Southern Bight Research Articles

An idealized model of tidal dynamics in the North Sea: resonance properties and response to large-scale changes

An idealized process-based model is developed to investigate tidal dynamics in the North Sea. The model geometry consists of a sequence of different rectangular compartments of uniform depth, thus, accounting for width and depth variations in a stepwise manner. This schematization allows for a quick and transparent solution procedure. The solution, forced by incoming Kelvin waves at the open boundaries and satisfying the linear shallow water equations on the f plane with bottom friction, is in each compartment written as a superposition of eigenmodes, i.e. Kelvin and Poincaré waves. A collocation method is employed to satisfy boundary and matching conditions. First, the general resonance properties of a strongly simplified geometry with two compartments, representing the Northern North Sea and Southern Bight, are studied. Varying the forcing frequency while neglecting bottom friction reveals Kelvin and Poincaré resonance. These resonances continue to exist (but with lower amplification and a modified spatial structure) when adding the Dover Strait as a third compartment and separating the solutions due to forcing from either the north or the south only. Including bottom friction dampens the peaks. Next, comparison with tide observations along the North Sea coast shows remarkable agreement for both semi-diurnal and diurnal tides. This result is achieved with a more detailed geometry consisting of 12 compartments fitted to the coastline of the North Sea. Further simulations emphasize the importance of Dover Strait and bottom friction. Finally, it is found that a sea level rise of 1 m, uniformly applied to the entire North Sea, amplifies the M2-elevation amplitudes almost everywhere along the coast, with an increase of up to 8 cm in Dover Strait. Bed level changes of ±1 m, uniformly applied to the Southern Bight only, imply weaker changes, with changes in coastal M2-elevation amplitudes below 5 cm.

Assessment of benthic ecosystem functioning through trophic web modelling: the example of the eastern basin of the English Channel and the Southern Bight of the North Sea

AbstractBenthic organisms appear to be accurate proxies for assessing coastal ecosystem structures and changes due to climatic and anthropogenic stresses. Functional studies of benthic systems are relatively recent, mainly because of the difficulties in obtaining the basic parameters for each benthic compartment (i.e. detritus, bacteria, meiofauna and macrofauna). Our study focuses on the eastern basin of the English Channel and the Southern Bight of the North Sea. Trophic web modelling was used to assess the functioning of the three main benthic community assemblages. To test and assess the relative importance of factors assumed to influence trophic structure (geographical environment and sedimentary particle size distribution), the study area was subdivided into divisions defined a priori according to the two main structural factors of community distribution; geographic distribution and sedimentary patterns. Then, a steady state trophic model utilising the inverse method was applied to a diagram composed of eight compartments, including detritus, bacteria, meiofauna, macrobenthos and fish. For each compartment, six physiological parameters were assessed, based on our own data, empirical relationships and literature data. This method allowed estimation of the flux of matter and energy within and between the units of the benthic system and assessment of the amount of trophic energy stored in these units (available mostly to fish). Our results showed that suspension‐feeders control most of the matter transfer through the macrobenthic food‐web, except in the fine sand community, where deposit‐feeders play a dominant role. The results also showed that, whatever the geographic area, trophic structure is strongly linked to the sedimentary conditions. As benthic communities are connected through hydrodynamics, a model of the entire eastern basin of the English Channel would appear to be acceptable. However, the main sediment types must be taken into account when establishing relationships between the functional units.

Mechanisms controlling the intra-annual mesoscale variability of SST and SPM in the southern North Sea

Thermal and optical remote sensing data were used to investigate the spatial and temporal distribution of sea surface temperature (SST) and of suspended particulate matter (SPM) in the southern North Sea. Monthly SST composites showed pronounced seasonal warming of the southern North Sea and delineated the English coastal and continental coastal waters. The East-Anglia Plume is the dominant feature of the English coastal waters in the winter and autumn SPM composites, and the Rhine region of freshwater influence (ROFI), including the Flemish Banks, is the dominant feature of the continental waters. These mesoscale spatial structures are also influenced by the evolution of fronts, such as the seasonal front separating well-mixed water in the southern Bight, from the seasonally stratified central North Sea waters. A harmonic analysis of the SST and SPM images showed pronounced seasonal variability, as well as spring–neap variations in the level of tidal mixing in the East Anglia Plume, the Rhine ROFI and central North Sea. The harmonic analysis indicates the important role played by the local meteorology and tides in governing the SST and near-surface SPM concentrations in the southern North Sea. In the summer, thermal stratification affects the visibility of SPM to satellite sensors in the waters to the north of the Flamborough and Frisian Fronts. Haline stratification plays an important role in the visibility of SPM in the Rhine ROFI throughout the year. When stratified, both regions typically exhibit low surface SPM values. A numerical model study, together with the harmonic analysis, highlights the importance of tides and waves in controlling the stratification in the southern North Sea and hence the visibility of SPM.

Null models reveal preferential sampling, spatial autocorrelation and overfitting in habitat suitability modelling

Nowadays, species are driven to extinction at a high rate. To reduce this rate it is important to delineate suitable habitats for these species in such a way that these areas can be suggested as conservation areas. The use of habitat suitability models (HSMs) can be of great importance for the delineation of such areas. In this study MaxEnt, a presence-only modelling technique, is used to develop HSMs for 223 nematode species of the Southern Bight of the North Sea. However, it is essential that these models are beyond discussion and they should be checked for potential errors. In this study we focused on two categories (1) errors which can be attributed to the database such as preferential sampling and spatial autocorrelation and (2) errors induced by the modelling technique such as overfitting, In order to quantify these adverse effects thousands of nulls models were created. The effect of preferential sampling (i.e. some areas where visited more frequenty than others) was investigated by comparing model outcomes based from null models sampling the actual sampling stations and null models sampling the entire mapping area ( Raes and ter Steege, 2007). Overfitting is exposed by a fivefold cross-validation and the influence of spatial autocorrelation is assessed by separating test and training sets in space. Our results clearly show that all these effects are present: preferential sampling has a strong effect on the selection of non-random species models. Crossvalidation seems to have less influence on the model selection and spatial autocorrelation is also strongly present. It is clear from this study that predefined thresholds are not readily applicable to all datasets and additional tests are needed in model selection.

Seasonal and inter-annual variability of air–sea CO 2 fluxes and seawater carbonate chemistry in the Southern North Sea

A 3D coupled biogeochemical–hydrodynamic model (MIRO-CO 2&CO) is implemented in the English Channel (ECH) and the Southern Bight of the North Sea (SBNS) to estimate the present-day spatio-temporal distribution of air–sea CO 2 fluxes, surface water partial pressure of CO 2 (pCO 2) and other components of the carbonate system (pH, saturation state of calcite ( Ω ca) and of aragonite ( Ω ar)), and the main drivers of their variability. Over the 1994–2004 period, air–sea CO 2 fluxes show significant inter-annual variability, with oscillations between net annual CO 2 sinks and sources. The inter-annual variability of air–sea CO 2 fluxes simulated in the SBNS is controlled primarily by river loads and changes of biological activities (net autotrophy in spring and early summer, and net heterotrophy in winter and autumn), while in areas less influenced by river inputs such as the ECH, the inter-annual variations of air–sea CO 2 fluxes are mainly due to changes in sea surface temperature and in near-surface wind strength and direction. In the ECH, the decrease of pH, of Ω ca and of Ω ar follows the one expected from the increase of atmospheric CO 2 (ocean acidification), but the decrease of these quantities in the SBNS during the considered time period is faster than the one expected from ocean acidification alone. This seems to be related to a general pattern of decreasing nutrient river loads and net ecosystem production (NEP) in the SBNS. Annually, the combined effect of carbon and nutrient loads leads to an increase of the sink of CO 2 in the ECH and the SBNS, but the impact of the river loads varies spatially and is stronger in river plumes and nearshore waters than in offshore waters. The impact of organic and inorganic carbon (C) inputs is mainly confined to the coast and generates a source of CO 2 to the atmosphere and low pH, of Ω ca and of Ω ar values in estuarine plumes, while the impact of nutrient loads, highest than the effect of C inputs in coastal nearshore waters, also propagates offshore and, by stimulating primary production, drives a sink of atmospheric CO 2 and higher values of pH, of Ω ca and of Ω ar.

Sediment-related distribution patterns of nematodes and macrofauna: Two sides of the benthic coin?

We investigated the sediment-related distribution of both nematodes and macrofauna on the Belgian part of the North Sea (Southern Bight of the North Sea) in order to evaluate whether both faunal groups reflect similar patterns in community composition and diversity. Fine-grained sediments (median grain size <200 μm) were inhabited by nematode communities characterised by a low diversity and dominated by non-selective deposit-feeding nematodes. Nematode communities from coarser sediments were significantly different in terms of community composition and diversity. Moreover, all nematode feeding types were present in coarser sediments. These differences were explained by the contrasting biogeochemical processes prevailing in both sediment types, rather than granulometry and food availability per se. Macrofaunal distribution patterns were different from those of the nematode communities and seem to be related to water column processes (SPM loading, food availability, hydrodynamic stress) that promote the establishment of diverse communities in the coarser sediments but not in the finest sediments. This suggests that data on nematodes and macrofauna reveal different, complementary aspects of the factors structuring the benthic ecosystem that can be of importance in assessing the ecological status of the seafloor.

Analysis of the spatial evolution of the 2003 algal bloom in the Voordelta (North Sea)

Phaeocystis blooms in the Southern Bight of the North Sea may cause damage to the aquatic ecosystem and to commercial mussel cultures at the entrance of the Oosterschelde estuary. In this paper the potential for early detection of Phaeocystis blooms in Dutch coastal waters is studied, using a combination of field data, satellite observations and hydrodynamic- and biological modelling. For the spring bloom period in the year 2003 MERIS chlorophyll-a maps, derived with the HYDROPT algorithm for coastal waters, were compared to in-situ measurements at stations off the coast in de Voordelta and the results of the GEM biogeochemical model. The analysis shows that the spatial and temporal variability in Phaeocystis abundance and total biomass (expressed by the Chl-a concentration) during spring is large. It is confirmed that blooms may develop off shore and show a tendency to accumulate within 10 km from the coastline, giving rise to rapid biomass accumulation at the mouth of the multiple estuaries in the Voordelta. Based on the outcome of this study an early warning system is proposed that notifies local water managers and shellfish growers for potentially harmful Phaeocystis bloom formation.

Mapping nematode diversity in the Southern Bight of the North Sea

In order to protect the biodiversity of the seas from, e.g., overexploitation, the spatial distribution of biodiversity and the mapping of biodiversity hotspots are of great importance. In the present paper we discuss different methods to develop full coverage biodiversity maps of free-living marine nematodes in the Southern Bight of the North Sea. A database with sampling data, gathered over 3 decades (1972 to 2004), combined with exhaustive environmental data, was employed to predict species richness and the expected number of species by different methods: ordinary kriging (OK) and regression kriging (RK) with ordinary least squares (OLS) and generalised least squares (GLS). The predictive value of these methods was evaluated by an independent validation set. Replicate samples were used to make an accurate estimation of the nugget variance, since replicates reveal local variability. Accordingly, it was feasible to find a spatial pattern in the residuals of the regression models. Our analysis pointed out that GLS improved the OK models substantially, while RK only slightly improved the GLS model. The diversity of marine nematodes is substantially influenced by the silt-clay fraction and the amount of total suspended matter, which is also reflected in the resulting map with a species-poor area near the coast line, especially near the south of the mouth of the Scheldt estuary. Off coast diversity and evenness are generally higher.

Microchemical variation in juvenile Solea solea otoliths as a powerful tool for studying connectivity in the North Sea

Estimating connectivity between juvenile and adult fish habitats can provide an important contribution to effective fisheries management, through a better understanding of population resilience to harvesting pressure. Indirect methods for quantifying connectivity, such as geochemical or genetic techniques, allow us to assign adults from various sampling regions to their natal location, provided that natal origin data can be defined. The elemental composition of otoliths from juvenile sole Solea solea collected at 4 sampling locations in the Southern Bight of the North Sea was measured using laser-abla- tion inductively-coupled plasma mass spectrometry (LA-ICPMS), to determine elemental fingerprints indicative of distinct nursery grounds. Significant differences in elemental composition were detected among the 4 locations, with Na, Sr, Ba, Mn and Rb concentrations varying the most between groups. A discriminant model resulted in high assignment proportions of the juvenile fish to their respective nursery grounds with a total jackknife reclassification success of 88%. Even though some interannual variability in otolith chemistry was observed in juveniles from the Scheldt estuary, spatial patterns seemed to dominate. Our results constitute a firm basis for future investigations on nursery area contri- butions and quality, adult dispersal history and applications of population traceability.

Interannual variability of alongshore spring bloom dynamics in a coastal sea caused by the differential influence of hydrodynamics and light climate

Abstract. Timing and spatial distribution of phytoplankton blooms in coastal oceans are highly variable. The interactions of various biological and physical factors leading to the observed variability are complex and remain poorly understood. We present an example for distinct differences in the spatio-temporal chlorophyll a (CHL-a) distribution on an interannual scale, integrating high-frequency data from an autonomous measuring device (FerryBox), which operated on an alongshore route in the coastal German Bight (North Sea). While in one year the distribution of CHL-a was spatially homogeneous (2004), a bloom only developed in one part of the transect in the following spring period (2005). We use a one-dimensional Lagrangian particle tracking model, which operates along the mean current direction, combined with a NPZ-model to identify the mechanisms controlling the observed interannual bloom variability on the alongshore transect. Our results clearly indicate that in 2004 the local light climate determined the spatial and temporal dynamics of the spring bloom. In contrast, the import of a water mass with elevated CHL-a concentrations from the adjacent Southern Bight triggered the spring bloom in 2005. The inflow event did, however, not last long enough to spread the bloom into the eastern part of the study area, where high turbidity prevented local phytoplankton growth. The model identifies two interacting mechanisms, light climate and hydrodynamics, that controlled the alongshore dynamics. Especially the occurrence of a pronounced spring bloom despite unfavourable light conditions in 2005 underlines the need to carefully consider hydrodynamics to understand the dynamics of the plankton community in coastal environments.

Role of macrofauna functional traits and density in biogeochemical fluxes and bioturbation

The importance of different functional traits of macrobenthos in benthic processes of the Southern Bight of the North Sea was investigated to estimate the effects of density declines and species loss on benthic ecosystem functioning. Two laboratory experiments were performed: before (winter, temperature = 10°C) and after (summer, temperature = 18°C) sedimentation of the spring phytoplankton bloom. Single species treatments of key species (Abra alba, Lanice conchilega and Nephtys sp.) with different functional traits were added to microcosms at 3 density levels (natural, lower, lowest) to account for possible density declines. Sediment-water exchanges of oxygen and nutrients, denitrification and bioturbation were measured. In absence of fauna, benthic minerali- sation in the summer experiment was 2.0 times higher than in winter. Fauna stimulated microbial respiration more in summer (up to 100% in L. conchilega treatments) than in winter (negligible fauna effect). As chlorophyll a concentrations were similar in both seasons, the stronger fluxes in summer must be explained by a higher macrobenthic activity owing to the elevated temperature and better condition of the animals. Stimulation of mineralisation by the 3 species in the microcosms was different, and behaviour-related. Owing to its irrigation activity, the tube dweller L. conchilega had more pronounced influences on benthic respiration, nutrient release and denitrification than did the biodiffusers, A. alba and Nephtys sp. A. alba appeared to be a more effective bioturbator than Nephtys sp. Processes such as benthic respiration, nutrient fluxes, denitrification and bioturbation seem to be related to animal densities and therefore decreases in densities can possibly have impli- cations for ecosystem functioning.

Assessing the effect of nutrient mitigation measures in the watersheds of the Southern Bight of the North Sea

The Seine, Somme, and Scheldt Rivers (France, Belgium, and Netherlands) are the major delivering rivers flowing into the continental coastal zone of the Southern Bight of the North Sea, an area regularly affected by eutrophication problems. In the present work, the Seneque–Riverstrahler model was implemented in a multi-regional case study in order to test several planned mitigation measures aimed at limiting stream nutrient contamination and restoring balanced nutrient ratios at the coastal zone. This modeling approach, which is spatially distributed at the basin scale, allows assessing the impact of any change in human activities, which widely differ over the three basins. Here, we define realistic scenarios based on currently proposed measures to reduce point and non-point sources, such as the upgrading of wastewater treatment, the introduction of catch crops, and the development of extensive farming. An analysis of the current situation showed that a 47–72% reduction in P point-source emissions within the three basins could be reached if the intended P treatment was generalized to the largest treatment plants. However, only an overall 14–23% reduction in N could be achieved at the outlet of the three basins, by combining improved wastewater treatment and land use with management measures aimed at regulating agricultural practices. Nonetheless, in spite of these efforts, N will still be exported in large excess with respect to the equilibrium defined by the Redfield ratios, even in the most optimistic hypothesis describing the long-term response of groundwater nitrate concentrations. A comprehensive assessment of these mitigation measures supports the need for additional reductions of nutrient losses from agriculture to control harmful algae development. It also stresses the relevance of this mechanistic approach, in which nutrient transfers from land to sea can be calculated, as an integrated strategy to test policy recommendations.

Horizontally viscous effects in a tidal basin: extending Taylor's problem

The classical problem of Taylor (Proc. Lond. Math. Soc., vol. 20, 1921, pp. 148–181) of Kelvin wave reflection in a semi-enclosed rectangular basin of uniform depth is extended to account for horizontally viscous effects. To this end, we add horizontally viscous terms to the hydrodynamic model (linearized depth-averaged shallow-water equations on a rotating plane, including bottom friction) and introduce a no-slip condition at the closed boundaries.In a straight channel of infinite length, we obtain three types of wave solutions (normal modes). The first two wave types are viscous Kelvin and Poincaré modes. Compared to their inviscid counterparts, they display longitudinal boundary layers and a slight decrease in the characteristic length scales (wavelength or along-channel decay distance). For each viscous Poincaré mode, we additionally find a new mode with a nearly similar lateral structure. This third type, entirely due to viscous effects, represents evanescent waves with an along-channel decay distance bounded by the boundary-layer thickness.The solution to the viscous Taylor problem is then written as a superposition of these normal modes: an incoming Kelvin wave and a truncated sum of reflected modes. To satisfy no slip at the lateral boundary, we apply a Galerkin method. The solution displays boundary layers, the lateral one at the basin's closed end being created by the (new) modes of the third type. Amphidromic points, in the inviscid and frictionless case located on the centreline of the basin, are now found on a line making a small angle to the longitudinal direction. Using parameter values representative for the Southern Bight of the North Sea, we finally compare the modelled and observed tide propagation in this basin.

A reanalysis of North Sea plaice spawning-stock biomass using the annual egg production method

Abstract van Damme, C. J. G., Bolle, L. J., Fox, C. J., Fossum, P., Kraus, G., Munk, P., Rohlf, N., Witthames, P. R., and Dickey-Collas, M. 2009. A reanalysis of North Sea plaice spawning-stock biomass using the annual egg production method. – ICES Journal of Marine Science, 66: 1999–2011. Uncertainty about the quality of current virtual population analysis-based stock assessment for North Sea plaice (Pleuronectes platessa) has led to various abundance indices. We compared biomass estimates from the annual egg production (AEP) method with current stock assessments based on catch-at-age to validate the current and historical perception of exploitation. The AEP method was also used to investigate the dynamics of the spatial components of plaice in the North Sea. We corrected for fecundity down-regulation and changes in sex ratio. Estimates from both methods were similar in trend and absolute biomass. On the Dogger Bank, there was a dramatic decline in biomass from 1948 and 1950 to 2004, and in the Southern Bight, the stock appeared to increase from 1987 and 1988 to 2004, although not reaching the historically high levels of 1948 or 1950. The timing of spawning of North Sea plaice does not appear to have changed throughout the period of high exploitation. We conclude that the AEP method is a useful way to hindcast the spatial dynamics of heavily exploited flatfish stocks.

Aminostratigraphy of Middle and Late Pleistocene deposits in The Netherlands and the southern part of the North Sea Basin

A review of all available amino acid racemization D (alloisoleucine)/L (isoleucine) data from the whole shell of four molluscan species from Late and late Middle Pleistocene deposits of the Netherlands is presented. The data allow the distinction of 5 aminostratigraphical units, NAZ (Netherlands Amino Zone) A–E, each representing a temperate stage. The zones are correlated with marine isotope stages 1, 5e, 7, 9, and 11 respectively. Apart from NAZ-D (MIS 9), in all aminozones the marine transgression reached the present-day onshore area of the Netherlands. The transgression during NAZ-C (Oostermeer Interglacial: MIS 7) seems to be at least as widespread as its counterpart during NAZ-B (Eemian: MIS 5e) in the southern bight of the North Sea Basin. The stratigraphic position of the Oostermeer Interglacial is just below deposits of the Drente phase of the Saalian and because of this position the interglacial marine deposits have formerly erroneously considered to be of Holsteinian age. Neede, the ‘classic’ Dutch Holsteinian site, is dated in NAZ-E (MIS 11), like Noordbergum. Although the validity of these zones has been checked with independent data, some overlap between succeeding zones may occur. The relation between amino acid data from elsewhere in the North Sea Basin and the Netherlands amino zonation is discussed. The deposits at the Holsteinian stratotype Hummelsbüttel in North West Germany are dated in NAZ-D. This interglacial correlates with MIS 9. The Belvédère Interglacial, which is of importance for its archaeology, is in NAZ-D (MIS 9) and therefore of Holsteinian age as well. The lacustroglacial ‘pottery clays’ in the Noordbergum area are deposits from two glacial stages, which can be correlated with MIS 8 and 10 (the Elsterian). The pottery clay that is considered equivalent to the German ‘Lauenburger Ton’ correlates with MIS 10.

Lack of population genetic structuring in the marine planktonic diatom Pseudo-nitzschia pungens (Bacillariophyceae) in a heterogeneous area in the Southern Bight of the North Sea

Several marine holoplanktonic organisms show a high degree of geographically structured diversity for which it often remains unclear to what extent this differentiation is due to the presence of cryptic taxa. For the genetically distinct diatom Pseudo-nitzschia pungens var. pungens, we used six microsatellite markers to investigate the spatial and temporal genetic composition in the heterogeneous Southern Bight of the North Sea. Although our sampling area (ca. 100 km) comprised water bodies with different environmental conditions (marine, estuary, saline lake) and different degrees of connectivity (from complete isolation to supposedly free exchange between environments), no evidence of genetic differentiation was found. Expanding our sampling area (ca. 650 km), suggested a homogenous population structure over even larger areas in the North Sea. Our results suggest that the population structure of this diatom is mainly shaped by strong homogenizing effects of gene flow preventing genetic drift, even in water bodies with limited connectivity.

Nutrient transfer in three contrasting NW European watersheds: The Seine, Somme, and Scheldt Rivers. A comparative application of the Seneque/Riverstrahler model

An understanding of the ecological functioning of an aquatic continuum on a multi-regional scale relies on the ability to collect suitable descriptive information. Here, the deterministic Seneque/Riverstrahler model, linking biogeochemistry with the constraints set by geomorphology and anthropogenic activities, was fully implemented to study the Seine, Somme, and Scheldt Rivers. Reasonable agreement was found between calculated and observed nutrient fluxes for both seasonal and inter-annual variations along the networks. Nutrient budgets underline: i) a clear partition of diffuse and point sources with respect to the specific activities of the watersheds, ii) the importance of riparian retention, responsible for 25–50% of nitrogen retention, iii) the role played by benthic processes, resulting in the retention of up to 45% of the phosphorus and 35% of the silica entering the river systems. Nutrient ratios confirmed that fluxes to the Eastern Southern Bight of the North Sea are imbalanced, supporting the potential for undesirable algal blooms.

EPXMA survey of shelf sediments (Southern Bight, North Sea): A glance beyond the XRD-invisible

Shelf sediments of the southern North Sea, were studied with a microanalytical [electron probe X-ray microanalysis (EPXMA)] and two bulk [X-ray diffraction (XRD) and X-ray fluorescence (XRF)] techniques. The investigation proved that the promptness of the microanalytical method is combined with a reasonable analytical reliability. XRD studies of such a type of sediments with monotonous mineral composition are not able to provide mineralogical information beyond the main well-crystalline minerals and the mineralogical quantitative characteristic of the sediment based on XRD estimations are incorrect. The EPXMA mineralogical interpretations are based on the statistical evaluation of a huge data set (thousands of mineral particles) and provide a rather correct quantitative determination of the main minerals. The comparative EPXMA–XRF study revealed that the Al, Si, K, Ca, Fe and to some extent Ti contents estimated by EPXMA are fairly reliable. In this respect the accuracy of the EPXMA-based mineral identification of the pure silicates, pure aluminosilicates, and Al-, Ca-, Fe- and Ti-containing minerals with simple composition is very high. Mg-calcite, augite and apatite determinations are assessed to be correct. The supposed accuracy of the clay mineral determinations is slightly lower (70–80%) than that of the other main minerals due to the complex and varying composition of the clays. The identification of XRD-invisible accessory minerals and quantification of their presence in the sediments is an essential advantage of the EPXMA, which makes it a useful approach in tracing the origin of the sediments, the pathways of their transport and the geochemical processes they have undergone. However, the EPXMA has several flaws, which need to be solved in the future sediment investigations: (1) calibration with natural standards is needed in order to provide a higher accuracy of the mineral determinations; (2) any EPXMA study of sediments needs to be secured with XRF examinations of selected samples since EPXMA gives only semi-quantitative information about the abundance of the elements; (3) ultra-thin window EPXMA of low- Z elements has to be used since some of them (O, C) are always present in the main sediment components: silicates, aluminosilicates, carbonates and metal oxyhydroxides; (4) the interpretations of the clay fraction have to be supported with detailed XRD investigations of selected samples, while the mineralogy of the silt and sand fractions needs to be backed up with optical microscopy studies. The information from different analytical techniques (EPXMA with XRF–XRD-optical microscopy of selected samples) combined with the knowledge about the most possible minerals in a given environment, would give the most reliable results in studying mineralogical composition of shelf sediments.

Response of the Rhine–Meuse fluvial system to Saalian ice‐sheet dynamics

A new reconstruction of the interaction between the Saalian Drente glaciation ice margin and the Rhine–Meuse fluvial system is presented based on a sedimentary analysis of continuous core material, archived data and a section in an ice‐pushed ridge. Optically Stimulated Luminescence (OSL) was applied to obtain independent age control on these sediments and to establish a first absolute chronology for palaeogeographical events prior to and during the glaciation. We identified several Rhine and Meuse river courses that were active before the Drente glaciation (MIS 11‐7). The Drente glaciation ice advance into The Netherlands (OSL‐dated to fall within MIS 6) led to major re‐arrangement of this drainage network. The invading ice sheet overrode existing fluvial morphology and forced the Rhine–Meuse system into a proglacial position. During deglaciation, the Rhine shifted into a basin in the formerly glaciated area, while the Meuse remained south of the former ice limit, a configuration that persisted throughout most of the Eemian and Weichselian periods. An enigmatic high position of proglacial fluvial units and their subsequent dissection during deglaciation by the Meuse may partially be explained by glacio‐isostatic rebound of the area, but primarily reflects a phase of high base level related to a temporary proglacial lake in the southern North Sea area, with lake levels approximating modern sea levels. Our reconstruction indicates that full ‘opening’ of the Dover Strait and lowering of the Southern Bight, enabling interglacial marine exchange between the English Channel and the North Sea, is to be attributed to events during the end of MIS 6.

Phylogeography of the Rhabditis (Pellioditis) marina species complex: evidence for long‐distance dispersal, and for range expansions and restricted gene flow in the northeast Atlantic

Pinpointing processes that structure the geographical distribution of genetic diversity of marine species and lead to speciation is challenging because of the lack of obvious dispersal barriers and the likelihood of substantial (passive) dispersal in oceans. In addition, cryptic radiations with sympatric distributions abound in marine species, challenging the allopatric speciation mechanism. Here, we present a phylogeographical study of the marine nematode species complex Rhabditis (Pellioditis) marina to investigate processes shaping genetic structure and speciation. Rhabditis (P.) marina lives on decaying macroalgae in the intertidal, and may therefore disperse over considerable distances. Rhabditis (P.) marina consists of several cryptic species sympatrically distributed at a local scale. Genetic variation in the COI gene was screened in 1362 specimens from 45 locations around the world. Two nuclear DNA genes (ITS and D2D3) were sequenced to infer phylogenetic species. We found evidence for ten sympatrically distributed cryptic species, seven of which show a strong genetic structuring. A historical signature showed evidence for restricted gene flow with occasional long-distance dispersal and range expansions pre-dating the last glacial maximum. Our data also point to a genetic break around the British Isles and a contact zone in the Southern Bight of the North Sea. We provide evidence for the transoceanic distribution of at least one cryptic species (PmIII) and discuss the dispersal capacity of marine nematodes. The allopatric distribution of some intraspecific phylogroups and of closely related cryptic species points to the potential for allopatric speciation in R. (P.) marina.