Southern German Bight Research Articles

Risk assessment of war wrecks – a comprehensive approach investigating four wrecks containing munitions in the German Bight/North Sea

AbstractShipwrecks and dumped munition continue to be a major hazard, both in the North Sea but also on a global scale. Research within the EU Interreg project North Sea Wrecks (NSW), in cooperation with the German Aerospace Centre, Institute for the Protection of Maritime Infrastructures (DLR), is generating new insights into the status of wrecks, the potential leakage of pollutants from remaining munitions loads and the effects of contamination on exposed marine organisms in the North Sea environment. Further, historical documents are generated from archives to describe ship's history and sinking scenario. These historical findings were compared to models and images of the visual inspections of the wrecks. Further, samples of water, sediment and organisms are being analysed for traces of explosives. Combining the results of these different fields of research allows for a better understanding of the environmental risks deriving from these wrecks. This process is shown below by focusing on the wreck of the German light cruiser SMS MAINZ, which sank in 1914. Data were compared to three additional wrecks situated also within the southern German Bight. Available data about the wrecks were preliminary assessed using a wreck risk model. Finally, wrecks were ranked according to their potential environmental risk.

RCP8.5-projected changes in German Bight storm surge characteristics from regionalized ensemble simulations for the end of the twenty-first century

This study investigates climate-induced changes in height, frequency and duration of storm surges in the German Bight. The regionally coupled climate model system MPIOM-REMO with a focus on the North Sea has been utilized to dynamically downscale 30 members of the global climate model system MPI-ESM1.1-LR for the historical period 1950–2005 and a continuation until 2099 with the RCP8.5 scenario. Results of all members have been collected into the historical (1970–1999) and the rcp85 (2070–2099) data pools amounting to 900 years of the corresponding climate state. The global mean sea level rise was not considered. Nevertheless, the mean ensemble German Bight SSH trend amounts to about 13 ± 1 cm/century (PI control: 3 cm/century) due to adaptation of the ocean circulation to the changing climatic conditions. Storm surges were defined as SSH above mean high tidal water plus 1.5, 2.5, 3.5 m for “regular”, heavy, extreme storm surges, and then clustered to events. Our simulated storm surge events show a clear location-dependent increase in frequency (6–11%), median duration (4–24%), and average duration (9–20%) in the German Bight. Only along the central German Bight coast (Cuxhaven), longer lasting events gain more relevance. Heavy storm surge events show also a strong increase in frequency (7–34%) and average duration (10–22%). Maximum sea levels during storm events increase strongest and most significant along the northern German Bight and Danish coasts with more than 30 cm/century for the 60-year return period at Hörnum and 10–15 cm/century for shorter return periods. Levels of return periods shorter than a few years significantly increase everywhere along the southern German Bight coasts (around 5 cm/century for the 2-year return period). Highest SSH maxima do not change, and consequently, extreme storm surge events show hardly any response to climate change. Furthermore, our results indicate a shift of seasonality from the last to the first quarter of a year. As the main driver for the encountered alteration of German Bight storm surge characteristics, we identified a change in wind conditions with a pronounced increase of frequency of strong westerly winds.

Detailed Patterns of Methane Distribution in the German Bight

Although methane is a widely studied greenhouse gas, uncertainties remain with respect to the factors controlling its distribution and diffusive flux into the atmosphere, especially in highly dynamic coastal waters. In the southern North Sea, the Elbe and Weser rivers are two major tributaries contributing to the overall methane budget of the southern German Bight. In June 2019, we continuously measured methane and basic hydrographic parameters at a high temporal and spatial resolution (one measurement per minute every 200–300 m) on a transect between Cuxhaven and Helgoland. These measurements revealed that the overall driver of the coastal methane distribution is the dilution of riverine methane-rich water with methane-poor marine water. For both the Elbe and Weser, we determined an input concentration of 40–50 nmol/L compared to only 5 nmol/L in the marine area. Accordingly, we observed a comparatively steady dilution pattern of methane concentration toward the marine realm. Moreover, small-scale anomalous patterns with unexpectedly higher dissolved methane concentrations were discovered at certain sites and times. These patterns were associated with the highly significant correlations of methane with oxygen or turbidity. However, these local anomalies were not consistent over time (days, months). The calculated diffusive methane flux from the water into the atmosphere revealed local values approximately 3.5 times higher than background values (median of 36 and 128 μmol m–2 d–1). We evaluate that this occurred because of a combination of increasing wind speed and increasing methane concentration at those times and locations. Hence, our results demonstrate that improved temporal and spatial resolution of methane measurements can provide a more accurate estimation and, consequently, a more functional understanding of the temporal and spatial dynamics of the coastal methane flux.

The long-term variability of extreme sea levels in the German Bight

Abstract. Extreme high sea levels (ESLs) caused by storm floods constitute a major hazard for coastal regions. We here quantify their long-term variability in the southern German Bight using simulations covering the last 1000 years. To this end, global earth system model simulations from the PMIP3 past1000 project are dynamically scaled down with a regionally coupled climate system model focusing on the North Sea. This approach provides an unprecedented long high-resolution data record that can extend the knowledge of ESL variability based on observations, and allows for the identification of associated large-scale forcing mechanisms in the climate system. While the statistics of simulated ESLs compare well with observations from the tide gauge record at Cuxhaven, we find that simulated ESLs show large variations on interannual to centennial timescales without preferred oscillation periods. As a result of this high internal variability, ESL variations appear to a large extent decoupled from those of the background sea level, and mask any potential signals from solar or volcanic forcing. Comparison with large-scale climate variability shows that periods of high ESL are associated with a sea level pressure dipole between northeastern Scandinavia and the Gulf of Biscay. While this large-scale circulation regime applies to enhanced ESL in the wider region, it differs from the North Atlantic Oscillation pattern that has often been linked to periods of elevated background sea level. The high internal variability with large multidecadal to centennial variations emphasizes the inherent uncertainties related to traditional extreme value estimates based on short data subsets, which fail to account for such long-term variations. We conclude that ESL variations as well as existing estimates of future changes are likely to be dominated by internal variability rather than climate change signals. Thus, larger ensemble simulations will be required to assess future flood risks.

Choosing the lesser evil – A case study on quantification of sediment reworking rates in multi corer type sediment cores

Macrofaunal sediment reworking activity is a key driver of ecosystem functioning in marine systems. So far sediment reworking rates can only accurately be assessed by measurements as inference from community parameters is limited. In this case study we test the applicability of 2-D optical florescent sediment profile imaging (f-SPI) on multi corer type incubation cylinders. f-SPI has to date been applied to flat-surfaced (i.e. rectangular) cores only, while multi corer type incubation cylinders were analyzed by the spatially low resolved and invasive slicing technique. Here we apply both methods to cylindrical sediment cores (10 cm diameter). Cores were taken from by two common communities (i.e. Nucula-community and Amphiura-community) in the southern German Bight. Both f-SPI and the slicing technique showed similar vertical luminophore profiles. However the slicing technique found no significant differences between the two communities, whereas f-SPI showed significant differences for all investigated sediment reworking parameters: sediment reworking rate, non-locality index, mean weighted luminophore depth, and the maximal luminophore depth. Consequently, this may lead to different conclusions about the sediment reworking behaviors of the two communities. Likely the slicing method failed to detect significant differences between the Nucula- and Amphiura-community, owing to insufficient spatial accuracy. The f-SPI method, on the other hand, did not capture the full extent of maximal sediment reworking depth due to wall-effects. We conclude that both methods have specific drawbacks and advantages. While slicing is preferable when focusing on the absolute maximal sediment reworking depth especially with predominantly sessile communities, f-SPI is better suited to capture general sediment reworking patterns of most other communities. We demonstrate further that the bias, which is introduced by the distortion effect on imaging due to optical perspective and cylinder wall curvature of rounded cylinders using f-SPI, is negligible. Accordingly our results indicate that the distortion effects by curvature of the rounded cylinder walls will not cause underestimations of sediment reworking parameters in the f-SPI approach. Consequently f-SPI is suitable for the investigation of sediment reworking in natural communities by means of multi corer type samples.

Mobile demersal megafauna at common offshore wind turbine foundations in the German Bight (North Sea) two years after deployment - increased production rate of Cancer pagurus

Within the next decades the construction of thousands of different types of large wind turbine foundations in the North Sea will substantially increase the amount of habitat available to reef fauna. To gain first insights which effect these substantial changes in habitat structure and diversity might have on faunal stocks settling on hard substrata, we compared the mobile demersal megafauna associated with the common types of wind turbine foundations ('jacket', 'tripod' and 'monopile with scour protections of natural rock') in the southern German Bight, North Sea. Monopiles with scour protection were mostly colonized by typical reef fauna. They were inhabited by an average of about 5000 edible crabs Cancer pagurus (per foundation), which is more than twice as much as found at the foundation types without scour protection. Strong evidence was found that all three foundation types not only function as aggregation sites, but also as nursery grounds for C. pagurus. Assuming equal shares of the three foundation types in future wind farms, we project that about 27% of the local stock of C. pagurus might be produced on site. When, for example, comparing the existing fauna at 1000 ship wrecks and on the autochthonous soft substrate with those which probably will establish at the foundations of 5000 hypothetically realized wind turbines, it becomes clear that the German Bight in the future will provide new artificial reef habitats for another 320% crabs (C. pagurus) and 50% wrasse (Ctenolabrus rupestris) representing substrata-limited mobile demersal hard bottom species. Further research is urgently required in order to evaluate this overspill as it would be an important ecological effect of the recent offshore wind power development.

Long-term change in the copepod community in the southern German Bight

The North Sea has undergone considerable change in recent years, with several reported regime shifts in the last decades, the most recent of which is thought to have occurred in the final years of the last century. As biological evidence corroborating this most recent regime shift is still rare, we investigated the reaction of the copepod community of the Helgoland Roads sampling site to this perceived shift. We observed that the densities of calanoid copepods have declined to values which are roughly 25% of the peak densities in the mid 1980s and link the decrease to the decreasing nutrient inputs into the North Sea. The initial increase in the densities of non-calanoid copepods seems to have reversed, and currently most of the copepods of the community in the southern North Sea are below their long-term average. These strong declines in densities could have major consequences for recruitment of higher trophic levels. We expect a stronger dependence of copepod densities to the larger oceanographic phenomena such as inflows of Atlantic water into the North Sea, as now that the large anthropogenic riverine inputs of nitrogen and phosphorus have decreased and these inflows were the main source of nutrients into the North Sea.

Multi-biomarker approach using the blue mussel (Mytilus edulis L.) to assess the quality of marine environments: Season and habitat-related impacts

Using a comprehensive approach, intertidal, near- and offshore sites in the German Bight were analysed for their environmental quality by assessing the health of blue mussels (Mytilus edulis). During a ten month sampling period mussels were studied with a set of biomarkers comprising lysosomal membrane stability and accumulation of lipofuscin, supplemented by biomarkers indicating nutritional status such as neutral lipids and glycogen in the cells of the digestive gland. Data were analysed in relation to sex, gonadal status, condition index and for the presence of parasites, to determine the overall health status of mussels at the respective sites. Mussels from all sites showed clear signs of stress, indicating an inferior environmental quality throughout the southern German Bight. Further, habitat characteristics such as inundation time and growing on- or off-bottom, as well as seasonal factors, can clearly influence the response of biomarkers in mussels exposed to similar levels of chemical environmental stress.

Phycotoxin composition and distribution in plankton fractions from the German Bight and western Danish coast

From the German Bight along Jutland to the western Skagerrak, we found representatives of almost all groups of phycotoxins known to occur in North Sea plankton. Identification was by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in plankton size fractions, with domoic acid and 20-me G the most abundant toxins. The dominance of 20-me G in the spirolide (SPX) composition of plankton from the Jutland current system matched very well with that of an isolate of the dinoflagellate Alexandrium ostenfeldii. The SPXs of the A. ostenfeldii strain S6_P12_E11, previously isolated from the western North Sea along the Scottish coast, comprised 100% 20-me G, suggesting toxin homogeneity among North Sea populations of this species. We detected highest amounts of azaspiracid-1 in the 3–20-mm size fraction at offshore stations, where the Jutland coastal current converges with the westward North Sea flow off Skagerrak. Azadinium spinosum was subsequently identified by clonal isolation from crude cultures established from these stations. Except for lipophilic toxins usually produced by the dinoflagellate Dinophysis spp., dinophysistoxin-1 (DTX-1) and DTX-2, we detected no other phycotoxins in plankton from the southern German Bight. The spatial distribution of the phycotoxins in the eastern North Sea was apparently related to the hydrographical conditions, identified from salinity and coloured dissolved organic matter profiles. The biogeographical distribution of phycotoxins indicates a strong association with the northward advection by the Jutland current and the mixing of German Bight and North water masses along the northwest Danish coast towards the Skagerrak.

Mobile demersal megafauna at artificial structures in the German Bight – Likely effects of offshore wind farm development

Within the next few decades, large underwater structures of thousands of wind turbines in the northern European shelf seas will substantially increase the amount of habitat available for mobile demersal megafauna. As a first indication of the possible effects of this large scale habitat creation on faunal stocks settling on hard substrata, we compared selected taxa of the mobile demersal megafauna (decapods and fish) associated with the foundation of an offshore research platform (a wind-power foundation equivalent) with those of five shipwrecks and different areas of soft bottoms in the southern German Bight, North Sea. When comparing the amount of approximately 5000 planned wind-power foundations (covering 5.1 × 106 m2 of bottom area) with the existing number of at least 1000 shipwrecks (covering 1.2 × 106 m2 of bottom area), it becomes clear that the southern North Sea will provide about 4.3 times more available artificial hard substratum habitats than currently available. With regard to the fauna found on shipwrecks, on soft substrata and on the investigated wind-power foundation, we predict that the amount of added hard substrata will allow the stocks of substrata-limited mobile demersal hard bottom species to increase by 25–165% in that area. The fauna found at the offshore platform foundations is very similar to that at shipwrecks. Megafauna abundances at the foundations, however, are lower compared to those at the highly fractured wrecks and are irregularly scattered over the foundations. The upper regions of the platform construction (5 and 15 m depth) were only sparsely colonized by mobile fauna, the anchorages, however, more densely. The faunal assemblages from the shipwrecks and the foundations, respectively, as well as from the soft bottoms clearly differed from each other. We predict that new wind-power foundations will support the spread of hard bottom fauna into soft bottom areas with low wreck densities.

BREAKS AND LONG TERM TRENDS OF TIDAL CHARACTERISTICS IN THE SOUTHERN GERMAN BIGHT

The recent influence of climate change and land subsidence on the water levels and tidal characteristics in the German Bight is documented by regional tide gauge observations. However, in any long time series the chance arises that measurement conditions may change over time. Some of these changes occur instantaneously like the change of the sensor or corrections of the zero-point of the gauge. Other changes occur subtly, such as alterations of the regional morphology and therefore the mode of behavior at the measurement site. We present long-term changes of tidal characteristics in the waterways of the southern German Bight, by detecting abrupt breakpoints and resulting homogenized trends. In order to understand processes of the changes of the tidal characteristics significant trends in the time series for measured and calculated tidal parameters are analyzed.

Infestation of oysters and mussels by mytilicolid copepods: differences between natural coastal habitats and two offshore cultivation sites in the German Bight

Summary The aim of this study was to determine the macro-parasitic infestation level of oysters from the southern German Bight focussing on copepods of the genus Mytilicola. Crassostrea gigas, Ostrea edulis and Mytilus edulis were collected at five locations: three nearshore sites in the eastern Wadden Sea and two offshore cultivation sites in the German Bight. To reveal seasonal variations one sampling site was investigated in winter and summer. At the nearshore sites, Mytilicola orientalis was regularly detected in C. gigas. Prevalences ranged between 32.3% and 45.1%, intensity between 3.0 ± 0.6 and 8.2 ± 1.5. Infestation rates of C. gigas within the southern German Bight decreased from west to east: Apparently, M. orientalis has started its range extension along the German coast with gradual retardation eastwards but generally followed the invasion route of its main host, the Pacific oyster. Interestingly, we detected not only M. intestinalis but also M. orientalis as an intestinal parasite in M. edulis, which has sofar not previously been described as host within this region. We conclude that M. orientalis is flexible in its host choice. Furthermore, in the eastern Wadden Sea infestation rates of oysters and mussels by copepods are similar. These results deviate from the patterns observed for the northern Wadden Sea in terms of infestation level and host specificity. No macro-parasites were found in oysters and mussels from the offshore sites. This absence can be considered as potentially beneficial for aquaculture activities in the open ocean in terms of stamina and physiological performance.

Recently established Crassostrea-reefs versus native Mytilus-beds: differences in ecosystem engineering affects the macrofaunal communities (Wadden Sea of Lower Saxony, southern German Bight)

Since 1998 the non-indigenous Pacific oyster Crassostrea gigas (Thunberg 1793) has been invading the Wadden Sea of Lower Saxony, southern German Bight. C. gigas settles predominantly on intertidal Mytilus-beds (M. edulis L.) and subsequently create rigid reef-like structures. Both bivalve species are ecosystem engineers in sedimentary tidal flats. They provide hard substrate for sessile species, mobile organisms find refuge within the habitat matrix of dense suspension feeders, and biodeposits enrich the sediments with organic matter. The transformation of Mytilus-beds into Crassostrea-reefs gives rise to the question whether the invader may affect the native community. We investigated two parts of a changing bivalve bed in the backbarrier area of the island of Juist in March 2005. One part was still dominated by M. edulis whereas the other part was already densely colonized by C. gigas. Crassostrea-reefs compensate for the conceivable loss of Mytilus-beds in the intertidal of the Wadden Sea by replacing the ecological function of M. edulis. There was no indication of a suppression of indigenous species. This even applied to M. edulis, which persisted at the site invaded by C. gigas. The associated macrofaunal community showed increased species richness, abundance, biomass, and diversity in the Crassostrea-reef. The latter particularly favored sessile species like anthozoans, hydrozoans, and barnacles. Higher abundance and biomass for vagile epizoic species like the shore crab Carcinus maenas and the periwinkle Littorina littorea also occurred among oysters. Abundance of deposit feeding oligochaetes was enhanced by oysters as well. More opportunistic, facultative filter-feeding polychaetes occurred in the Crassostrea-reef.

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.

Processing of 13C-labelled phytoplankton in a fine-grained sandy-shelf sediment (North Sea): relative importance of different macrofauna species

On-board and in situ experiments with 13 C-labelled diatoms were carried out to investigate the processing of algal carbon by the macrofauna community of a fine sandy-shelf site in the southern German Bight (North Sea). The time series (12, 30, 32 and 132 h incubations) was supplemented by additional laboratory experiments on the role of the dominant macrofauna organism, the bivalve Fabulina fabula (Bivalvia: Tellinidae), for particulate organic matter subduction to deeper sediment layers. The specific uptake of algal 13 C by macrofauna organisms was visible after 12 h and constantly increased during the incubation periods. F. fabula, a facultative (surface) deposit- and suspension-feeder, Lanice conchilega (Polychaeta: Terebellidae), a suspension-feeder and the (surface) deposit-feeder Echinocardium cordatum (Echinodermata: Spatangidae) were responsible for the majority of macrofaunal carbon processing. Predatory macrofauna organisms like Nephtys spp. (Polychaeta: Nephtyidae) also quickly became labelled. The rapid subduction of fresh organic matter by F. fabula down to ca. 4 to 7 cm sediment depth could be demonstrated, and it is suggested that entrainment by macrofauna in this fine-grained sand is much more efficient than advective transport.

Decomposition of diatoms and nutrient dynamics in permeable North Sea sediments

This study addresses the decomposition of diatoms in different permeable North Sea sand beds. During three cruises in 2001 to the southern German Bight, the regeneration of nutrients was assessed after the experimental deposition of organic matter corresponding to a typical spring diatom bloom in in situ and on-board chamber experiments. The diatom pulse was followed by a high regeneration of nutrients during the first day: 5–10% d −1 of the added nitrogen was converted to NH 4 + and up to 0.67% d −1 of the added biogenic silica was dissolved to Si(OH) 4. These results are used to interpret the response of pore water nutrient concentrations in permeable North Sea sands to seasonal nutrient and phytoplankton dynamics in the water column. The rapid advective solute exchange in these permeable sediments reduces the accumulation of regenerated nutrients, and, thus pore water concentrations of Si(OH) 4, PO 4 3− and NH 4 + decreased with increasing permeability. All sands were characterized by relatively high NO 3 − concentrations down to 10 cm sediment depth, indicating that the upper sediment layers are oxidized by advective flushing of the bed. Our results demonstrate that biogenic silica and organic matter are rapidly degraded in permeable coastal sands, revealing that these sediments are very active sites of nutrient recycling.

Seasonal variability of epibenthic communities in different areas of the southern North Sea

AbstractBetween November 2000 and May 2002, epibenthos was sampled monthly with a 2-m beam trawl at three stations along a transect from the southern German Bight towards the northeastern part of the Dogger Bank (North Sea) in order to investigate the seasonal variability of the epibenthic communities. The stations were chosen to reflect a gradient in the hydrigraphic regime, organic matter supply, and fishing effort. The epibenthic community of the southern German Bight was characterized by high biomass and abundance, dominated by Asterias rubens and Ophiura albida. In contrast, at the northern stations in the Oyster Ground and at the Dogger Bank, epibenthic biomass and abundance were substantially lower and the dominant species were mainly crustaceans such as Corystes cassivelaunus, Liocarcinus holsatus, and Pagurus bernhardus. In terms of seasonal variability, mean abundance and biomass in the southern German Bight showed highest values in the summer months and lowest values in the winter months. A similar pattern, but less distinct, was observed in the Oyster Ground. But at the Dogger Bank the pattern was different, with highest abundance and biomass values in the winter months. The differences in spatial and temporal patterns are discussed in relation to differences in temperature, thermal stratification and fishing effort at the three study sites.

Effect of advective pore water transport on distribution and degradation of diatoms in permeable North Sea sediments

This contribution addresses the incorporation and degradation of diatoms in coastal fine, medium and coarse North Sea sands. During 3 cruises in 2001 to a highly dynamic, non-depositional area in the southern German Bight, the transport of 13 C-labeled diatoms into these different permeable sand beds was assessed by in situ and on-board chamber experiments. Enhanced advective transport of diatom frustules and 13 C-enriched diatom carbon into sandy sediments with increasing permeability was demonstrated. Highest transport rates were observed in medium and coarse sand, where 6 % of the added algae were found below 1 cm after 20 h incubation. In the coarse sand, the high ratio between sand grain and particle size enhanced the delivery of algae to the sediment, but seemed to reduce the filtration efficiency and thus algal retention. Broken frustules of Thalassiosira sp., the diatom which dominated the diatom spring bloom in 2001, were found in the medium and coarse sand in autumn. This indicates that advective transport and, to some limited extent, bioturbation, deposits phytoplankton into these sandy sediments, where strong bottom currents theoretically would prevent the sedimentation of low-density organic material. The trapped cells are rapidly degraded, as observed in our chamber experiments, where 28 % of the added diatom carbon was released as dissolved organic carbon (DOC) per day after the third incubation day. We conclude that permeable sediments represent expansive coastal filter systems, where high advective flushing rates boost remineralization of trapped algal cells. These processes promote a fast recycling of organic matter and, thus, may be important for maintaining high primary production rates in shelf environments.

Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin

Sediments (568) and suspended particulate matter (SPM, 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (Al, Fe, K), trace metals (Mn, Pb), and 206Pb/ 207Pb ratios using XRF, ICP–OES, ICP–MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected) <recent tidal flat sediments (RTF) <Helgoland Island mud hole sediments (MH) <nearshore SPM (SPM concentration>5 mg l −1) < offhore SPM (<5 mg l −1). Besides pollution, RTF contain elevated amounts of natural Pb-rich materials (K-feldspars and heavy minerals) due to a man-made high-energy environment (dike building) in comparison to HTF. 206Pb/ 207Pb ratios of RTF (1.192±0.019) are similar to the local geogenic background, determined from HTF (1.207±0.008). In contrast, Pb isotope ratios of nearshore SPM (1.172±0.007) and offshore SPM (1.166±0.012) show a distinct shift towards the anthropogenic/atmospheric signal of 1.11–1.14. This difference between RTF and SPM supports the assumption of low deposition rates of fine material in the intertidal systems. As the 206Pb/ 207Pb ratios of SPM do not reach the pure anthropogenic signal, the adsorbed Pb fraction was examined (leaching). However, the leachates also contained large amounts of geogenic Pb (SPM ≈40%, recent sediments ≈60%). The authors assume that the uptake of natural Pb occurs in nearshore waters, presumably in the turbid intertidal systems. Possible sources for dissolved Pb are mobilisation during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input. Comparatively small parts of SPM leave the coastal water mass and reach the open North Sea. This process therefore leads to a decontamination of the tidal flat sediments. Due to more pronounced atmospheric input, the offshore SPM becomes enriched in anthropogenic Pb as indicated by decreasing 206Pb/ 207Pb ratios with increasing distance from the coast.

Temporal and spatial variability of bacterial abundance and production in the German Wadden Sea

The bacterial abundance at Busum (northern German Wadden Sea) in 1995 varied between 1.2 and 66 x 105 ml-1. Bacterial production fluctuated between 3.9 and 95 Μg C l-1 d-1. The development of bacterial blooms in spring and summer was reflected rather in bacterial numbers than in bacterial production, although the latter showed stronger seasonal variations. This points to seasonally changing turnover rates of bacterial biomass. The dynamics of cell numbers and productivity rates were dominated by bacteria attached to particles. This temporal variability was compared with the spatial variance observed at 152 stations in the southern German Bight in January, May and August 1995. Cell numbers varied between 0.5 and 102 x 105 ml-1, and production between 0.5 and 74 Μg C I“1 d-1. The data suggest faster turnover of bacterial biomass in the inner tidal basins than in offshore areas. The temporal variability observed at Busum was smaller than the spatial variability in the German Bight. Thus, several time-series stations are needed to monitor bacterial dynamics in the whole area.