The response of salt marsh vegetation to tidal reduction caused by the Oosterschelde storm-surge barrier

In 1986 a sluice gate barrier was completed in the mouth of the Oosterschelde estuary. The barrier has been partially or completely closed during 1986 and the first months of 1987. Consequently the high tides were reduced to such a level that the salt marshes were scarcely flooded. Since April 1987 the barrier has been closed on average during two out of 706 high tides a year. Although the barrier allows tidal exchange the tidal flow has been restricted as a result of the reduced width of the mouth of the estuary. This restriction of the tidal flow caused a 26 cm decrease of mean high water in the eastern part of the estuary. As a result the inundation frequencies of the salt marshes decreased.The response of salt-marsh vegetation to this tidal reduction was analyzed using annual records (1982 till 1990) of species composition in 57 permanent plots in two marshes at the southern shore of the estuary. Analysis of the response of individual species to marsh elevation in the pre and the post-barrier situation revealed that most species moved down the marsh elevation gradient. The first axis of an ordination (DCA-1) was significantly negatively related to inundation frequency. Between 1984 and 1990 all plots were displaced towards a significantly higher ordination score, indicating a trend towards a species composition from higher up the marsh. The position of most plots along DCA-1 remained stable until 1985 and started to increase in 1986 or 1987. The vegetation in plots dominated either by Halimione portulacoides or by Spartina anglica, started to change in 1985. This premature change was attributed to frost damage in January 1985. The initially high rate of change along DCA-1 decreased in 1989 and 1990. This would suggest that the vegetation re-equilibrated with the newly established tidal conditions. Further analysis revealed no significant difference in the relation between inundation frequency and sample score along DCA-1 between 1984 and 1990. This corroborates the view that species composition had re-equilibrated with the tidal conditions. Along DCA-2 the samples were displaced towards a significantly higher score as well. The change was attributed to an increase of the perennial Halimione and the annual Suaeda maritima. The annual Atriplex hastata displayed an increase from 1986 till 1988, but strongly declined in 1990. The transient response of this annual was described by DCA-3.During the pre-barrier phase several attempts had been made to predict the vegetation response to tidal reduction. We developed a multivariate model (CCA) with inundation frequency as a constraining factor to describe the relation between inundation frequency and species composition in the pre-barrier phase in 1984. Next we used this model to predict the score of the samples along CCA-1 in the post-barrier phase from the inundation frequencies in 1990. The actual ordination score in 1990 was calculated from the observed species composition. The predicted ordination scores did not differ significantly from the observed ones. In retrospect it is concluded that the response of the vegetation to the reduction of the tides, as far as the fraction of species composition which is related to inundation frequency is concerned, could have been predicted by our model.

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Composition,distribution and biomass of meiobenthos in the Oosterschelde estuary (SW Netherlands)

Meiofauna composition, abundance, biomass, distribution and diversity were investigated for 31 stations in summer. The sampling covered the whole Oosterschelde and comparisons between the subtidal — intertidal and between the western-central — eastern compartment were made.Meiofauna had a community density ranging between 200 and 17 500 ind 10 cm^–2, corresponding to a dry weight of 0.2 and 8.4 gm^–2. Abundance ranged between 130 and 17 200 ind 10 cm^–2 for nematodes and between 10 and 1600 ind 10 cm^–2 for copepods. Dry weight biomass of these taxa was between 0.5–7.0 gm^–2 and 0.008–0.3 gm^–2 for nematodes and copepods respectively.The meiofauna was strongly dominated by the nematodes (36–99%), who's abundance, biomass and diversity were significantly higher intertidally than subtidally and significantly higher in the eastern part than in the western part. High numbers were positively correlated with the percentage silt and negatively with the median grain size of the sand fraction. The abundance and diversity of the copepods were highest in the subtidal, but their biomass showed an inverse trend being highest on the tidal flats.The taxa diversity of the meiofauna community and species diversity of both the nematodes and the copepods were higher in subtidal stations than on tidal flats. In the subtidal, the meiofauna and copepod diversity decreased from west to east, whereas nematode diversity increased.The vertical profile clearly reflected the sediment characteristics and could be explained by local hydrodynamic conditions.Seasonal variation was pronounced for the different taxa with peak abundance in spring, summer or autumn and minimum abundance in winter.Changes in tidal amplitude and current velocity enhanced by the storm-surge barrier will alter the meiofauna community structure. As a result meiofauna will become more important in terms of density and biomass, mainly due to increasing numbers of nematodes, increasing bioturbation, nutrient mineralisation and sustaining bacterial growth. A general decrease in meiofauna diversity is predicted. The number of copepods is expected to decrease and interstitial species will be replaced by epibenthic species, the latter being more important in terms of biomass and as food for the epibenthic macrofauna and fishes.     

Changes in area,geomorphology and sediment nature of salt marshes in the Oosterschelde estuary (SW Netherlands) due to tidal changes

As a result of the construction of a storm-surge barrier across the mouth of the Oosterschelde (SW Netherlands) in 1987, the tidal range and mean high water level in the estuary have been reduced permanently to about 88% of their original values. During the final stage of construction (1985–1987) the tidal range and mean high water level were reduced even further for more than 18 months, by up to about 65% of their original values. This paper describes the consequences of these reductions for some abiotic aspects of the salt marshes.Strong ripening of the soil, especially in basins of the middle high salt marshes, resulted in the soils in these basins having more or less the dry nature of levees. This may cause moisture deficits for the vegetation during dry periods locally, and may lead locally to acidification of the soil as a result of oxidation of pyrite.Erosion of the edges of the salt marshes has increased in many places since 1986, both due to lowering of the surface level of the foreland, causing wave action to affect the marsh cliff more strongly than before, and weakening of cliff strength as a consequence of desiccation of the salt marsh soil and subsequent withering of plants and plant roots. In addition, the gradual salt marsh gradients have decreased on a large scale, as a consequence of increased wave attack and frost damage to Spartina. Finally, also due to desiccation and plant withering, levees have degraded and eroded, forming shoulders in the creeks.Settling, especially in the basins, has steepened and narrowed the height gradients between basin and levee.     

Changes in basin geomorphology after implementation of the Oosterschelde estuary project

The completion in 1986/87 of an open storm-surge barrier in the inlet and of secondary dams in the landward parts of the Oosterschelde tidal basin (SW Netherlands) has had and will continue to have a significant impact on geomorphological developments. An analysis of historic data, and of recent detailed bathymetric and morphodynamic process data, indicates that former trends have reversed. At present the Oosterschelde is a sedimentation basin with a degrading intertidal area and silting up of channels. The continuing reduction in intertidal area, the decreasing geomorphological gradients, the increasing fine sediment content of channel deposits, combined with a general reduction in hydrodynamics, imply significant ecological effects.     

Biomass and standing stock on sublittoral hard substrates in the Oosterschelde estuary (SW Netherlands)

From 1979 to 1991 the species composition of communities living on hard substrata (hardsub) in the Oosterschelde has been studied — in both the littoral and sublittoral zones. From 1984 onwards, biomass was also measured. This paper deals mainly with the distribution and the development of biomass on sublittoral hardsub in the Oosterschelde. Analysis has shown that the most important abiotic factors regulating the flora and fauna are: quantity and nature of the substrate; sedimentation; exposure to water movement (mainly currents); and light. The construction of the storm-surge barrier has influenced those factors. The main consequences for the flora and fauna on sublittoral hard substrata have been through the increased amount of available hard substratum by about 10% until 1984 and a further 20% from 1984 to 1987, the main barrier construction period). Within the same period (until 1987) the biomass per square metre also increased. This caused a net increase of hardsub biomass — in the sublittoral — of about 35%.After the barrier was completed sedimentation increased; in some parts of the basin hardsub organisms were covered by sediment and have not recovered; the total quantity of available hard substratum decreased by an amount yet to be established. For the purpose of this paper it is tentatively estimated at 20%, but the process is still going on.Tidal current velocities are smaller in the post-barrier situation, which caused a shift from more passive suspension feeders to more actively filtering species. The relative importance of suspension feeders on hard substrata has decreased by about 20% after the building of the storm-surge barrier. In 1990 and 1991 it increased again.Overall water transparency increased, but the lower limit of macroalgal growth has not gone deeper, as nearshore turbulence and turbidity did not change significantly.Effects on hardsub were small in the beginning. During the construction period (1985–1987) no clear effects were registered. After the completion of the barrier total species diversity increased at first, followed by a decrease from the second half of 1988 onwards. Biomass increased rather sharply, at first, but decreased very sharply in 1989. In 1990 a recovery in biomass became apparent. Developments in biomass and species composition differed per sampling location. An attempt is made to explain some of those developments, in relation to the abiotic changes brought about by the storm-surge barrier. This appeared difficult, because climatic influences obscured the effects of the barrier. The most explicit of those masking effects was brought about by a temporary, huge increase of the brittlestar (Ophiothrix fragilis). This animal covered the substratum in relatively thick layers (up to 5 cm) and more or less suffocated the other fauna. It was therefore difficult to quantify the effect of increased sedimentation on the fauna. The increase of Ophiothrix is probably not caused by the storm-surge barrier, but by a succession of several mild winters.It is clear that a new equilibrium in the basin is still to be reached. Total effects in terms of species richness and of biomass will continue to be monitored, and the results used to advise the water authorities as to management and nature friendly dike building methods.     

Nutrients and phytoplankton primary production in the marine tidal Oosterschelde estuary (The Netherlands)

In a shallow marine tidal area, the eastern part of Oosterschelde estuary in the S.W. Netherlands, phytoplankton primary production amounted to 176–338 g C.m^?2.y^?1 during the period 1981–1985. The influence of nutrient concentrations on the phytoplankton primary production is discussed. Phosphate and inorganic nitrogen generally were amply available. Import of inorganic nitrogen into the basin was shown and an intense delivery of ammonia by zoobenthos was suggested. Nitrate was considered to be slightly influenced by phytoplankton consumption and mainly by nitrate reduction at the bottom. Silicate may have played a limiting role in phytoplankton primary production. The first phytoplankton bloom in spring (diatom bloom) always terminated when silicate concentration decreased below K_s values. Further on in 1983 and 1984 both primary production and chlorophyll curves showed a dip when silicate was not available. The influence of available light on the primary production was demonstrated during situations with a low extinction coefficient when primary production reached maximum values. Further on during 1985 the spring bloom occurred already in March when winter extinction coefficients were lower than during preceding winters. Long term production studies are necessary to understand the extreme fluctuations of annual production patterns in relation to the prevailing environmental conditions.     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

Effect of sedimentological and hydrodynamical changes in the intertidal areas of the Oosterschelde estuary (SW Netherlands) on distribution,density and biomass of five common macrobenthic species: Spio martinensis (Mesnil), Hydrobia ulvae (Pennant), Arenicola marina (L.), Scoloplos armiger (Muller) and Bathyporeia sp.

In order to evaluate the impact of the construction of the storm-surge barrier and secondary dams on macrobenthos of the tidal flats in the Oosterschelde (SW Netherlands), changes in distribution, density and biomass of five common species (Spio martinensis, Hydrobia ulvae, Arenicola marina, Scoloplos armiger and Bathyporeia sp) were analysed.Data on macrobenthos were collected from 1979 to 1989 on five different tidal flats. Changes in sediment texture and hydrodynamic factors during the construction and after the completion of the coastal engineering project were taken into account.Three severe winters in a row caused more disturbance in the population of the main predator of S. armiger than did the hydrodynamical changes. A temporary prolongation of the emersion time (in 1986 and 1987) caused a temporary decrease in juvenile A. marina. But afterwards they still occupy the same nursery grounds. Increased wave action on the edges of the flats probably created new niches for Bathyporeia sp. and Spio martinensis, replacing other benthic species.It is not yet clear what has caused the decline of H. ulvae in many places in the Oosterschelde estuary. Parasitic infestation is one of the possibilities.     

Changes in the sublittoral hard substrate communities in the Oosterschelde estuary (SW Netherlands), caused by changes in the environmental parameters

In order to assess the effects of the execution of the Delta Project, the sessile sublittoral communities on hard substrates in the Oosterschelde estuary and the environmental parameters were quantitatively investigated from 1985 till 1990. During the construction period of the barrier, three communities were sampled in the photic zone and four in the aphotic zone. The distribution of the communities in the photic zone seemed to be determined by the exposition to water movement and depth, while the communities in the aphotic zone were restricted to geographic areas, with differences in tidal current velocities: the mouth of the estuary, the Hammen, the central part and the Zijpe. Two years after the completion of the enclosure works, the community structure changed rapidly, caused by decreases of tidal current velocities, increases of the amounts of sedimentation, especially of fine sediments, and an increase of the transparency of the water. Changes within the associated vagile animals showed the same tendency as the sessile communities: under less exposed conditions the number of organisms remained the same or increased, while at some locations this increase was nullified by increasing amounts of sedimentation.     

Population dynamics of annual Salicornia species in the tidal salt marshes of the Oosterschelde,The Netherlands

1. The population dynamics of two Salicornia species from the Bergen op Zoom salt marsh (south-west Netherlands) was examined. Based on the results of several field studies three preliminary life tables were constructed, two for S. procumbens agg. populations growing respectively on the mud flats and in the salt marsh, and one for S. europaea agg. living in the upper marsh.
2. The life cycles are described and quantified in terms of eight phases and the transition probabilities between them, starting from a notional individual representative of each population.
3. The models depicting the life cycle of S. procumbens show a mean offspring number of 4.26 individuals per parent for the mud-flat population and 0.18 for the salt-marsh population. The S. europaea model gives an output of 0.44 individuals per parent. These results reflect the fluctuations in population size observed in sample plots over the years 1976–78.
4. Comparison of the transition probabilities reveals that on the mud flats most S. procumbens individuals die during pollination and seed germination, while the population in the salt marsh proper is thinned especially during the seed phase in winter time and during the growth from established seedlings to maturation. S. europaea behaves in a similar but less pronounced way to S. procumbens in the salt marsh.
5. Probabilities for one flower or one seed to produce a mature flowering plant were calculated, and were compared with those found in the literature. They are roughly of the same order of magnitude as the probabilities for other annual species, but much higher than those reported for biennial species.

     

Diatom (Bacillariophyceae) assemblages in tidal environments of Vancouver Island,British Columbia,Canada

To understand distributions of coastal diatoms along Vancouver Island, British Columbia, Canada, this paper describes diatom assemblages observed in 47 surface sediment samples from intertidal environments. One hundred and eighty-four diatom taxa were identified from five transects crossing tidal flats, salt marshes, and freshwater forests in Tofino, Ucluelet, and Port Alberni. Distributions of the diatom assemblages were consistent with those reported elsewhere in the Pacific Northwest, but a few diatom taxa show different trends in their distributions. For example, one benthic species Denticula subtilis shows widespread distributions along the transect in Tofino. An ordination shown by Detrended Correspondence Analysis (DCA) using a combined dataset indicated overlapped scatter plots of diatom assemblages, suggesting that assemblages with similar species compositions are observed in more than one location. Hierarchical and k-means clustering analyses using Euclidean distance recognized unique small groups along each transect. Rank abundance curves show different trends for richness and evenness of diatom assemblages among the five transects.     

Nutrient concentrations and planktonic diatom-flagellate relations in the Oosterschelde (SW Netherlands) during and after the construction of a storm-surge barrier

Nutrients The inflow of Rhine water into the Oosterschelde was strongly reduced from 1987 onwards. This caused the winter concentrations of silicate and nitrate to decrease in the Eastern compartment, while those in the deeper Western compartment, more dependent on North Sea concentrations, hardly changed. The result was a levelling of the former East-West gradients for these nutrients. In East, summer concentrations of nitrate reached limiting levels in the post-barrier period and molar nitrate/ammonium ratios became < 1, indicating that any release of nitrogen must be important to stimulate phytoplankton growth in this area. Silicate summer concentrations in East, on the other hand, were higher in the new situation. In West, differences in summer nutrient concentrations between the old and new situation were smaller than in East, due to the still continuing exchange with the North Sea. Phytoplankton diatoms and flagellates In East during summer, N-depletion and longer residence times caused the phytoplankton to become strongly dependent on nutrient regeneration processes and increased zooplankton grazing. Average diatom biomass declined, but flagellate biomass rose during summer. Spring conditions for phytoplankton development in this area improved due to the increased water transparency, nutrients being present in excess, and this resulted in a higher new production of diatoms than before.In West, summer biomass of diatoms decreased, probably due to increased consumption by mussels under conditions of longer residence times; nutrients were not limiting, due to important benthic mineralization processes and exchange with the North Sea. The previously existing West-East biomass gradients disappeared, or sometimes reversed.Experimental (mesocosm studies) as well as field data, reported in the literature, give evidence for the given explanations.     

A new trend in the development of the phytoplankton in the Oosterschelde (SW Netherlands) during and after the construction of a storm-surge barrier

During the pre-barrier period (1982–83), the Oosterschelde phytoplankton were a diatom-dominated community, comprising a species-rich assemblage throughout the year. Assemblages of spring, early summer and summer, developed in response to a gradually evolving turbidity-light gradient during the course of the year.During the barrier-construction period (1984–87), characterized by decreasing current velocities, increasing sedimentation of suspended matter, increasing water transparencies and unchanged nutrient conditions, the growth season for the phytoplankton started earlier and lasted longer. Some flagellate species responded by much higher biomass than before. The impact of short-term climatic factors during this period, notably severe winters, could be illustrated with examples of clear responses of some species (e.g. Biddulphia aurita).In the post-barrier years (1987–90) a changed light-nutrient-salinity regime (i.e. much light, limitation of nitrate, high salinity) was demonstrated and an extended summerseason developed, without the original gradual transitions. This was reflected in an a-seasonal trend of the phytoplankton assemblage, where summer species were already observed in spring and spring species decreased in abundance. In summer small flagellates increased and some weakly silicified diatom species made their appearance. In the eastern compartment no colony formation of Phaeocystis occurred in summer and this was thought to be due to nitrate limitation. Changes in abundance of some species (Phaeocystis, Ditylum brightwellii, Skeletonema costatum), occurring during the entire period of investigation (1982–90), could be explained using field observations compared with experimental evidence from the literature.The relationship between species composition and biomass on the one hand and environmental variables on the other hand, was analysed in a Canonical Correspondence Analysis, for both compartments separately.     

潮汐作用对黄河三角洲盐沼湿地甲烷排放的影响

盐沼湿地作为陆海交互作用的过渡带是CH₄重要的自然来源。潮汐活动通过影响CH₄的产生、氧化和传输驱动了湿地CH₄间歇性、周期性的排放。利用涡度相关和微气象监测技术,对黄河三角洲一个盐地碱蓬生态系统CH₄通量、环境因子和水文要素（潮汐）进行了长期连续监测分析了该生态系统生长季CH₄排放的季节动态及潮汐作用对CH₄排放的影响。结果表明：生长季该生态系统是CH₄的排放源,排放日均值为0.063 mg m^-2 h^-1,（范围为-0.36-0.57 mg m^-2 h^-1）。潮汐淹水阶段和落潮后湿润阶段表现为CH₄的显著源。此外我们发现,短期潮汐活动引起土壤干湿状况的变化促进了CH₄脉冲式的排放,因此未来气候变化下温度升高和降雨季节分配引起的土壤干湿变化将会对该区域CH₄排放甚至碳循环产生积极影响。     

Field observations in the Oosterschelde (The Netherlands) onCoscinodiscus concinnus andCoscinodiscus granii (Bacillariophyceae) infected by the marine fungusLagenisma coscinodisci (Oomycetes)

During 1985–1990Coscinodiscus concinnus andCoscinodiscus granii from the Oosterschelde were infected by the marine fungusLagenisma coscinodisci, although not every year with the same intensity. Infected cells were only observed during the period July–October at water temperatures between 13.2 and 20.2°C. In 1986 and 1987 the course of the infection withL. coscinodisci inC. concinnus andC. granii, was recorded at three stations. The highest infection percentages varied between 22.2 and 58.3% inC. concinnus and between 7.1 and 41.9% inC. granii. It is concluded that the water temperature may play an important role in the appearance ofL. coscinodisci inC. concinnus andC. granii and that silicate limitation is of minor importance. Finally it is discussed that the infection was not of great importance for the phytoplankton community in the Oosterschelde.     

Restoration of the Sieperda Tidal Marsh in the Scheldt Estuary, The Netherlands

Because of land reclamation, reinforcement of dikes, and the deepening of shipping channels, large areas of tidal marshes have been removed or eroded from the Scheldt estuary during the last two centuries. Tidal wetland restoration contributes toward compensating this loss of habitat. Not all restoration projects are meticulously planned, however; some are forced by nature. During a severe storm in 1990, a dike was breached in the brackish part of the Scheldt estuary and returned tidal influence to the Sieperda polder. In the 10 years since the dike breach, the former polder has changed into a brackish tidal marsh. Here we report on the geomorphologic and ecological developments that have taken place in the marsh. Tidal intrusion into the former polder turned crop fields into mudflats and changed pastures into salty marsh vegetation. The digging of a new creek improved marsh hydrology and enhanced tidal intrusion further into the marsh. Macrofauna typical of estuarine mudflats established rapidly in the developing marsh. Vegetation succession took place rapidly. Within 5 years, large areas of mudflats became covered with marsh vegetation. Birds characteristic of salt marshes were observed breeding or seen foraging in the marsh. The number of wading birds declined as areas of mudflat became overgrown. It is demonstrated that tidal flow is the engine to tidal marsh restoration. Tidal influence caused geomorphologic changes, which directed ecological developments in the former polder.     

The effect of fluctuations in tidal inundation frequency on a salt-marsh vegetation

Over a period of 15 years recordings were made of the species cover in permanent plots on the salt marsh of one of the West Frisian Islands, Schiermonnikoog (The Netherlands). Correlations between annual changes in the cover of the major species, and fluctuations in the monthly frequency of inundation by seawater were studied. First, a spectral analysis was carried out on the inundation frequency data to look for predictable patterns. Subsequently, fluctuations were defined as deviations from these predictable patterns. In a repeated multiple regression model, the effects of the season in which the fluctuations occurred, and the elevational position of the plots on the salt marsh were studied as factors influencing the correlation patterns. The behaviour of various species is discussed in relation to their seed bank characteristics and their salt tolerance.     

Restoration of salt marshes in the Netherlands

The conquest of land from the sea has been a long tradition in the Netherlands. When salt marshes were high enough, they were embanked when it was economically feasible, and transformed into intensively exploited agricultural land. This resulted in the transformation of halophytic communities to glycophytic communities. Often as an alternative, a low levee, a summerdike was built, which greatly reduced the flooding frequency of the landward summerpolder, hence creating a sedimentation deficit therein. Such summerpolders now cover 1200 ha in the Netherlands, 2100 ha in NW-Germany and small areas in England. Due to continuous embankments, the present salt-marsh area is relatively small with respect to the tidal basins. Discussions have been started how to increase the salt-marsh area. Two options will be discussed, firstly de-embankment of summerpolders and maintenance of the protective seawall, secondly increase of the effects of saline seepage behind the seawall by top soil removal. Both options include the restoration of salt-marsh communities (target communities) in intensively agriculturally exploited sites that have been salt marshes before. From the few examples abroad and experiments it is discussed (1) to which extent the sedimentation deficit in summerpolders could be compensated for, (2) if the soil seed bank is likely to contribute to re-establishment of salt-marsh communities, (3) if the dispersal of propagules of halophytic plants will be possible by hydrochory when the summerdike is breached, (4) to what extent is dispersal by endozoochory through waterfowl important in case re-establishment in a saline seepage area behind the seawall without open connection to the sea is envisaged. Two case studies of de-embanked summerpolders in the Netherlands revealed that the sedimentation deficit can be counteracted by rapid sedimentation, provided enough transport is possible from the foreshore. Dispersal by incoming tidal water from the nearby salt-marsh source area into the target area is possible for many salt-marsh plant species. The rate of success seems to depend on the relative position of source area and target area. A case study in a saline seepage area after top soil removal in the Netherlands, showed that the number of viable seeds dispersed by droppings from waterfowl is limited. Hence the possibilities for restoration of inland halophytic plant communities seem much lower than after de-embankment of summerpolders.     

The infection dynamics and dispersion pattern of Lernaeocera branchialis L. on 0+ whiting (Merlangius merlangus L.) in the Oosterschelde (SW Netherlands)

Size and structure of the Lernaeocera branchialis population infecting 0+ whiting in the Oosterschelde were studied during 1989. Two periods of successful transmission were distinguished. The first transmission wave occurs in late spring when the post-larval whiting enter the Oosterschelde. A second wave occurs in autumn. This pattern in the infection dynamics is possibly related to seasonal variations in spatial overlap of the intermediate ( Platichthysflesus ) and the final host ( Merlangius merlangus ). The dispersion pattern of Lernaeocera branchialis within the whiting population can be described by the Poisson distribution. Possible explanations for the observed dispersion pattern are given. Evidence is presented that the rejection of pennella larvae is a key mechanism determining the abundance of L. branchialis in whiting. The potential impact of parasite-induced host mortality on population size and dispersion pattern of the parasite is discussed.