Dutch North Sea Coast Research Articles

Spatial patterns in age‐ and colony‐specific survival in a long‐lived seabird across 14 contrasting colonies

Demographic rates such as recruitment and survival probability can vary considerably among populations of the same species due to variation in underlying environmental processes. If environmental processes are spatially correlated, nearby populations are expected to have more similar demographic rates than those further apart. Breeding populations and foraging ranges are spatially segregated in colonial seabirds, making them ideal for studying spatial patterns in demographic rates and their effects on local population dynamics. Here we explored variation in age‐dependent survival probabilities across 14 colonies of Herring GullsLarus argentatusbreeding along the Dutch North Sea coast. We used long‐term mark–recapture data of marked fledglings to estimate survival, and estimated spatial autocorrelation of survival probabilities. We assessed whether survival until recruitment age or until 10 years old (close to their expected lifespan) explained variation in population trajectories of each colony. Juvenile and adult survival showed a strong, but different, north‐to‐south gradient in survival probability, with lower juvenile but higher adult survival in northern colonies than southern colonies, whereas the spatial pattern of immature survival was less distinct. Neither recruitment nor the proportion of 10‐year‐old adults alive predicted whether a colony collapsed, declined, remained stable or increased. The distinct spatial pattern in survival suggests variation in regional food availability, which do not seem to drive local population dynamics. The absence of a link between survival and colony trajectories implies that connectivity between populations plays an important role affecting population dynamics.

Using machine learning and beach cleanup data to explain litter quantities along the Dutch North Sea coast

Abstract. Coastlines potentially harbor a large part of litter entering the oceans, such as plastic waste. The relative importance of the physical processes that influence the beaching of litter is still relatively unknown. Here, we investigate the beaching of litter by analyzing a data set of litter gathered along the Dutch North Sea coast during extensive beach cleanup efforts between the years 2014 and 2019. This data set is unique in the sense that data are gathered consistently over various years by many volunteers (a total of 14 000) on beaches that are quite similar in substrate (sandy). This makes the data set valuable to identify which environmental variables play an important role in the beaching process and to explore the variability of beach litter concentrations. We investigate this by fitting a random forest machine learning regression model to the observed litter concentrations. We find that tides play an especially important role, where an increasing tidal variability and tidal height leads to less litter found on beaches. Relatively straight and exposed coastlines appear to accumulate more litter. The regression model indicates that transport of litter through the marine environment is also important in explaining beach litter variability. By understanding which processes cause the accumulation of litter on the coast, recommendations can be given for more effective removal of litter from the marine environment, such as organizing beach cleanups during low tides at exposed coastlines. We estimate that 16 500–31 200 kg (95 % confidence interval) of litter is located along the 365 km of Dutch North Sea coastline.

Temporal patterns in offshore bird abundance during the breeding season at the Dutch North Sea coast

The expanding development of offshore wind farms brings a growing concern about the human impact on seabirds. To assess this impact a better understanding of offshore bird abundance is needed. The aim of this study was to investigate offshore bird abundance in the breeding season and model the effect of temporally predictable environmental variables. We used a bird radar, situated at the edge of a wind farm (52.427827° N, 4.185345° E), to record hourly aerial bird abundance at the North Sea near the Dutch coast between May 1st and July 15th in 2019 and 2020, of which 1879 h (51.5%) were analysed. The effect of sun azimuth, week in the breeding season, and astronomic tide was evaluated using generalized additive modelling. Sun azimuth and week in the breeding season had a modest and statistically significant (p < 0.001) effect on bird abundance, while astronomic tide did not. Hourly predicted abundance peaked after sunrise and before sunset, and abundance increased throughout the breeding season until the end of June, after which it decreased slightly. Though these effects were significant, a large portion of variance in hourly abundance remained unexplained. The high variability in bird abundance at scales ranging from hours up to weeks emphasizes the need for long-term and continuous data which radar technology can provide.

Meteorologically generated long-period waves and their impact on the Dutch primary national flooding protection system

During storms, long-period water level oscillations can occur on the North Sea. The meteorological phenomena that cause these oscillations are known to a large extent and include atmospheric single pulse perturbations or oscillations of longer duration and larger spatial scale (De Jong in Origin and prediction of seiches in Rotterdam harbour basins, Delft University of Technology, ISBN 90-9017925-9, 2004). During such events, standing waves, or ‘seiches’, can occur in the ports along the Dutch North Sea coast. These seiches need to be considered in the height criteria for the dikes and other flood protection works around the port basins. Over the last decades, several projects on the climatology of seiching have been performed by Deltares under assignment by the Dutch Ministry of Public Works. Results of these projects served to update the height criteria for the storm surge barriers in the Port of Rotterdam, and as input to the design of two large new sea locks for other coastal ports, in IJmuiden and Terneuzen. This paper describes the three project locations: the statistical and hydrodynamical analyses of seiche events at these locations and the translation of the results into a buffer, or ‘seiche allowance’, to the height criteria.

An evolutionary divergent pestivirus lacking the Npro gene systemically infects a whale species

ABSTRACT Pestiviruses typically infect members of the order Artiodactyla, including ruminants and pigs, although putative rat and bat pestiviruses have also been described. In the present study, we identified and characterized an evolutionary divergent pestivirus in the toothed whale species, harbour porpoise (Phocoena phocoena). We tentatively named the virus Phocoena pestivirus (PhoPeV). PhoPeV displays a typical pestivirus genome organization except for the unique absence of Npro, an N-terminal autoprotease that targets the innate host immune response. Evolutionary evidence indicates that PhoPeV emerged following an interspecies transmission event from an ancestral pestivirus that expressed Npro. We show that 9% (n = 10) of stranded porpoises from the Dutch North Sea coast (n = 112) were positive for PhoPeV and they displayed a systemic infection reminiscent of non-cytopathogenic persistent pestivirus infection. The identification of PhoPeV extends the host range of pestiviruses to cetaceans (dolphins, whales, porpoises), which are considered to have evolved from artiodactyls (even-toed ungulates). Elucidation of the pathophysiology of PhoPeV infection and Npro unique absence will add to our understanding of molecular mechanisms governing pestivirus pathogenesis.

Microplastics en route: Field measurements in the Dutch river delta and Amsterdam canals, wastewater treatment plants, North Sea sediments and biota

Environmental contamination by plastic particles, also known as ‘microplastics’, brings synthetic materials that are non-degradable and biologically incompatible into contact with ecosystems. In this paper we present concentration data for this emerging contaminant in wastewater treatment plants (WWTPs) and freshwater and marine systems, reflecting the routes via which these particles can travel and the ecosystems they potentially impact along their path. Raw sewage influents, effluents and sewage sludge from seven municipal WWTPs in the Netherlands contained mean particle concentrations of 68–910L−1, 51–81L−1 and 510–760kg−1 wet weight (ww), respectively (particle sizes between 10 and 5000μm). Even after treatment, wastewater constitutes a source of microplastic pollution of surface waters, and via biosolids applications in farming and forestry, plastic retained in sewage sludge can be transferred to terrestrial environments. The WWTPs investigated here had a mean microplastics retention efficiency of 72% (s.d. 61%) in the sewage sludge. In the receiving waters of treated and untreated wastewaters, we detected high microplastic levels in riverine suspended particulate matter (1400–4900kg−1 dry weight (dw)) from the Rhine and Meuse rivers. Amsterdam canal water sampled at different urban locations contained microplastic concentrations (48–187L−1), similar to those observed in wastewater that is emitted from sewage treatment facilities in the area. At least partial settling of the particles occurs in freshwater as well, as indicated by microplastics in urban canal sediments (<68 to 10,500particleskg−1dw). Microplastics in suspension in the water column have the potential to be discharged into the sea with other riverine suspended particulates. We report microplastic concentrations from 100 up to 3600particleskg−1 dry sediment collected at 15 locations along the Dutch North Sea coast. The high microplastic enrichment in marine sediments compared to most literature data for seawater at the surface supports the hypothesis of a seabed sink for these materials. Marine species are heavily exposed to plastic particles. Body residues between 10 and 100particlesg−1dw were measured in benthic macroinvertebrate species inhabiting the Dutch North Sea coast: filter-feeding mussels and oysters (species for human consumption) as well as other consumers in the marine food chain.

Observations and analytical modeling of freshwater and rainwater lenses in coastal dune systems

Observations are reported on (i) groundwater recharge rates under various types of vegetation as measured with megalysimeters in the dunes, (ii) freshwater lenses along the Dutch North Sea coast in the early 1900s, and (iii) rainwater lenses that develop on top of laterally migrating, artificially recharged riverwater. Subsequently analytical methods are presented to estimate annual natural groundwater recharge as function of rainfall and vegetation, and to calculate the size, shape and transition zone of freshwater lenses on saline groundwater and rainwater lenses on infiltrated riverwater. An empirical correction factor, based on the hydraulic resistance of an aquitard within the freshwater lens, is proposed to account for the frequently observed reduction of the Ghyben-Herzberg ratio of 40. This factor raises the groundwater table, reduces the depth of the fresh/salt interface and increases the lens formation time. The suite of methods offers a tool box for knowledge based water management of dune systems, by rapidly predicting: (i) more or less autonomous changes due to sealevel rise, climate change and vegetation development; and (ii) the potential (side) effects of interventions. Knowing what happened or will happen to the fresh water lens or a rainwater lens is important, because changes impact on important natural habitat parameters such as salinity, depth to groundwater table, decalcification rate (and thus on pH, Ca/Al, PO4, NH4) and nutrient availability, and on drinking water supply. The analytical models are applied to predict effects of sealevel rise, coastal progradation, vegetation changes, and increased temperature of coastal air and river water to be infiltrated.

Fully probabilistic assessment of safety against flooding along the Dutch coast

AbstractThis paper describes a fully probabilistic safety assessment of the Dutch North Sea coast, in which stochastic properties of both hydraulic loads and strength of the flood defences have been taken into account. The study has led to an overview of failure probabilities along the coast with high spatial resolution. Both dikes and dunes have been considered. Failure probabilities at individual locations have been combined to flooding probabilities per dike ring area. The vast majority of the Dutch coastal defences is quite secure in terms of flooding. This study demonstrates that generally, the Dutch dunes provide a higher degree of safety than the sea dikes. When incorporating the consequences of flooding to the analysis, the calculated flooding probabilities can be used to determine flood risks. The probabilistic method, presented in this paper, enables accurate balancing between avoided flood risks and investments to reinforce the flood defences.

Evaluation of eutrophication variables and thresholds in the Dutch North Sea in a historical context — A model analysis

This study presents an evaluation of eutrophication variables and thresholds in the Dutch North Sea in a historical context by means of a model analysis. An ecosystem model (Delft3D-GEM for the North Sea) was used to simulate four different years, representing periods with little anthropogenic input (1930), medium anthropogenic input (1960), and high anthropogenic input (1985), as well as current (2007) conditions. Model runs differed with respect to nutrient inputs (riverine input and atmospheric deposition) but physical and hydrodynamic conditions were kept unchanged, to enable a clear assessment of the impact of changes in nutrient inputs. Model results showed that current (2007) dissolved inorganic phosphate (DIP) concentrations along the Dutch North Sea coast are similar to levels in 1930, while dissolved inorganic nitrogen (DIN) concentrations have been reduced to levels that were present in 1960. Chl a concentrations did not show clear differences between 2007, 1930 and 1960, but Phaeocystis levels were higher in 2007 than in 1930 and 1960 in all areas, although these differences were not statistically significant. Furthermore, model results indicate that DIN (rather than DIP) should be the focus of nutrient reduction efforts, since reducing nitrogen will cause a shift from a P-limited coastal ecosystem towards a more pristine, N-limited coastal ecosystem. Finally, it was found that assessment parameters and threshold levels should be revised and the harmonization between them should be improved, for which several recommendations are given.

Harbour Porpoises Phocoena phocoena in the Eastern Scheldt: A Resident Stock or Trapped by a Storm Surge Barrier?

Coastal protection measures are planned and executed worldwide to combat the effects of global warming and climate change, in particular the acceleration of sea level rise, higher storm surge flooding and extensive coastal inundation. The extent to which these defensive measures may impact coastal and estuarine ecosystems is still poorly understood. Since the building of a storm surge barrier, movement of harbour porpoises Phocoena phocoena in and out of the Eastern Scheldt tidal bay (SW-Netherlands) may be limited. To measure residency, porpoises stranded along the Dutch North Sea coast between 2006 and 2008 were sampled for muscle (n = 102) and bone tissue (n = 118), of which 9 muscle (8.8%) and 12 bone samples (10.2%) were collected from animals stranded within the Eastern Scheldt. Stable carbon (δ13C) was analysed to get insight into the habitat use and residency of porpoises in the Eastern Scheldt. Our data showed significantly higher δ13C values in the muscle of porpoises stranded within the Eastern Scheldt (µ = −17.7‰, SD = 0.4‰) compared to animals stranded along the Dutch coast (µ = −18.3‰, SD = 0.5‰). This suggests that most porpoises stranded in the Eastern Scheldt foraged there for a longer period. The distinct δ13C signature of animals from the Eastern Scheldt was not observed in bone tissue, suggesting a relatively recent shift in habitat use rather than life-long residency of porpoises within the Eastern Scheldt. The high number of strandings within the Eastern Scheldt suggests a higher mortality rate compared to the Dutch coastal zone. Our study indicates that along with other changes in the physical environment, the storm surge barrier may play an important role in determining the residency of porpoises in the Eastern Scheldt, and that the area might act as an ecological trap for porpoises entering it.

Data-based yearly forecasting of beach volumes along the Dutch North Sea coast

Beach and nearshore levels have been measured yearly along the entire Dutch North Sea coast since the mid 1960s (the ‘Jarkus’ data set). This data set has been processed to create separate time series of beach volumes at longshore intervals of about 250 m, giving over 2000 time series in total. These time series typically show a high annual variability with weak long-term trends. The present Dutch national coastal management strategy involves making year-ahead forecasts of beach volumes by extrapolating a linear least squares trend through the previous ten years' data separately for each longshore location. In this paper, these forecasts are shown to be worse than the trivial forecast in which the most recently measured beach volume persists unchanged into the future, with a mean square error (MSE) about 13.5% worse (equivalent to a root mean square error (RMSE) 6.5% worse). Improvements to these forecasts are sought by testing six different univariate forecasting methods. The two best methods improve on the persistence of the most recently measured beach volume by about 15% MSE (8% RMSE), and on the presently used linear least squares trend method by about 25% MSE (13.5% RMSE). Further comparisons are made between the forecasting methods to investigate several factors. These include varying the amount of fitting data for the forecasting methods, smoothing of the fitting data, different methods for interpolating gaps in the data, the longshore aggregation of data, making forecasts for coastal profiles with and without nourishments, and making forecasts up to five years ahead. These forecasting methods are designed as a coastal management tool to provide yearly forecasts quickly and routinely for the whole Dutch North Sea coast.

Physical and optical aerosol properties at the Dutch North Sea coast based on AERONET observations

Abstract. Sun photometer measurements at the AERONET station at the North Sea coast in The Hague (The Netherlands) provide a climatology of optical and physical aerosol properties for the area. Results are presented from the period January 2002 to July 2003. For the analysis and interpretation these data are coupled to chemical aerosol data from a nearby station of the Dutch National Air Quality Network. This network provides PM10 and black carbon concentrations. Meteorological conditions and air mass trajectories are also used. Due to the location close to the coast, the results are strongly dependent on wind direction, i.e. air mass trajectory. In general the aerosol optical properties are governed by industrial aerosol emitted form various industrial, agricultural and urban areas surrounding the site in almost all directions over land. For maritime air masses industrial aerosols are transported from over the North Sea, whereas very clean air is transported from the NW in clean polar air masses from the North Atlantic. In the winter the effect of the production of sea salt aerosol at high wind speeds is visible in the optical and physical aerosol data. In these cases fine and coarse mode radii are similar to those reported in the literature for marine aerosol. Relations are derived between the Ångström coefficients with both the fine/coarse mode fraction and the ratio of black carbon and PM10.

Late Quaternary sea‐level change and isostatic and tectonic land movements along the Belgian–Dutch North Sea coast: geological data and model results

AbstractEarly and middle Holocene sea‐level data from Belgium plot ca. 2 m above data from the western Netherlands, reflecting differential tectonic and isostatic movements. The total uplift rate of Belgium relative to the western Netherlands decays gradually from 0.66 m kyr−1 at 7500 cal. yr BP to less than 0.25 m kyr−1 since 5000 cal. yr BP. The tectonic component of this relative movement, inferred from Eemian sea‐level data, is in the order of 0.06 to 0.16 m kyr−1. The isostatic component of the uplift is related to the last phase of the collapse of a peripheral bulge under The Netherlands and the Dutch and German sectors of the North Sea, and to hydroisostatic subsidence of the North Sea basin caused by water loading as sea‐level rose. Comparisons with isostatic rebound models show that they predict well the Holocene isostatic movement between Belgium and the western Netherlands, and that the predictions are improved when the tectonic component is removed from the observations. Preliminary evaluation of sea‐level data from the Dutch sector of the North Sea shows that glacio‐hydroisostatic subsidence relative to Belgium was significantly greater than for the western Netherlands coastal area, and spatially highly variable. Copyright © 2002 John Wiley &amp; Sons, Ltd.

The pharmaceutical exploration of cold water ascidians from the Netherlands: a possible source of new cytotoxic natural products

Six ascidian species from the Dutch North Sea coast were screened for cytotoxic activity. Freeze-dried biological material was extracted with solvents of different polarity followed by determination of the cytotoxicity. The most active extracts were further separated using different chromatographic techniques. The microculture tetrazolium (MTT) assay was used to determine the cytotoxicity against two human tumor cell lines: COLO 320 (a colon adenocarcinoma) and GLC 4 (a small cell lung carcinoma). The two cell lines were selected for their different response towards known cytostatics. GLC 4 is sensitive and COLO 320 is resistant to most of the known cytostatics. Three of the species tested yielded interesting fractions. From the in colonial Didemnum lahillei we have isolated a compound which is more active against the COLO 320 cell line (IC 50:33 μg ml −1) than the GLC 4 (IC 50:49 μg ml −1). The structure is currently elucidated. Another colonial species, Aplidium glabrum, yielded a very cytotoxic fraction, with an IC 50 of 5 μg ml −1 against the COLO 320 cell line. From our results it can be concluded that the North Sea could be another interesting source of new compounds with pharmaceutical potential.

Introductions and developments of oysters in the North Sea area: a review

To replenish the exploited native stocks ofOstrea edulis, imports from almost all European coasts have arrived in the North Sea, particularly in the Dutch Oosterschelde estuary. The American oysterCrassostrea virginica and the Portuguese oysterC. angulata have also been imported into the North Sea several times. However, only the introductions of various genetic strains of the Pacific oysterC. gigas have been of lasting success. Spat from British Columbia (Canada) was first imported to the Oosterschelde in 1964. First spatfalls in this area took place in the warm summers of 1975 and 1976. Further larval outbursts occurred in 1982 and 1989, and good settlements took place on culture plots as well as along the dikes of the Oosterschelde. Since 1977 no more cupped oysters have been imported from overseas. The population maintained itself and was able to spread in a northern direction along the Dutch North Sea coast. In Britain, hatchery-producedC. gigas were transferred to several sites, in the 1970s including the British North Sea coast. Here, occurrence in the wild seems to be rather limited up to now. Into the German Wadden Sea,C. gigas larvae and spat from a Scottish hatchery have been introduced since 1971, as were medium-sized oysters from a variety of European sources in the 1980s. Strong spatfalls on intertidal mussel beds in the northern German Wadden Sea occured in 1991 and 1994. For the introductions ofC. gigas along the west European coasts, precautionary measures to minimize unintentional transfers of associated organisms, as recommended by the ICES Code of Practice in 1994, came too late.

Diseases of flounder ( Platichthys Flesus L.) in the Dutch Wadden Sea, and their relation to stress factors

In 1988 a survey was conducted on the occurrence of gross pathologies in flounder ( Platichthys flesus L.) in the Dutch Wadden Sea in relation to stress factors, especially near fresh-water drainage sluices. A total of 9608 flounder were collected from 16 sites in June and September. Diseases recorded were: skin ulcers (affecting 7.7% of fish); fin rot (2.8%; only included in the study in September); lymphocystis (3.7%); skeletal deformities (0.3%); liver nematode infections (5.0%); intestinal cysts of the parasite Glugea sp. (0.4%); and liver nodules (pre-neoplastic and neoplastic lesions) (< 0.1%). Prevalences of skin ulcers and fin rot in Wadden Sea flounder are considerably higher than in those captured off the Dutch North Sea coast or in the Elbe estuary in Germany. These high disease levels in the Wadden Sea, however, are largely due to extremely high prevalences of skin ulcers observed near the drainage sluices of Lake IJssel, indicating a localized effect. Fin rot occurs at relatively high prevalences over the whole study area, but the data should be interpreted with some caution, due to diagnostic problems. Associations with possible disease determinants were investigated and causal hypotheses formulated. In general fish seem to develop skin diseases at sites with strong salinity fluctuations. Nutritional deficiencies possibly related to strong salinity fluctuations, high bacterial loads in the environment and obstacles to fish migration through sluices are amongst the most likely factors determining the locally high levels of skin ulcers. Although not established, it is possible that pollution-related factors and contaminant bioavailability are important. The stress imposed by a combination of these factors may adversely affect the immune system, making the fish vulnerable to opportunistic bacterial infection.

Measurement of gaseous nitric acid by a laser-photolysis fragment-fluorescence (LPFF) method in the black forest and at the North Sea cost

Mixing ratios of gaseous nitric acid (ranging from 0.8 to 2.2 ppbv) were measured at the UBA station in the Black Forest, F.R.G., and (ranging from 0.12 to 0.9 ppbv) at the Dutch North Sea coast in Petten (ECN). A novel-laser photolysis fragment-fluorescence (LPFF) method was used which monitors HNO3 in air by OH fluorescence after irradiation by ArF laser light. Simultaneous measurements of HNO3, other trace species, and further parameters were additionally performed by the host institutions (UBA and ECN). The HNO3 data show variable agreement.

Monitoring bio-available marine contaminants with mussels (Mytilus edulis L) in the Netherlands

After discussing the usefulness of mussels (e.g. Mytilus edulis) for investigating contamination in the marine environment, four cases are presented: (1) A survey of Cu bioavailability in and around the Eastern Scheldt during one winter (1974-1975); (2) A surveillance with a six years interval (1974; 1980) of the bioavailability of Zn along the Dutch North Sea coast. (3) A monitoring programme over the period 1972-1980 showing the decrease in bioavailable Hg in the water column of the Ems-Dollard estuary after decreased emissions at Delfzijl. (4) Retrospective monitoring of two PCB's, as a background for the effectiveness of legal restrictions to PCB use. These four cases are considered a first step in understanding the effects of pollutants in marine ecosystems. In depth physiological, histological and biochemical studies could lay the basis for improved routine biological effect monitoring.

Sigmoidal tidal bundles and other tide-generated sedimentary structures of the Curtis Formation, Utah

The Curtis Formation in central Utah contains a rich suite of sedimentary structures that provides persuasive evidence for a tidal mode of origin. The most diagnostic of these structures are sigmoidal tidal bundles: sandstone packages deposited during one dominant tidal episode that comprise a set of large-scale cross-lamination enclosed by two relatively gently dipping, sigmoid-shaped pause planes. Bundles are especially recognizable as tidally generated features because they exhibit an internal progression, from gentle, drape-like sigmoids to avalanche-type cross-bed foresets back to sigmoidal drapes. This progression reflects the sequential change in tidal flow velocities from slack water to maximum flow back to slack water. In addition, cyclic variability in the thickness and in internal structures among tidal bundles is clearly related to periodicity in the lunar month. These features of the Curtis Formation are remarkably similar to sedimentary structures observed in Holocene tidal sediments along the Dutch North Sea coast.

LOW FREQUENCY OSCILLATIONS ON THE DUTCH COAST

During several months in 1981 and 1982/1983 two extensive programs of field measurements were carried out at Egmond on the Dutch North Sea coast, one during the spring of 1981 and the other during the winter of 1982/83. In these programs researchers of the Dutch "Rijkswaterstaat", of The Delft University of Technology and of the Delft Hydraulics Laboratory cooperated in the planning and execution of the measurements and are now involved in data analysis and evaluation of results. During the year 1983 the senior author spent a sabbatical year with the Rijkswaterstaat in the Netherlands and joined the team effort in analyzing the data. Reference is made to other papers presented at this conference, highlighting some aspect of this research program: Van Heteren and Stive (1984), Derks and Stive (1984). The present paper deals with low frequency oscillations in the surfzone and their potential impact on certain aspects of coastal morphology. The conclusions of this paper have to be considered as tentative and should be followed by further study. Its main intention is to point to certain possible relationships regarding coastal morphology so that in future measurements the program can be designed in such a way that certain relationships can clearly and unmistakenly be identified. The programs of measurements carried out in 1981 and 1982/83 were primarily designed to evaluate the usefulness and accuracy of various types of instruments in the surfzone. The measurements were never designed for the use we have made of them and results may therefore not be fully conclusive and thus subject to discussion. Nevertheless the authors found their tentative findings of sufficient interest to share them with their colleagues.