Dutch Coast Research Articles

Clay mineral provinces in tidal mud flats at Germany's North Sea coast with illite K–Ar ages potentially modified by biodegradation

Mineralogical studies, chemical analyses and K–Ar dating were carried out on clay fractions from tidal mud flats along the Lower Saxony coast and its bays to identify material sources and sedimentary processes at this dynamic interface between air, land and sea.From the coast into the bays, sediments are enriched in fine-grained smectite relative to the coarser grained illite, chlorite and kaolinite, due to the weakening of the tidal current energy in the bays. In addition, the study area can be divided into two provinces on the basis of the illite K/Rb ratios and Mg contents. To the west [Schiermonnikoog, Dollart, Ley Bay up to Norderney island], longshore currents carry suspensions from the Belgian and Dutch coasts; to the east [from Langeoog island, Jade Bay to the Helgoland mud area] suspensions from the Elbe and Weser rivers are mixed with submarine reworked glacial sediments, whereas the portion of longshore current suspensions from the west decreases, becoming negligible in the Helgoland mud area off the Elbe and Weser estuaries. The illite K–Ar data vary considerably and fail as source indicators due to differential settling and mixing of the clay material and probably to Ar loss from illite by biodegradation during digestive processes. Only further offshore, outside the zone of dynamic sediment dispersion, do the K–Ar data fit provenance patterns.

Recent Coastal Dune Development: Effects of Sand Nourishments

BAKKER,M.A.J.; VANHETEREN, S.; VONHÖGEN, L.M.; VAN DER SPEK, A.J.F., and VAN DER VALK, L., 2012. Recent coastal dune development: effects of sand nourishments. Much of the Dutch coast has been subject to structural erosion. From 1990 onward, sand nourishments have been used under a government policy of dynamic preservation. Annual monitoring and field inspections show that the structural erosion has decreased or even turned into coastal progradation after 1990. The monitoring data concern only morphodynamics and thus supply limited information on system-related geological processes driving the observed changes. Recently acquired ground-penetrating radar (GPR) data help establish the origin of sedimentary elements within the beach-foredune area, determine their decadal-scale preservation potential under the present nourishment policy, and demonstrate temporal and spatial accretion/erosion variability along nourished coasts. GPR images from a nonnourished retrograding barrier section show historical storm surge deposits within the eroding foredune and accumulations of natural eolian sediment farther landward. GPR images from a heavily nourished, prograding site show that the accreted foredune and beach consist of nourishment embankments (20%), wind-blown units derived from nourished sand (70%), and progradational beach deposits (10%). The net volume of accretion at this site is approximately 200 m3/m. Remarkably, almost all sand nourished before 2000 has been washed away, except for embankments constructed in 1990. Analysis of meteorological data suggests that 1999 storm surges are responsible for this erosion. The relative longevity of post-2000 nourishments can be attributed to a combination of shoreface nourishment and favorable meteorological conditions. During a storm surge in 2007, water-lain embankments proved to be more resistant against wave erosion than nourished sand redistributed by wind, indicating the importance of grain size, roundness and packing in the durability of nourishments.

The Effect of the 18.6-Year Lunar Nodal Cycle on Regional Sea-Level Rise Estimates

BAART, F.; VAN GELDER, P.H.A.J.M.; DE RONDE, J.; VAN KONINGSVELD, M., and WOUTERS, B., 2012. The effect of the 18.6-year lunar nodal cycle on regional sea-level rise estimates.Sea-level rise rates have become important drivers for policy makers dealing with the long-term protection of coastal populations. Scenario studies suggest that an acceleration in sea-level rise is imminent. The anticipated acceleration is hard to detect because of spatial and temporal variability, which consequently, have become important research topics. A known decadal-scale variation is the 18.6-year nodal cycle. Here, we show how failing to account for the nodal cycle resulted in an overestimation of Dutch sea-level rise. The nodal cycle is present across the globe with a varying phase and a median amplitude of 2.2 cm. Accounting for the nodal cycle increases the probability of detecting acceleration in the rate of sea-level rise. In an analysis of the Dutch coast, however, still no significant acceleration was found. The nodal cycle causes sea level to drop or to rise at an increased rate; therefore, accounting for it is crucial to accurately estimate regional sea-level rise.

Settlement and possible competition for space between the invasive violet tunicate Botrylloides violaceus and the native star tunicate Botryllus schlosseri in The Netherlands

Settlement and competition for space of two colonial sea squirts, the non-native violet tunicate Botrylloides violaceus and the native golden star tunicate Botryllus schlosseri, were compared in The Netherlands. In each year, from March 2006 to March 2010, 125-150 grey, 14  14 cm, PVC plates were deployed along the Dutch coast at 13 localities, at a depth of 1 m, and checked for species after three and six months. New plates were deployed every three months. While comparing plates with only one species represented to plates with both species represented, it appeared that Botrylloides violaceus outcompeted Botryllus schlosseri for space. Botryllus schlosseri is nevertheless expected to remain abundant along the Dutch coast because it can inhabit places with low or fluctuating salinities where Botrylloides violaceus is at a disadvantage. Settlement and the interactions between these species in The Netherlands resembled the situation in North America where both of them are considered non-native. The interactions between the two species seemed to be independent of their being native or introduced in a particular area.

Ascidians in the succession of marine fouling communities

Approximately 200 grey PVC settlement plates were hung along at sites along the Dutch coast at a depth of 1 meter. The cover of each species that could be identified on the plates was recorded every three months from March 2009 to March 2010 to study the role of native versus non-native ascidians in the succession of marine fouling communities. Three native ascidian species, Ciona intestinalis, Ascidiella aspersa and Botryllus schlosseri, and five species of cryptogenic and/or non-native origin, Molgula socialis, Styela clava, Diplosoma listerianum, Botrylloides violaceus and Didemnum vexillum, were recorded. After the Cnidaria, Ascidiacea were found to be the group that explained most of the differences found in species communities on settlement plates at different stages in the succession. Regardless of the fact that non-native ascidian species were very abundant at times, their role in the succession of the studied marine fouling communities appears to be limited. After the polyp stage of the moon jelly Aurelia aurita and hydroid species of the genus Obelia, the two native ascidian species Ciona intestinalis and Ascidiella aspersa explained most of the differences that were found between the successive stages in the fouling communities. In general, the three native ascidian species appeared to determine the succession of the fouling communities more than the five cryptogenic and/or nonnative ascidian species. Whether this is linked to species being either native or non-native is questionable however. The various ascidian species differed strongly in the period of settlement, the ability to settle on prior settlers and the ability to survive the winter. The non-native ascidians settled mostly in June to December, while the native ones settled from March to December. Although the native ascidians M. socialis and B. schlosseri were found to be restricted in their settlement by the presence of prior settlers, the non-native species B. violaceus and D. vexillum were not. Native and non-native ascidian species survived the winter equally well. Native C. intestinalis and A. aspersa and non-native B. botrylloides and D. vexillum were most successful in surviving winter water temperatures of just below 0°C. Thus the results of this study, based on one year of data, suggest that the roles of ascidian species in the succession of fouling communities vary, and that being native or non-native is not necessarily linked to the degree to which species influence the development of marine fouling communities. Their individual seasonality, abilities to overgrow prior settlers and survival of winter temperatures appear to be more important.

ISPRS Commission V Symposium: Close Range Image Measurement Techniques

The Photogrammetric RecordVolume 26, Issue 136 p. 381-383 Editorial ISPRS Commission V Symposium: Close Range Image Measurement Techniques Jon P. Mills, Jon P. Mills Newcastle upon Tyne, Tyne and Wear, EnglandSearch for more papers by this authorDerek D. Lichti, Derek D. Lichti Calgary, Alberta, CanadaSearch for more papers by this authorNorbert Pfeifer, Norbert Pfeifer Vienna, AustriaSearch for more papers by this author Jon P. Mills, Jon P. Mills Newcastle upon Tyne, Tyne and Wear, EnglandSearch for more papers by this authorDerek D. Lichti, Derek D. Lichti Calgary, Alberta, CanadaSearch for more papers by this authorNorbert Pfeifer, Norbert Pfeifer Vienna, AustriaSearch for more papers by this author First published: 19 December 2011 https://doi.org/10.1111/j.1477-9730.2011.00662.x Frontispiece. Close range image measurement techniques. An image from each of the six papers in this ISPRS Commission V 2010 Newcastle Symposium special issue. Top left: cross sections of colour-coded vector lengths of a simulated vortex ring (Westfeld and Maas). Top right: the Adorazione dei Magi painting by Giorgio Vasari in the church of Bosco Marengo in Alessandria, Italy, surveyed using a ShapeGrabber laser scanner (Remondino et al.). Centre left: the hilltop archaeological site of Drapham Dzong, Bhutan, surveyed (inset) by a Microdones md4-200 quadrotor UAV (Eisenbeiss and Sauerbier). Centre right: interactive web application for analysing Buddhist stone inscriptions of sutra in China with overlaid interpretation text (Schmidt et al.). Bottom left: modelling using Rhino software of the octagonal first Belvedere of the main spire of Milan Cathedral, Italy (Fassi et al.). Bottom right: changes in the volume of sand deposited near a screen on the Dutch coast over a 32-hour period as monitored by a terrestrial laser scanner (Lindenbergh et al.). Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume26, Issue136Special Issue: ISPRS COMMISSION V SYMPOSIUM 2010 CLOSE RANGE IMAGE MEASUREMENT TECHNIQUESDecember 2011Pages 381-383 RelatedInformation

Sustainable Development of the Dutch Coast: Present and Future

GIARDINO, A., MULDER, J., DE RONDE, J and STRONKHORST, J., 2011. Sustainable Development of the Dutch Coast: Present and Future. In: Micallef, A. (ed.), MCRR3–2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 166–172 Grosseto, Tuscany, Italy, ISSN 0749'0208.The Netherlands is a low-lying country, where approximately 27 percent of the territory is located below mean sea level and 55 percent is prone to flooding. Protection against flooding is traditionally the primary objective of coastal policy in the Netherlands. However, since 1990 coastal policy has been subject to a number of modifications, and new objectives have been added to cope with the structural erosion problems of the Dutch coast. To fulfil these new objectives, the yearly volumes of sand for nourishment have been constantly increasing. Even higher volumes will be necessary in the future due to more severe sea level rise scenarios predicted.In this paper, sand volumes in the coastal area were analysed in combination with numerical modelling results of the sediment transport along the entire coast, to assess the effectiveness of the current nourishment policy, in the short and long term. One reference scenario of the current situation and one long-term prediction, including nourishment and sea level rise, were simulated. The results of this analysis show that nourishment has been successful in coping with the past erosion problems. Furthermore, sand volumes in the upper shoreface are now increasing, creating an additional buffer of sand, which might be needed in the future, as shown by the future scenario.

The Implications of Skewed Risk Perception for a Dutch Coastal Land Market: Insights from an Agent-Based Computational Economics Model

Dutch coastal land markets are characterized by high amenity values but are threatened by potential coastal hazards, leading to high potential damage costs from flooding. Yet, Dutch residents generally perceive low or no flood risk. Using an agent-based land market model and Dutch survey data on risk perceptions and location preferences, this paper explores the patterns of land development and land rents produced by buyers with low, highly skewed risk perceptions. We find that, compared to representative agent and uniform risk perception models, the skewed risk perception distribution produces substantially more, high-valued development in risky coastal zones, potentially creating economically significant risks triggered by the current Dutch flood protection policy.

Using 18th century storm-surge data from the Dutch Coast to improve the confidence in flood-risk estimates

Abstract. For the design of cost-effective coastal defence a precise estimate is needed of the 1/10 000 per year storm surge. A more precise estimate requires more observations. Therefore, the three greatest storm surges that hit the northern part of the Holland Coast in the 18th century are reconstructed. The reconstructions are based on paintings, drawings, written records and shell deposits that have recently appeared. The storm-surge levels of these storms have been estimated using numerical modelling of the coastal processes. Here we show how these reconstructions can be used in combination with extreme value statistics to give a more confident estimate of low probability events.

On the influence of changes in the drag relation on surface wind speeds and storm surge forecasts

In this study it is investigated how uncertainties in the magnitude of the drag coefficient translate into uncertainties in storm surge forecasts in the case of severe weather. A storm surge model is used with wind stress data from a numerical weather prediction (NWP) model, to simulate several recent storms over the North Sea. For a fixed wind speed, the wind stress is linear in the drag coefficient. However, in the NWP model the wind speed is not fixed and increasing the drag in the NWP model results into reduced wind speeds. The results from simulations show that for given increase in the drag coefficient, the weakening of the 10-m wind field reduces the increase in the stress considerably. When the Charnock parameter is increased in the NWP model, the resulting relative changes in the wind stress are almost independent of the wind speed. This is related to the fact that the depth of the surface boundary layer depends on the wind speed. The ratio between relative changes in the wind stress and relative changes in the drag coefficient depends on the wind speed. For 10-m wind speeds larger than 20 m s−1 the ratio is 0.52; for lower wind speed criteria the ratio is somewhat larger (∼0.60). Approximately 36% of the relative change in the drag coefficient translates into a relative change in the surge in stations at the Dutch coast. The relative increase in the storm surge is approximately 68% of the relative increase in the stress.

Assessment of dune failure along the Dutch coast using a fully probabilistic approach

This paper describes an investigation into the added value of a fully probabilistic approach to dune resilience assessment over the currently applied deterministic and semi-probabilistic approaches. The method is applied to the Dutch coast but is generically applicable, provided of course the sufficient availability of data. The DUROS+ model in its most basic form was used to quantitatively assess dune resilience. The Monte Carlo method was used for the probabilistic investigation. Important research questions were (1) where can the DUROS+ model in combination with the fully probabilistic approach be applied along the Dutch coast? and (2) what is the alongshore variability of failure probability using this probabilistic approach?The main conclusion of the work presented in this paper is that the fully probabilistic approach provides valuable added insight with respect to the actual failure probability of transects. At the same time it is noted that the current dune erosion model in its most basic form is not able to cover all of the Dutch coast. Reasons lie in the availability of sufficient quality boundary conditions, applicability limits associated with model assumptions and insufficient quality coastal profile information.To extend the coverage of the analysis of failure probabilities along the Dutch coast it is recommended (1) to involve more process-based model concepts that can cope with the situations DUROS+ cannot, and (2) to expand currently available data on boundary conditions.

Effect of surge uncertainty on probabilistically computed dune erosion

To assess the flood protection capacity of dunes in The Netherlands, a semi-probabilistic dune-erosion prediction method is currently in use in which uncertainties in input parameters of an empirical dune erosion model were taken into account, with the exception of the uncertainty in the extreme surge distribution. Previous research has shown that the surge is by far the most influential parameter affecting erosion in the currently used erosion model, which is due both to the influence of the surge level itself and to the conditional dependence of the wave height and period on the surge level in the probabilistic model used for the assessment. Furthermore, the distribution of extreme surge levels has been shown to contain large statistical uncertainty. The inclusion of uncertainty in input variables into probabilistic models results in more extreme events (in this case erosion) for the same exceedance probability, largely due to the incorporation of higher values of the input variables. The goal of the research described in this paper was to determine the impact of the inclusion of uncertainty in the extreme surge distribution on the estimate of critical erosion (erosion associated with an exceedance frequency of 10 − 5 per year). The uncertainty in the surge distributions was estimated and parameterized, and was incorporated into the probabilistic model. A reduction in uncertainty was subsequently imposed to estimate what value a reduction in uncertainty can offer, in terms of the impact on critical erosion. The probabilistic technique first-order reliability method (FORM) was applied to determine the relative contribution of the uncertainty in the surge distribution (as well as the remaining stochastic variables) to the critical erosion. The impact of the inclusion of uncertainty in the surge distribution on the critical retreat distance was found to be substantial with increases ranging from 34% to 93% of the original estimate at five locations along the Dutch coast. The reduced uncertainty showed a more subtle impact, with increases in critical retreat distance ranging from 10% to 26% of the original estimate. The relative importance analysis showed that the uncertainty in the surge distribution has a strong influence, with the relative importance ranging from 10% to 23% for an exceedance frequency of 10 − 5 per year.

Evaluation of a LIDAR Land-Based Mobile Mapping System for Monitoring Sandy Coasts

The Dutch coast is characterized by sandy beaches flanked by dunes. Understanding the morphology of the coast is essential for defense against flooding of the hinterland. Because most dramatic changes of the beach and the first dune row happen during storms, it is important to assess the state of the coast immediately after a storm. This is expensive and difficult to organize with Airborne Laser Scanning (ALS). Therefore, the performance of a Land-based Mobile Mapping System (LMMS) in mapping a stretch of sandy Dutch coast of 6 km near the municipality of Egmond is evaluated in this research. A test data set was obtained by provider Geomaat using the StreetMapper LMMS system. Both the relative quality of laser point heights and of a derived Digital Terrain model (DTM) are assessed. First, the height precision of laser points is assessed a priori by random error propagation, and a posteriori by calculating the height differences between close-by points. In the a priori case, the result is a theoretical laser point precision of around 5 cm. In the a posteriori approach it is shown that on a flat beach a relative precision of 3 mm is achieved, and that almost no internal biases exist. In the second analysis, a DTM with a grid size of 1 m is obtained using moving least squares. Each grid point height includes a quality description, which incorporates both measurement precision and terrain roughness. Although some problems remain with the scanning height of 2 m, which causes shadow-effect behind low dunes, it is concluded that a laser LMMS enables the acquisition of a high quality DTM product, which is available within two days.

Impact of mega-scale sand extraction on tidal dynamics in semi-enclosed basins: An idealized model study with application to the Southern North Sea

Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology. Because of the unprecedented extent of the extraction scenario, we follow a generic approach aimed at understanding the physical mechanisms behind the changes, the orders of magnitude and area of influence. Hence, rather than applying an existing numerical model, we develop an idealized model for tide propagation in semi-enclosed rectangular basins. The model geometry consists of three adjacent compartments with a realistic cross-basin depth profile, the trench being located in the second compartment, while assuming along-basin uniformity of depth within each compartment. The problem is forced by an incoming Kelvin wave. The solution in each compartment, satisfying the linear depth-averaged shallow water equations on the f plane including bottom friction, is written as the superposition of semi-numerically obtained wave solutions. A collocation technique is employed to satisfy no-normal flow across the basin's closed end as well as continuity of elevation and flux across the interfaces between the compartments. The results indicate changes in tidal range, phase and currents throughout the whole basin, which shows the importance of a model domain on the scale of the basin. Changes in coastal tidal range show zones of decrease and increase (order cm). Changes in coastal shore-parallel tidal current amplitudes are of the order of cm s − 1 . A sensitivity analysis for various trench geometries shows that the hydrodynamic impact generally increases with increasing extraction volume, being more sensitive to trench depth and width than trench length. The changes in tidal currents may structurally alter sediment transport rates with several percents. As the bathymetry and coastal morphology result from subtle balances in long-term sediment transport, the trench may indirectly affect various user functions and values (coastal safety, navigation, marine ecology, cables and pipelines) and, hence, be of concern to coastal management.

Occurrence of perfluorinated organic acids in the North and Baltic seas. Part 1: distribution in sea water

Due to their high water solubilities and mobilities, persistent, polar perfluorinated compounds (PFCs) such as perfluorinated carboxylates and sulfonates are likely to end up in the oceans. In part 1 of this study, their distribution in North and Baltic Sea water is reported, being of special interest because these seas are surrounded by highly industrialized countries with high population densities. A combination of solid-phase extraction and liquid chromatography coupled with tandem mass spectrometry was used after optimisation to determine nine PFCs with chain lengths of C(4) to C(10) in water samples at ultra-trace levels. The observed concentration distribution and gradients were explained by oceanographic mixing processes and currents. The big rivers were identified as major input sources. At the mouth of the river Elbe, concentrations of 9ng/L were observed for perfluorooctanoate (PFOA), and 8ng/L for perfluorooctylsulfonate (PFOS); all other PFC concentrations ranged from 0.6 to 1.7ng/L. At coastal stations, concentrations decreased to 3.8ng/L (PFOA) and 1.8ng/L (PFOS), dropping to 0.13 and 0.09 ng/L, respectively, towards the open sea. Along the Dutch coast, high perfluorobutylsulfonate concentrations (3.9ng/L) were observed as regional characteristics. In the Baltic Sea, fairly even PFC distributions with low gradients were observed. Again, PFOA and PFOS were the major compounds (up to 1.1 and 0.9ng/L). The results underline the necessity to include PFCs in marine monitoring programs. Water was found to be a good matrix for monitoring environmental levels, sources, and transport pathways of PFCs.

A FIRST INVESTIGATION INTO THE IMPACT OF VERY LARGE-SCALE OFFSHORE SAND MINING ALONG THE DUTCH COAST

This paper investigates the impact of very large-scale offshore sand mining along the Dutch coast on waves, currents, sand transport rates and morphology. To this end, we simulate different extreme sand extraction trench geometries (depths between 1 and 17 m, volumes between 10 and 63 x 109 m3) with the process-based modelling system Delft3D. The simulations show that the waves become 0-20% higher, currents change with 0-0.3 m/s and transport rates with 10-80%. In general, the trench has a negative effect on the sand balance of coastal sections, especially near the narrow part of the deepening. The estimated trench impact on dune erosion during normative storm conditions with a frequency of occurrence of 1/10,000 yr is 5-10%. The impact of the deepening is generally seen to increase with depth.

AN IDEALIZED MODEL OF TIDAL DYNAMICS IN SEMI-ENCLOSED BASINS: THE EFFECTS OF A MEGA-SCALE SAND EXTRACTION TRENCH IN THE NORTH SEA

We investigate the effects of a mega-scale sand extraction trench (length ~200 km, width ~10 km, depth ~10 m, located in front of the Dutch coast) on tidal dynamics in the Southern North Sea. To this end, an idealized modeling approach is adopted, extending the classical problem of Kelvin wave reflection in rectangular semi-enclosed basins to account for lateral depth variations and the presence of a trench. The model results indicate changes in tidal range (zones of decrease and increase of the order of several cm), phase and currents (order cm/s) throughout the whole basin, which emphasizes the importance of a model domain on the scale of the basin. The changes in currents may structurally change sediment transport rates by several percents, which is relevant to coastal morphology. It is argued that this idealized model can be used to support a companion study carried out with a more detailed numerical engineering model. This is because of its ability to (1) provide insight into the physical mechanisms, (2) reveal the order of magnitude and area of influence of the effects and (3) allow for a quick sensitivity analysis with respect to trench geometry.

Seabirds and chronic oil pollution: Self-cleaning properties of gulls, Laridae, as revealed from colour-ring sightings

Mystery oil spills off the Dutch coast affected colonial, adult Lesser Black-backed Gulls prior to and within the breeding season. From colour-ringed individuals, it was demonstrated that most oiled birds survived and were clean within a few weeks and often bred successfully. Further evidence of self-cleaning properties of Larus-gulls is provided from a long-term colour-ringing project (1984–2009). In total 46 birds were reported ‘oiled’, two died, but the majority cleaned itself and survived for up to 20years after being oiled. From colour-ring data and 30years of beached bird surveys (1980–2010) it is demonstrated that the effects of chronic oil pollution is larger in winter than in summer; a reflection of seasonal differences in exposure and environmental conditions. The self-cleaning properties of gulls are such that long-term survival is not necessarily jeopardised and even in a breeding season, not all is lost in case of a spill.

Are white-beaked dolphinsLagenorhynchus albirostrisfood specialists? Their diet in the southern North Sea

The white-beaked dolphinLagenorhynchus albirostrisis the most numerous cetacean after the harbour porpoisePhocoena phocoenain the North Sea, including Dutch coastal waters. In this study, the diet of 45 white-beaked dolphins stranded on the Dutch coast between 1968 and 2005 was determined by analysis of stomach contents. Although 25 fish species were identified, the diet was dominated by Gadidae (98.0% by weight, 40.0% in numbers), found in all stomachs. All other prey species combined contributed little to the diet by weight (2.0%W). The two most important prey species were whitingMerlangius merlangus(91.1% frequency of occurrence (FO), 30.5%N, 37.6%W) and codGadus morhua(73.3%FO, 7.4%N, 55.9%W). In numbers, gobies were most common (54.6%N), but contributed little to the diet by weight (0.6%W). Three stomachs contained different prey compared to the others: one animal had taken 2250 gobies, accounting for 96.4% of all gobies found; one animal had fed on 29 small sepiolids; and one animal had solely taken haddockMelanogrammus aeglefinus.Squid and haddock were not found in any other stomach. The overall diet showed a lasting predominance of whiting and cod, without clear changes over time (35 years) or differences between sexes or size-classes of dolphins. This study adds to earlier published and unpublished data for Dutch coastal waters and agrees well with studies of white-beaked dolphins from other parts of the species’ range, in the North Sea and in Canadian waters, with Gadidae dominating the diet on both sides of the Atlantic.

On the occurrence of the thumbnail crab Thia scutellata in the inner German Bight

The thumbnail crab Thia scutellata occurs in the German Bight in at least two stable populations: at Borkum Riff and Loreley Bank. The western (Borkum Riff) population is larger than the eastern (Loreley Bank) one, probably because of the more frequent ingression of south-western warmer water along the Dutch coast. The sediments of these two localities are well-sorted middle sands with a very low amount of finer fractions.