Evaluating impacts of shellfish and baitworm digging on bird populations: short-term negative effects on the availability of the mudsnail Hydrobia ulvae to shorebirds

Many shorebird populations show evidence of declines. To identify the causes is a key issue in developing comprehensive shorebird conservation plans. In coastal areas, shorebirds are vulnerable to effects of shellfish and baitworm digging, including reduction of the food supply. The mudsnail Hydrobia ulvae is often reported to be the dominant inhabitant of intertidal mudflats, and is common in the diet of migrating and wintering shorebirds. This prosobranch mollusc lives at or just below the surface of intertidal mudflats, so it might be directly damaged and killed or buried within the mud by hand diggers. We studied the short-term effects of digging by hand on the availability of mudsnail to shorebirds. Twenty centimetres deep core samples were collected from undisturbed and recently disturbed intertidal mud. The total mudsnail density and biomass per core sample was similar in disturbed and undisturbed mud. However, mudsnail density and biomass were significantly lower in disturbed mud than in undisturbed mud when only the upper five centimetres of the mud were compared. If only the mudsnails found in this surface layer are potentially available for shorebirds, the available mudsnail density and biomass fraction for shorebirds had decreased by 62.6? ?±? 11.4% and 75.7? ?±? 7.2% in disturbed mud, respectively. The potential impact of this decreasing mudsnail fraction on shorebirds is addressed.     

Structure and Dynamics of Exopolymers in an Intertidal Diatom Biofilm

Diatom biofilms growing at the surface of the intertidal mudflat of Marennes Oléron, France, were incubated for 48 h in the laboratory under simulated conditions of high- and low tide (immersed and emersed in seawater) and day and night (illuminated or dark conditions). The biofilms were subsequently sampled using the cryolander technique, without disturbing the structure. The samples were kept in liquid nitrogen until they were transferred to the cooled stage of a field-emission cryo-scanning electron microscope, which was used to study the structural relationships between the sediment particles, the diatoms and the different types of extracellular polymeric substances (EPS) produced by these organisms. The diatoms were most abundant at the sediment surface when incubated in the light under emersed conditions. In the dark or when immersed, the diatoms migrated into the sediment. In the light, the diatoms were coated with EPS, while this was not the case when incubated in the dark. When immersed, the sediment surface appeared smooth as the result of the deposition of mud. Under emersed conditions, the coarser silt grains were prominently present. These grains were wrapped with organic matter and bound together through threads of EPS. This was the case both in light and in dark incubated sediment. It is proposed that this latter type of EPS contributes to the increased erosion threshold of intertidal mudflats colonized by biofilms of diatoms.     

Predicting Epipelic Diatom Exopolymer Concentrations in Intertidal Sediments from Sediment Chlorophyll a

Abstract In many intertidal cohesive—sediment habitats, epipelic diatoms are the dominant microphytobenthic organisms. In such sediments, concentrations of colloidal carbohydrate [including the exopolymeric substances (EPS) produced by diatoms during motility] are closely correlated with the biomass (chlorophyll a) of epipelic diatoms. A model describing this relationship (log (conc. coll. carbo. + 1) = 1.40 + 1.02(log (chl. a conc. + 1)) was derived from published data. It was validated against published and unpublished data from 6 different estuaries, and accounted for 64.6% of the variation in sediment colloidal carbohydrate concentrations. The model was valid for intertidal habitats with cohesive sediments where epipelic diatoms constituted >50% of the microphytobenthic assemblage. In sites with noncohesive sediments, or where the microphytobenthic assemblage was dominated by other algal groups, the model was not applicable. The mean percentage of EPS in colloidal carbohydrate extracts varied between 11 and 37% for axenic cultures of epipelic diatoms (with higher values obtained during stationary phase), and between 22.7% and 24.3% for natural sediments dominated by epipelic diatoms. Assuming an EPS percentage of 25% in colloidal extracts yielded an EPS chl. a ratio of 2.62:1. Maximum rates of EPS production in diatom cultures occurred at the beginning of stationary phase (1.6–5.09 μg EPS μg⁻¹ chl a d⁻¹), with Nitzschia sigma having a significantly (P < 0.05) higher rate of production than N. frustulum, Navicula perminuta and Surirella ovata. Similar rates of EPS production were measured in the field. The dynamics of EPS production and loss on mudflats is discussed, with reference to the model and these production rates. Received: 25 February 1997; Accepted: 23 May 1997     

Microphytobenthos as a biogeomorphological force in intertidal sediment stabilization

The properties and behavior of intertidal marine sediments cannot be understood without taking their biology into account. Biological factors are important for the stability and erosion threshold of intertidal sediments as well as for sediment transport. In this paper I focus on intertidal sediments that are colonized and dominated by phototrophic microorganisms and their impact on the morphodynamics and sediment stabilization. The emphasis is on epipelic diatoms. These organisms exude copious amounts of extracellular polymeric substances (EPS) that may contribute to the stability of the sediment by gluing and binding. I review the factors that lead to the development of such microphytobenthic communities and the processes that lead to the exudation of EPS and its fate in intertidal mudflats. Epipelic diatoms exude EPS partly as the result of unbalanced growth. Extraction of EPS from cultures of epipelic diatoms yields two operational fractions. While one fraction contains largely neutral EPS, which may serve as a carbon- and energy reserve for the organism, the other is acidic and more recalcitrant to degradation. The latter EPS fraction is therefore predominant in the muddy sediment and may be responsible for increasing the erosion threshold. However, since extracted EPS alone is incapable of increasing the erosion threshold, diatoms are apparently actively involved in the structuring of the biofilm matrix. Therefore, sediment stabilization cannot be attributed simply to EPS alone.     

Diatoms in recent Atlantic (20° S to 70° N latitude) sediments: abundance patterns and what they mean

Valve abundances of marine planktonic, marine benthic and freshwater diatoms in 780 sediment surface samples from the North and Equatorial Atlantic between 20° S 75° W and 70° N 10° E display patterns that do not correspond to published records of primary production in the photic zone. On the contrary, their abundance is more closely related to dissolution and dilution by terrigenous minerogenic and biogenic carbonate material. Highest abundances of marine planktonic diatoms occur in the northern North Atlantic south of Iceland and in the equatorial regions. The central North Atlantic and the central Caribbean, which feature lower primary production in the photic zone, contain few or no diatoms in surface sediments. African derived freshwater diatoms are concentrated in a lobe between 5° S and 20° N latitude decreasing in numbers from the African continent westward.Lars Gronlien died unexpectedly in January 1992.     

Qualitative and Quantitative Interactions between Microphytobenthos and Herbivorous Meiofauna on a Brackish Intertidal Mudflat

Micro- and meiofauna are the predominant consumers of diatoms on a brackish intertidal mudflat. The impact of grazing on the benthic diatom populations was studied by field observations and feeding experiments on a few abundant members of the community. Only small fractions of the microphytobenthic biomass and production are converted by herbivores. A hypothesis is presented explaining the growth kinetics and productivity of diatom populations and the inefficient transfer of carbon into herbivore foodchains. Data on feeding rate and population dynamics of the nematode species, Eudiplogaster pararmatus, are discussed in view of the seasonal succession of edible diatom species.     

Climate change, parasitism and the structure of intertidal ecosystems

Evidence is accumulating rapidly showing that temperature and other climatic variables are driving many ecological processes. At the same time, recent research has highlighted the role of parasitism in the dynamics of animal populations and the structure of animal communities. Here, the likely interactions between climate change and parasitism are discussed in the context of intertidal ecosystems. Firstly, using the soft-sediment intertidal communities of Otago Harbour, New Zealand, as a case study, parasites are shown to be ubiquitous components of intertidal communities, found in practically all major animal species in the system. With the help of specific examples from Otago Harbour, it is demonstrated that parasites can regulate host population density, influence the diversity of the entire benthic community, and affect the structure of the intertidal food web. Secondly, we document the extreme sensitivity of cercarial production in parasitic trematodes to increases in temperature, and discuss how global warming could lead to enhanced trematode infections. Thirdly, the results of a simulation model are used to argue that parasite-mediated local extinctions of intertidal animals are a likely outcome of global warming. Specifically, the model predicts that following a temperature increase of less than 4 degrees C, populations of the amphipod Corophium volutator, a hugely abundant tube-building amphipod on the mudflats of the Danish Wadden Sea, are likely to crash repeatedly due to mortality induced by microphallid trematodes. The available evidence indicates that climate-mediated changes in local parasite abundance will have significant repercussions for intertidal ecosystems. On the bright side, the marked effects of even slight increases in temperature on cercarial production in trematodes could form the basis for monitoring programmes, with these sensitive parasites providing early warning signals of the environmental impacts of global warming.     

Topographic effects of coastal seas on the composition of the culturable bacterial communities in marine sediments

Five published indirect methods to estimate benthic secondary production of intertidal mudflats and a new proposed formulation based on quarter-power allometric scaling and the “Universal Temperature Dependence” of biological processes (UTD) were compared. For this purpose, a dataset consisting of an annual series of samples, taken from the Lagoon of Venice from March 1996 to March 1997, at sites characterised by different seagrass coverage was used. All methods resulted in that biomass and secondary production decreased progressively when moving from the seagrass meadow toward areas of unvegetated substrate, suggesting an influence of the available marine phanerogams on the neighbouring sites. The equation proposed in this paper gives results comparable with those obtained using empirical regression models from literature. The main conclusion from this study is that general equations proposed by the “metabolic theory of ecology” can be applied for indirect estimations of secondary production of benthic communities.     

Spatial Synchrony in Intertidal Benthic Algal Biomass in Temperate Coastal and Estuarine Ecosystems

Microphytobenthos plays a vital role in estuarine and coastal carbon cycling and food webs. Yet, the role of exogenous factors, and thus the effects of climate change, in regulating microphytobenthic biomass is poorly understood. We aimed to unravel the mechanisms structuring microphytobenthic biomass both within and across ecosystems. The spatiotemporal distribution of the biomass of intertidal benthic algae (dominated by diatoms) was estimated with an unprecedented spatial extent from time-series of Normalized Differential Vegetation Index (NDVI) derived from a 6-year period of daily Aqua MODIS 250-m images of seven temperate, mostly turbid, estuarine and coastal ecosystems. These NDVI time-series were related to meteorological and environmental conditions. Intertidal benthic algal biomass varied seasonally in all ecosystems, in parallel with meteorology and water quality. Seasonal variation was more pronounced in mud than in sand. Interannual variation in biomass was small, but synchronized year-to-year biomass fluctuations occurred in a number of disjointed ecosystems. Air temperature explained interannual fluctuations in biomass in a number of sites, but the synchrony was mainly driven by the wind/wave climate: high wind velocities reduced microphytobenthic biomass, either through increased resuspension or reduced emersion duration. Spatial variation in biomass was largely explained by emersion duration and mud content, both within and across ecosystems. The results imply that effects on microphytobenthic standing stock can be anticipated when the position in the tidal frame is altered, for example due to sea level rise. Increased storminess will also result in a large-scale decrease of biomass.     

Feeding effects of the keystone deposit feeder Ilyanassa obsoleta (Neogastropoda,Gastropoda) on sedimentary diatoms

Microphytobenthos (MPB), typically comprised mainly of diatoms, is a key contributor to nearshore energy flow and nutrient cycles. Deposit-feeding invertebrates are known to alter the structure and activity of MPB. The eastern mud snail Ilyanassa obsoleta can reach extremely high densities in estuaries of the northwestern Atlantic, and their deposit-feeding and locomotion strongly influence other invertebrates and microbes. Our objective was to explore quantitative and qualitative effects of this keystone deposit-feeder on diatoms of intertidal sediments. We gathered snails from mudflat and sandflat habitats and collected their fresh fecal pellets in the laboratory. DNA metabarcoding allowed us to characterize diatom assemblages of ingested sediments and feces. We noted selective feeding such that reduction in MPB biomass with gut passage was difficult to quantify. Diatom α-diversity was reduced with gut passage in snails from both sedimentary regimes. Mudflat and sandflat diatom assemblages were distinct and differed markedly between feces and sediment in mud-feeding snails, whereas the difference in sand-feeding snails was minor. The sandy habitat was dominated by a mix of epipelic and epipsammic diatoms. In contrast, mudflat samples were dominated by epipelic and planktonic diatoms. Compositional differences between sediment and feces reflected preferential removal of planktonic taxa. Our results suggest the importance of phytodetritus to the mud snail diet, particularly in hydrodynamically quiescent environments. Due to the natural spatial patchiness of the snails and the capacity for rapid microbial recolonization, field experiments are recommended to determine whether MPB community changes attributed to gut passage are manifested at the landscape scale.     

Evidence for gravitactic behaviour in benthic diatoms

Vertical migration by diatoms is a well-known phenomenon, occurring in intertidal and subtidal benthic biofilms. It is partially endogenously driven, as cell movements can be observed in the absence of external stimuli such as light, temperature or water cover. Although vertical migration of diatoms under constant conditions has often been attributed to geotactic orientation, this hypothesis has never been experimentally demonstrated. Our study tested the gravitactic nature of the vertical migratory behaviour of benthic diatoms in sedimentary biofilms, using an experimental setup designed to distinguish gravitaxis from surface-oriented cell movements. The hourly variation of surface diatom biomass during migratory cycles was compared in homogenized sediment samples kept facing upwards (surface-oriented and gravity stimuli coinciding; controls) and facing sideways or downwards (surface-oriented and gravity stimuli not coinciding). During the experiments, sediment samples were kept in complete darkness in custom-made, sealed measuring chambers designed to avoid any contact with atmospheric air and the formation of physico-chemical gradients near the surface. Microalgal biomass was monitored non-intrusively using PAM fluorometry, by measuring dark-level fluorescence, F_o. The results showed a clear effect of sample orientation in relation to the gravitational stimulus. In the controls, a biphasic pattern in surface biomass was observed, with the formation of a clear biomass peak (three- to six-fold increase) followed by a slower decrease. In contrast, in samples facing sideways or downwards, surface biomass also varied but to a much lesser extent (typically < two-fold). These results strongly suggest that, in the absence of light, upward vertical migration of benthic diatoms is mostly guided by negative gravitaxis, supporting the often hypothesized capacity of these cells to sense and use gravity to move vertically within the sediment.     

Spatial Patterns of Benthic Macroinvertebrates in Intertidal Areas of a Southern European Estuary: The Tagus, Portugal

This study characterizes the composition and spatial distribution patterns of the benthic macrofauna in the intertidal mudflats of the Tagus estuary, western Portugal. A total of 68 species, more than 226,000 specimens with a total wet weight biomass of approximately 1170 g were identified in 380 sites. The species Streblospio shrubsolii, Cyathura carinata, Tharyx sp., Hydrobia ulvae and Tubificids were the most common and abundant. Scrobicularia plana strongly dominated the biomass. The invertebrate macrofauna of the Tagus estuary shows similarities to what is known from other temperate mudflats. The diversity of species, their overall abundance and the ratio of Molluscs plus Crustaceans to Polychaete species corroborate the distinctiveness between temperate and tropical mudflats and sandflats. The spatial distribution of the fauna reflects the sediment characteristics but the relationship between the environmental and the biological data is not as strong as obtained for sublittoral areas. This relationship diminishes from the sublittoral shelf to sublittoral estuarine areas, showing minimum values in this study, suggesting that such a relationship is less straightforward as natural disturbance increases. Nevertheless, a mixture of grain-size, elevation (inundation time) and particular habitats (relic oyster beds) form the best explanatory factors for the spatial distribution patterns of the intertidal benthic macrofauna of the Tagus estuary.     

<Emphasis Type="Italic">Spartina maritima</Emphasis> influence on the dynamics of the phosphorus sedimentary cycle in a warm temperate estuary (Mondego estuary,Portugal)

During the last decades the Mondego estuary has been under severe ecological stress mainly caused by eutrophication. In this salt march system, Spartina maritima covers about 10.5 ha of the intertidal areas. The objective of the present study was to evaluate the effect of Spartina maritima marshes on the dynamics of phosphorus (P) binding in the surface sediment. We compare phosphate and oxygen fluxes, P-adsorption capacity, phosphate concentrations and total amount, and the extractable P forms in the upper 20 cm of sediment in vegetated sediment with adjacent mudflats without vegetation. Sediment pore-water profiles followed a clear trend, with lower P concentrations in more superficial layers, and increasing with depth. The vegetated mudflats presented lower concentrations of dissolved inorganic phosphorus than adjacent bare bottom mudflats, lower phosphate total amount, as well as higher P-adsorption capacity. Results from the extraction procedure show that the superficial layers are the most important for estuarine phosphorus dynamics, since maximum concentrations of labile P pools are present here. In contrast, higher proportions of refractory P pool are found in deeper layers. Spartina marsh sediments had less total P, less iron bound P, and less exchangeable P than adjacent bare bottom mudflats. Also the pool of loosely sorbed P is lower in the Spartina marsh. Phosphate regeneration from the sediment to the overlying water was only 11.8 kg ha⁻¹ year⁻¹ in vegetated sediment while 25.8 kg ha⁻¹ year⁻¹ in the bare mud flat. Plant uptake for growth combined with an enhanced P-adsorption capacity of the sediment, may explain these differences. Therefore, Spartina marshes are very important agents in the sedimentary P cycle worldwide, and can be considered a useful management tool in estuarine ecosystem recovery efforts.     

利用牧草生长-消费模型优化草场放牧方案

从理论上探讨了草场生态系统牧草的生长过程和消费过程,采用数学模型方法模拟了两者的动态变化规律,分析了两者在草场生态系统生态平衡中的作用机制．运用牧草的生长和消费模型模拟特定生产周期内草场生态系统的累积牧草消费量,提出优化的草场放牧方案。为实际生产提供参考依据．     

The surficial sediments of Königshafen-variations over the past 50 years

The investigation highlights the untypical nature of the intertidal sediments of the sheltered Königshafen embayment in the north of the island of Sylt. The area is characterized by medium sand, which might be explained by aeolian input from the adjacent dunes. Mudflats and sandy mudflats build up the western part of the bay. Other sandy mudflats, all with a small spatial distribution, are associated with mussel beds. A peculiarity of the sediments in the Königshafen embayment are the gravel and cobbles along the northern shore and parts of the southern shore. These can be interpreted as residual sediments from former stages of the “Ellenbogen” spit. A comparison of the present results with earlier studies by Wohlenberg (1937) and Felix (1981) shows a decline in mudflats and an increasing consolidation of the mud in the period up to 1981. Since 1981 the sediment distribution has not changed, hence this tidal embayment can today be qualified as representing a relatively stable depositional area.     

Environmental and behavioural factors affecting activity in the intertidal gastropod Hydrobia ulvae

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvae (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m² h⁻¹ snail⁻¹). Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1-15 μg g⁻¹. Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvae, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment-water interface for various in situ conditions.     

Modeling population dynamics in a microbial consortium under control of a synthetic pheromone‐mediated communication system

Microbial consortia can be used to catalyze complex biotransformations. Tools to control the behavior of these consortia in a technical environment are currently lacking. In the present study, a synthetic biology approach was used to build a model consortium of two Saccharomyces cerevisiae strains where growth and expression of the fluorescent marker protein EGFP by the receiver strain is controlled by the concentration of α‐factor pheromone, which is produced by the emitter strain. We have developed a quantitative experimental and theoretical framework to describe population dynamics in the model consortium. We measured biomass growth and metabolite production in controlled bioreactor experiments, and used flow cytometry to monitor changes of the subpopulations and protein expression under different cultivation conditions. This dataset was used to parameterize a segregated mathematical model, which took into account fundamental growth processes, pheromone‐induced growth arrest and EGFP production, as well as pheromone desensitization after extended exposure. The model was able to predict the growth dynamics of single‐strain cultures and the consortium quantitatively and provides a basis for using this approach in actual biotransformations.     

Response of an estuarine ecosystem to reduced organic waste discharge

Since the mid-19^th century large amounts of organic wastewere sluiced out into the Ems Estuary on the border between Germany and TheNetherlands. This waste originated from the regional potato flour and cardboardindustries making the inland waterways completely anoxic. In the estuaryseriousoxygen depletion occurred, especially during autumn. Most of the organic wasteentered the estuary in the Dollard, a brackish embayment of the Ems Estuary. Anintensive sanitation scheme was started in the 1970s, leading to a stepwisereduction of the organic waste load on the estuary. In this paper, a review isgiven of the response of the benthos living at intertidal mudflats,viz. microphytobenthos (diatoms), meiofauna (nematodes)andmacrozoobenthos. The benthos response is described mainly on the basis of dataobtained under conditions of high (ca. 1980), intermediate (1987) and largelyreduced (1993) organic waste loading.Reduction of organic loading caused significant changes in abundance, speciescomposition and standing stock of diatoms and nematodes. Macrobenthicpopulations recovered from being severely reduced regularly during the autumnalwaste discharges towards a more stable situation. Altogether, the intertidalmudflat benthos changed from organic waste loading stress towards a normal,estuarine environmental stress. This was especially the case at high and muddyflats in the southeast of the Dollard. At mixed sand/mud flats in the centre ofthe Dollard, a response of macrozoobenthos may have been obscured by theinvasion of the polychaete Marenzelleria cf.wireni.     

Microphytobenthos,their Extracellular Polymeric Substances,and the Morphogenesis of Intertidal Sediments

Intertidal sediments are important areas that separate the land from the sea and form natural coastal defenses. They are known as highly productive ecosystems, fueling the coastal food web. It is also conceived that microphytobenthos contribute to the stability of intertidal sediments by increasing the erosion threshold and that they are major players in coastal morphodynamics. Depending on the sedimentary composition of intertidal flats, different types of microphytobenthos colonize the sediment surface. Fine sand sediment is often colonized by cyanobacteria, prokaryotic algae, which form dense and rigid microbial mats. Mudflats on the other hand are characterized by the development of thin biofilms of epipelic diatoms. Both groups of phototrophic microorganisms excrete extracellular polymeric substances (EPS), but they do so in different ways and for different reasons. Two operationally defined fractions, water- and EDTA-extractable EPS, have been obtained from intertidal diatom biofilms and from cultures. They differ in composition and their production seems to be under different metabolic control. Water-extractable EPS are considered to be closely associated with the diatoms and are rich in neutral sugars, notably glucose. These EPS show a dynamic relationship with the microphytobenthic biomass. EDTA-extractable EPS are tightly bound to the sediment, probably through bridging by divalent ions. This material is rich in uronic acids and other acid sugars and is weakly related to chlorophyll. These EPS have been conceived to be a major factor in the structuring and diagenesis of coastal sediments and essential for increasing the sediment erosion threshold. However, this relationship is now questioned.     

The intertidal distribution of the grey mangrove (Avicennia marina) in southeastern Australia: The effects of physical conditions,interspecific competition,and predation on propagule establishment and survival

Abstract The upper and lower limits of the distribution of mature Avicennia marina lie between mean high water and mean sea level in open estuaries in southeastern Australia. Newly established seedlings are highly variable in abundance, but are rarely found in the saltmarsh or on mudflats. Their distribution is unlikely to be limited by dispersal because propagules disperse into the saltmarsh and to intertidal mudflats, but their establishment may be limited by physicochemical conditions, interspecific competition and predation. The model that physicochemical conditions control the intertidal limits of establishment of seedlings was accepted for propagules stranding in the saltmarsh but rejected for those stranding on mudflats. No seedlings established on saltmarsh sediments but similar numbers of seedlings established within light gaps in adult mangrove stands and on intertidal mudflats. The model that interspecific interaction with freeliving macroalgae (Hormosira banksii) reduces the establishment of seedlings on mudflats covered with macroalgae or in stands with a ground cover of macroalgae was accepted. Under controlled conditions five times as many propagules established on cleared ground compared with ground covered with macroalgae. Predators also reduce seedling establishment, but the model that they preferentially act on propagules stranding on the mudflat was rejected. The low number of seedlings found on mudflats without macroalgae appears to relate to wave and current effects on establishment and the effects of waterlogging or fouling on survival.