chemical components. The dynamic regime of the German Bight taken together with a low correlation between the major mode of variability and phytoplankton and zooplankton data suggests that the lower trophic levels of this ecosystem are remarkably resilient.     

Pelagic tar, dissolved/dispersed petroleum hydrocarbons and plastic distribution in the Cretan Sea, Greece

During the first cruise of R/V ‘Philia’ in July 1997 within the framework of the TALOS programme supported by the Greek Ministry of Physical Planning and Public Works, the sampling of floating tar, litter and sea water for dissolved/dispersed petroleum hydrocarbons (DDPH) was carried out in the Cretan Sea. Analysis of these data has allowed a first assessment of the status of floating marine pollution in the region. DDPH measurements showed a mean concentration of 0.145 μg/l of chrysene equivalents (n = 24). Tar and plastics concentrations were in the range of 1–4280 and 0–1160 μg/m², respectively. Mean pelagic tar concentration was 318 μg/m², more than two times higher than what was reported for the area in previous studies. Based on in situ hydrographic observations there is strong evidence that most of the floating tar enters the Cretan Sea through the Ionian Sea.     

On the relation between residual currents and the wind field in the German Bight

Summary The long-period part (T>25 h) of currents measured at close spacing in the German Bight in June 1968 is investigated. The north component of the bottom current is shown to be highly correlated with the east component of the wind in this area, the wind-generated changes of the mass-field giving rise to a quasi-geostrophic current near the bottom.This current responds without perceptible delay to the variations of wind fields with periods considerably longer than those of the seiches of the North Sea. The accelerations linked to such long periods are so small as to be negligable and the residual currents can be regarded as quasi-stationary.

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Über die Beziehung zwischen Reststrom und Windfeld in der Deutschen Bucht

Zusammenfassung Es werden engabständige Strommessungen in der Deutschen Bucht aus dem Juni 1968 in ihrem langperiodischen Teil (T>25 h) untersucht. Dabei ergibt sich, daß die Nordkomponente des Bodenstromes (v) stark korreliert mit der Ostkomponente des Windes (U) in diesem Gebiet. Die winderzeugten Veränderungen des Massenfeldes verursachen eine quasi-geostrophische Strömung am Boden.Diese Strömung reagiert ohne erkennbare Verzögerung auf solche Veränderungen des Windfeldes, die erheblich größere Perioden haben als die Eigenschwingungen der Nordsee. Die damit verbundenen Beschleunigungen sind sehr klein und so zu vernachlässigen; daher kann man die Restströmung auch als quasi-stationär ansehen.

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Sur la relation entre les courants résiduels et le champ du vent en baie Allemande

Résumé La partie à longue période (T>25 h) de courants measurés en des points rapprochés en baie Allemande en juin 1968 est étudiée. La composante Nord du courant de fond se révèle largement corrélée avec la composante Est du vent dans cette zone. Les modifications du champ de masse dues au vent donnent naissance à un courant quasi-géostrophique près du fond.Ce courant répond sans délai perceptible aux variations du champ du vent avec des périodes considérablement plus longues que celles des seiches de la mer du Nord. Les accélérations liées à de telles longues périodes sont si petites qu'elles peuvent être négligées et les courants peuvent être considérés comme quasi-stationnaires.

     

Total body burden and tissue distribution of polyfluorinated compounds in harbor seals (Phoca vitulina) from the German Bight

Total body burden and tissue distribution of polyfluorinated compounds (PFCs) were investigated in harbor seals (Phoca vitulina) from the German Bight in 2007. A total number of 18 individual PFCs from the following groups could be quantified in the different tissues: perfluorinated carboxylic acids (PFCAs) and perfluorinated sulfonates (PFSAs) and their precursors perfluorinated sulfinates (PFSiAs), perfluorinated sulfonamides, and sulfonamido ethanols. Perfluorooctanesulfonate (PFOS) was the predominant compound in all measured seal tissues (up to 1665 ng g⁻¹ wet weight in liver tissue). The dominant PFCAs were perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA), but their concentrations were much lower compared to PFOS. The mean whole body burden in harbor seals of all detected PFCs was estimated to be 2665 ± 1207 μg absolute. The major amount of the total PFCs burden in the bodies was in blood (38%) and liver (36%), followed by muscle (13%), lung (8%), kidney (2%), blubber (2%), heart (1%), brain (1%), thymus (<0.01%) and thyroid (<0.01%). These data suggest large differences in body burden and accumulation pattern of PFCs in marine mammals.