Distribution and abundance of the zooplankton of the Ems estuary (North Sea)

From 1974 to 1977 a study was made of the abundance and the distribution of the zooplankton species of the Ems estuary (The Netherlands and the Federal Republic of Germany), an area of about 500 km² with extensive tidal flats.The most important component of the zooplankton consisted of holoplanktonic calanoid copepods with, during the summer, a significant contribution of meroplankton, mainly consisting of polychaete and cirripede larvae. Zooplankton abundance showed a marked seasonality, with a pronounced spring peak and a smaller late summer/autumn peak. In the low salinity area the spring bloom was dominated by Eurytemora affinis, which persisted in the salinities below S=5 upriver during the whole year. In the polyhaline area Acartia bifilosa was the main component of the spring bloom. The late summer maximum from August to October was dominated by Acartia tonsa in the meso- and polyhaline area and by A. discaudata and Centropages hamatus in the euhaline part of the estuary bordering on the Wadden Sea proper. The number of abundant (> 1000 ind·m⁻³) species increased from the inner, fresher part of the estuary towards the Wadden Sea but, except for April, average zooplankton density in salinities <18 was similar (2768 ind·m⁻³) to the density in salinities >18 (2817 ind·m⁻³).The distribution patterns show that there are only a few autochthonous species and that the majority of species is allochthonous and penetrates more or less deeply into the estuary from the Wadden Sea and North Sea, varying with species and season.     

What determines the densities of feeding birds on tidal flats? A case study on dunlin,Calidris alpina,in the Wadden Sea

The tidal and seasonal pattern of habitat utilization by dunlin was studied in a tidal-flat area in a shallow bay called Königshafen, island of Sylt, by counting the number of dunlins on marked plots at 10-min intervals over whole tidal cycles. Sediment type, tidal elevation, and seasonal changes in food choice were found to influence the feeding densities of dunlin irrespective of total numbers present in the area. Densities of dunlin were generally highest on low muddy substrates. A preference for high sandy substrates was found in late summer. The tidal pattern of habitat utilization differed between areas. In preferred areas dunlin stayed during the whole emersion period. Other areas were only used by following the moving tide line. Seasonal changes in habitat utilization were apparently triggered by changes in food choice. In spring dunlin fed almost exclusively on polychaetes. In late summer a relatively high proportion of shrimps, Crangon crangon, was found in the diet of dunlins. The preference for shrimps may force the birds to stay away from the tide line, where the shrimps can escape into deeper waters. It is recommended that studies on habitat utilization on tidal flats should consider the tidal behaviour of the target species, as single low-tide counts may not give valid results.     

The importance of rivers as nursery grounds for 0- and 1-group flounder (Platichthys flesus L.) in comparison to the Wadden sea

From May 1988 to June 1989 selected areas of the German Wadden Sea, the Ems, Weser, Elbe and Eider estuaries as well as in Elbe tributaries were sampled for 0- and 1-group flounder (Platichthys flesus L.). Estuarine and Wadden Sea sampling was conducted on board commercial shrimp vessels equipped with beam trawls. The Elbe river system was sampled on board a research boat equipped with a similar 3-m beam trawl. Density indices were calculated for the various areas. To estimate the importance of rivers as nursery areas for flounder, river surface area was compared to the area of tidal flats in the Wadden Sea.In all surveys, there was no relationship between fish densities and tow directions with respect to tide. In the Elbe river system flounder densities were not correlated to substrate types but increased significantly with decreasing salinity. Abundances were always lowestt in polyhaline habitats and increased up to 10-fold in mesohaline estuarine areas. In the limnetic tidal sections of the Elbe river and its tributaries densities increased again by factors of 3 to 10. The succession of 1-group modal lengths from limnetic to polyhaline habitats demonstrated that smaller fish preferred less saline waters. 0-group specimens lagging behind in growth did not leave the limnetic river section in winter. The onset of the spawning migration in November was determined by monitoring the length-frequency distributions throughout the year.The possible sources of bias are discussed and gear efficiency is estimated from literature sources. The contribution of rivers to the 0- and 1-group flounder population on the tidal flats of the Wadden Sea is estimated at about 35.1%.     

Suspended-sediment transport in the Dollard estuary

For an analysis of suspended-sediment transport in the Dollard estuary, water samples were taken and current velocities and directions measured on the tidal flats and in the channels. Measurements carried out during fair-weather conditions in summer were compared with measurements carried out during a heavy gale in winter. During the onset of the flood, high suspended-sediment concentrations occurred as a result of resuspension of the top layer of the bottom sediment. Measurements carried out in the channels pointed to sediment transport from the estuary seaward, but inward transport over the flats during fair weather apparently compensates for the outflow during ebb and storms: the result is a net deposition in the order of 1 to 2 mm (up to 8 mm) per year, as previously observed in this area.     

Environmental characteristics and distribution of macrobenthos in a mudflat of the west coast of Korea (Yellow Sea)

The spatio-temporal changes of environmental factors and the distribution patterns of macrobenthos were investigated on an intertidal mudflat near Panweol on the west coast of Korea. Temperatures of the surface sediments and salinities of the intertitial water were highest in summer and lowest in winter, while water content of the sediment was lowest in summer and highest in winter. Hourly variations of sediment temperature, water content and interstitial salinity during the daytime depended largely upon both the timing of ebb tide and daylight intensity.Temperature, water content and interstitial salinity showed a marked variation with sediment depth. Temperatures decreased sharply from the surface to 10 or 15 cm depth, and below this depth they were nearly constant, except in summer. Water content and interstitial salinity underwent a rapid decline in this upper layer and began to rise from this depth. From the high tide mark toward the main tidal channel, water content increased but salinity decreased. The sand fraction increased with the decrease of clay content toward the main tidal channel. This tidal flat could be divided into two zones: the “upper intertidal zone” and the “lower intertidal zone”, based on the spatial differences of the water content, interstitial salinity and grain-size composition in the surface sediment.There were three distinct zones of macrobenthos. The upper intertidal zone was dominated by burrowing deposit feeders, such as Ilyoplax dentimerosa, Helice tridens sheni, Cleistostoma dilatatum and Perinereis vancaurica tetradentata. The middle intertidal zone was characterized by other burrowing deposit feeders, such as Ilyoplax pingi, Macrophthalmus japonicus and Periserrula leucophryna. And the lower intertidal zone was dominated by suspension feeders represented by Laonome tridentata and Potamocorbula amurensis. Each zone was named after these characteristic species.Although the species composition of the upper intertidal community was different from that of the middle intertidal, the way of living and the feeding behaviour of the species occurring in these two zones were very similar. But the lower intertidal zone showed a characteristic community structure differing not only in species composition but also in ways of living and feeding of the species. Some sets of environmental factors are considered to explain the zonation. Water content of the sediment and interstitial salinity will be the main factors causing the observed zonation.     

The influence of environmental conditions on settlement,distribution and growth of 0-group sole (Solea solea (L.)) in a macrotidal estuary (Vilaine,France)

In the Vilaine estuary, the environmental conditions at the onset of the settlement of metamorphosing sole have been surveyed for 5 years. The inshore migration occurs either in early or in late April depending on the hydroclimatic conditions: sole are observed in the estuary when bottom water salinity varies between 25 and 30 S and water temperature is up to 11°C. During spring, their distribution pattern is similar in all years: first, accumulation at the entrance of the estuary, then concentration in the upper parts and finally, colonization of the whole estuarine area by juveniles. Interannual variations in growth rates were observed during their first estuarine phase, in particular when the estuary is transformed into a ‘ria’ with high water temperature and salinity. These biological features are discussed according to ontogenic changes occurring during these early life stages. From field and experimental data and from literature information on sole and other pleuronectiform species, a conceptual model on relationships between environmental parameters and metamorphosis processes in estuarine areas is proposed.     

Changes in suspended-matter floc size during the tidal cycle in the dollard estuary

Measurements of in situ particle size of suspended matter in a tidal channel in the Dollard (Dutch Wadden Sea) indicated systematic variations in floc size during the tidal cycle that can be explained by assuming settling during slack tide, resuspension during the early ebb and early flood, flocculation of fine particles into large ones during most of the tide, and deflocculation of large flocs into smaller particles during or after settling to the bottom. There was a characteristic difference in floc-size variation during ebb and during flood. During the ebb maximum floc size coincided with maximum suspended-matter concentration (maximum collision frequency); during the flood maximum floc size continued to increase towards high-water slack tide. The adjacent tidal flats had a marked influence on floc size: where the flats were small, the highest percentages of large flocs (>F128 μm) occurred around slack tide when current velocities are low. Where the adjacent flats were broad and extensive, high percentages also occured during intermediate periods. Flocs of maximum size are probably not in equilibrium with the bulk of the suspended matter: they are mainly formed during periods of high suspended-matter concentrations of short duration.     

Animal traces on a tidal flat in Hangzhou Bay,China

Despite prevailing low salinity (S ranging from 1 to 10) of the Hangzhou Bay, its extensive (up to 2 km wide) flats along the northern coast have a species-rich macrobenthic fauna of crustaceans, bivalves, gastropods and polychaetes. These produce a variety of crawling, feeding, dwelling and resting traces, some comparable to those observed on the flats of Korea and Taiwan. The characteristic starlike feeding traces of the bivalve Tellina (Moerella) iridescens are described here for the first time. Sediment reworking appears to be largely confined to the surface layer. The burrow-inhabiting crabs are also mainly surface sediment feeders. These Chinese flats lack organisms reworking deeper sediment layers such as lugworms or callianassid crustaceans. As compared to the Wadden Sea, sedimentation rate is higher and subsurface sediment reworking less intense, resulting in relatively few bioturbation structures in vertical sections of the Hangzhou tidal flat sediments.     

Settlement,abundance, growth and mortality of juvenile flatfish in a subtropical tidal estuary (Georgia,U.S.A.)

The population dynamics of juveniles of some flatfish species were studied in the Duplin River, a tidal creek in a subtropical salt-marsh area in Georgia, U.S.A. from April until September 1990. Seven species were found. Paralichthys dentatus, Paralichthys lethostigma, Paralichthys oblongus and Trinectus maculatus were relatively rare. Etropus crossotus, Citharichthys spilopterus and Symphurus plagiusa were abundant and settled during the period studied. E. crossotus was the most abundant species with a mean abundance of 18 ind·10⁻² m⁻² (max. 287). Demersal settlement of E. crossotus took place in shallow areas and over sandy bottoms from mid-May to August. Prolonged settlement hampered the calculation of growth rate and instantaneous mortality rate. However, laboratory growth experiments indicated a mean growth of about 0.50 mm·d⁻¹ at 24–28°C. Juveniles of C. spilopterus were already present in the Duplin River in March. Settling continued until the end of April with a mean abundance of 3.5 ind·10⁻² m⁻² (max. 183). With increasing size the juveniles of this species tended to migrate to deeper waters and to the mouth of the river, possibly as a reaction to increasing water temperatures. Maximum growth rate was 1.4 mm·d⁻¹ at about 26°C. The mean instantaneous mortality rate (z) was estimated at 0.03·d⁻¹. Settling of S. plagiusa occurred from mid-May onwards. The mean abundance was 10.3 ind·10⁻² m⁻² (max. 98.3). Newly settled juveniles were most abundant on muddy sediments in the shallow river areas. The maximum growth rate was 1.3 mm·d⁻¹ at about 28°C. The mean instantaneous mortality rate (Z) decreased from 0.04·d⁻¹ in April to 0.01·d⁻¹ in August. At all sites the abundance of juveniles of this species decreased with increasing water depth. Predation experiments indicated that blue crabs (Callinectes similis and C. sapidus) and sea robins (Prionotus sp.) are potential predators on juvenile flatfish. The high abundances of juvenile flatfish indicate that the tidal creeks are an important nursery area. The correspondence between growth rates estimated from field data and those observed in the laboratory suggests that growth in the nursery is mainly related to water temperature and not food limited.     

Commercial fishing practices on offshore juvenile flatfish nursery grounds on the Grand Banks of Newfoundland

Analyses of the 1985 to 1989 juvenile flatfish survey data from the Grand Bank off the east coast of Newfoundland has delimited two major areas of juvenile American plaice, Hippoglossoides platessoides, concentrations: one on the northern slope and one on the southern edge of the bank. A major concentration of juvenile yellowtail flounder, Limanda ferruginea, was distributed in the area of the Southeast Shoal on the southern Grand Bank. Juveniles of both species shared their distribution with their adult population and there was some overlap by juveniles of both species on the southern Grand Bank. These areas of concentrations are hypothesized to be oceanic nursery sites for both Grand Bank species.The northern plaice nursery area is fished heavily by the Canadian otter-trawler fleet because the majority of the adult population is located in this area. The southern nursery area for plaice is fished heavily by many otter-trawl fleets of other nations since it is located outside of Canada's 200-mile fishing boundary. The southern nursery area for juvenile yellowtail flounder straddles the 200-mile boundary and, consequently, is fished heavily by Canadian (inside) and non-Canadian (outside) fleets. The implications of these dual fisheries on the southern Grand Bank is discussed in terms of discarding of under-sized fish by the Canadian fleet inside the 200-mile boundary and the directed effort on juveniles outside the boundary.     

气候因子对滩涂围垦区不同盐分水平下土壤盐分季节性变化的影响

为探明滩涂围垦区气候因子对不同盐分水平下土壤盐分的变化规律,于2014年5月—2015年5月在江苏如东滩涂开展试验,设低、中、高3个盐分水平,监测了土壤盐分和气候因子的动态变化。结果表明：（1）在中-高盐水平下,表层盐分均表现为在9—12月有明显积盐作用,且在10月达到最大值分别为26.66 g/kg和14.13 g/kg;在低盐下,表层和亚表层盐分均无明显的季节性变化规律,而分别在0.50~2.00 g/kg和1.20~2.90 g/kg之间波动;（2）在中-高盐水平下,表层盐分变化均与降雨量有显著的负相关,与蒸降比有极显著的正相关;在低盐下,土壤盐分与大气温度有明显的正相关。因此,若采取适当的人为措施如种植植被、覆盖等加剧降雨量对土壤盐分变化的影响,充分利用沿海降雨量比较集中的特点,可能是未来沿海滩涂降盐改土的重要措施。     

春季生物膜对营养盐在水-沉积物界面扩散的影响

为了明确春季大沽河河口潮滩养殖功能的退化是否由食物（底栖藻类）匮乏所致,2009年5月在大沽河河口潮滩采集柱状沉积物样,利用氯化汞抑制沉积物表层生物膜的活性,研究生物膜对营养盐在水-沉积物界面迁移扩散的影响程度。结果表明,沉积物既可以是水体中营养盐的“源”也可以是其“汇”;在河道的近海侧,沉积物表面的生物膜会促进磷酸盐自沉积物向水体扩散;在其余区域,沉积物表面的生物膜会抑制营养盐自沉积物向水体扩散;在该河口潮滩,沉积物表层的生物膜均会抑制营养盐自沉积物向水体扩散。研究表明,春季大沽河河口潮滩贝类底播养殖功能的退化,并非食物短缺所致。     

Community structure and biomass distribution of seagrasses and macrofauna in the flores sea,Indonesia

In several locations in the Flores Sea region the community structure and the biomass distribution of seagrasses were studied along transects perpendicular to the shoreline. The share of each species within a sample plot was estimated, divided in above- and below-ground biomass. Statistics regarding substrate coverage, shoot density and leaf-area index were sampled. A standard relation was calculated between seagrass dry weight, ash-free dry weight and organic carbon content.The biotic data were related to environmental factors: DOC and nutrients in the water, salinity, tidal amplitude, sediment composition. A relation was estimated between bottom coverage of seagrasses and standing stock. Further calculations of biomass-production ratios allow a quick and rough estimate of seagrass productivity. Maximum above-ground biomass values (500–700 g AFDW·m⁻²) together with qualitative data indicate resource (= space) partitioning among the component seagrasses within a community, and suggest a carrying capacity of the reefflat habitat for seagrass density and biomass.A tentative model was constructed, starting from a constant, non-distributed multispecies vegetation in the lower intertidal and subtidal zone on sand and coral rubble, and moving into several suboptimal situations. The upper shore carries an impoverished, constrained vegetation (irregular tides, desiccation, harvesting). Sediment reworking by animals and physical displacement of sand disturbs the vegetation and favours pioneer species. Muddy habitats bordering mangroves carry monospecific stands showing extremely high biomass (e.g. below-ground Enhalus acoroides 3500 g AFDW·m⁻²). Thalassia hemprichii and Enhalus acoroides are the most constant species in all habitats mentioned.Macrofauna biomass within the seagrass beds fluctuated widely (maximum values 50–70 g AFDW·m⁻² in mixed seagrass vegetations) and only a weak relation between benthic macrofauna biomass and seagrass community structure and biomass was found.     

Experimental determination of salt intrusion mechanisms in the Volkerak estuary

Salt transport mechanisms in the Volkerak estuary, The Netherlands, are analysed by use of the results of simultaneous measurements of current and salt concentrations in 32 verticals distributed over six cross-sections.Longitudinal salt fluxes in each cross-section are evaluated by interpolating measured values of current speed and salt concentration over the cross-section and in time.Salt fluxes are decomposed in an advective flux, a local salt flux (covariance of the deviations of velocity and salt concentration from the cross-sectional mean) and a non-local salt flux (covariance of the tidal variation in current speed and salt concentration). From a theoretical analysis it follows that the non-local flux in a cross-section equals the difference between the local salt flux in that cross-section and the local salt flux in a cross-section moving with the tidal velocity. Because of the relatively large tidal amplitude to depth ratio the existing decomposition techniques are expanded to include cross-sections with time-varying water levels.The field data and subsequent analysis reveal that in the seaward part of the estuary the advective salt flux is balanced by the non-local dispersive flux. Salt intrusion in this region is caused by large scale exchanges of water bodies between the Volkerak estuary and Eastern Scheldt estuary, between different tidal channels at channel junctions and between tidal channels and tidal flats. In the landward part of the estuary gravitational circulation is the dominant salt intrusion process, reflected most strongly in the local salt flux component associated with steady vertical variations in velocity and salt concentration.     

The distribution of suspended matter in the Dutch coastal zone

Results are presented of suspended matter observations taken at bi-weekly intervals during the period 1975–1983, in a strip 70 km wide along the Dutch coast. The average distribution showed a (weak) minimum zone located north of Noordwijk at a distance of about 30 km from and parallel to the Dutch coast, between the salinity maximum and the coast. Variability was large between different years, seasons and individual cruises. A clear seasonal cycle emerged from the observed suspended-matter patterns: in winter a pronounced minimum existed, while in summer generally a monotonically decreasing concentration was measured, to open-sea values much lower than in winter. This seasonal cycle may be explained by a combination of variation in wind, river discharge and the activity of suspended-matter sources. The Flemish Banks and Channel waters are the main sources supplying suspended matter to the Dutch coastal area. Especially the varying transport through the Strait of Dover, large in winter, small in summer, determines the seasonal variation in the total amount of suspended matter. The existence of a localized turbidity minimum is tied to relatively large discharges from the Rhine and associated steep salinity gradients, also occurring mainly in winter and early spring. A simple model of cross-shore density-driven circulation shows the possible influence of the strong salinity gradients on the sedimentation of suspended matter. Particles with a settling rate comparable to the vertical velocity component of the circulation are forced to move offshore until they reach an area they sink out of the surface layer. The strength of this circulation is determined by the cross-shore density gradient due to the inflow of fresh water from the Rhine-Meuse estuary.     

Carbon dioxide and oxygen disequilibrium in a tidal basin (Dutch wadden sea)

Measurements of total CO₂, alkalinity and dissolved oxygen were conducted in a tidal basin with estuarine properties during three cruises. In winter CO₂ and oxygen were close to equilibrium with the atmosphere. In May the basin was a large source for CO₂, where the fractional gain with respect to the freshwater CO₂ was as high as 42%. Also in September high pCO₂ indicated a release to the atmosphere; both in May and September there was accompanying low oxygen. Water originating from the tidal flats was shown to affect the water in the adjacent tidal channel significantly. Tidal-flat water was characterized by high TCO₂ and alkalinity and low oxygen as compared with the surrounding water. It develops through biological processes on the tidal flats (only in the vegetative season), as well as through the local discharge of a small freshwater source with very high alkalinity (and TCO₂). Using alkalinity as a tracer it was calculated that in March 12% of the fresh water in an adjacent tidal channel might originate from this source, although it constituted only 1.5% of the total freshwater discharge.     

Year-to-year variability in the biomass of macrobenthic animals on tidal flats of the Wadden Sea: How predictable is this food source for birds?

The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data series of annual estimates of biomass of macrobenthic animals collected on tidal flats in various parts of the Dutch and German Wadden Sea. Year-to-year fluctuations were stronger in nearly all individual benhos species than in total macrozoobenthic biomass. The various species differed significantly in their year-to-year variability. Everywhere the tellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conchilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within species, fluctuations in biomass of individual age or size classes were stronger than those in total biomass of the species.Several species showed minimal biomass values in the same years over vast areas. This synchronization of scarcity was caused particularly by similar responses to winter character, which was each year similar over the entire Wadden Sea. Such similar responses included low over-winter survival during severe winters and recruitment failure after exceptionally mild winters. Immediately after severe winters, such cold-sensitivee species as C. edule, L. conchilega and N. hombegii were scarce all over the Waden Sea, thus liimiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient quantitu. Simultaneous recruitment failure in several important bivalve species (C. edule, Mytilus edulis, and Mya arenaria limited the possibility for specialized bivalve consumers to switch to alternative prey types in certain years.     

Temperature beat on a tidal flat: Potential cue for harpacticoid bloom

Temperature was recorded in the sediment on a tidal flat in the Fraser estuary (Canada) from February 15 to July 1985, and from February 19 to July 9 in 1986, with continuous recording thermographs. Within these periods, the abundance of an assemblage of harpacticoid copepods (Tisbe spp., Zaus aurelii, Harpacticus uniremis) was monitored bi-weekly by sampling the water column with a sled-sampler. Temperature time-series of both years showed the expected fortnightly beats in the daily temperature cycle. A reversal in the direction of the change of temperature in the sediment during exposure was observed on March 12 in 1985, and on March 1 in 1986. This reversal initiated the seasonal warming beat in the sediment when exposed at lower low waters, and is caused by the precessing of the timing of the lower low waters relative to the solar day. Blooming period of harpacticoids was centred on March 21 in 1985, and on March 12 in 1986. Closeness and apparent co-phasing of warming beat initiation and bloom onset suggest that the reversal in the direction of temperature change at lower low waters exposure could be a cue for harpacticoid blooming. The predictability of the timing of this clear signal in the thermal regime could provide the basis for an unequivocal definition of the link between harpacticoid abundance and temperature on tidal flats.     

Effects on the benthic fauna of embanking an intertidal flat area (the Markiezaat,Eastern Scheldt estuary,The Netherlands)

Effects of embanking an intertidal-flat area in the south-western part of the Netherlands, the Markiezaat, on abundance and condition of benthic animals were followed at 3 stations during spring, 1983. Part of the former intertidal flats became permanently submerged with stagnant salt water, whereas another part became permanently emerged.At the submerged sampling station, no clear changes were found during the first 3 months of submersion. However, within 6 months, most species died off.At the drained stations, all individuals of most species died off within 2 months. During some preliminary experiments with drained sediment cores, a high mortality of the animals was reached within much shorter periods. During these experiments the animals showed a retarded mortality after they had been returned to a normal tidal cycle. Species living in the lower part of the intertidal zone were found to be less tolerant to drainage than those living in the higher part. The observed mortality rates in the Markiezaat probably represent minimal values, because of the heavy rainfall and the absence of extreme temperatures during drainage. At high (summer) and low (winter) temperature and low precipitation, benthic animals may be less tolerant to drainage.Neither permanent submersion nor emersion affected the condition determined as dry weight per unit of volume. Therefore, the suitability of this condition-index as an indicator of stress for intertidal benthic animals may be questioned.     

Immigration of larval plaice (Pleuronectes platessa L.) into the western wadden sea: A question of timing

Migration of larval plaice into a nursery area in the western Wadden Sea was monitored during three immigration seasons. Daily otolith increments were used to estimate age and hatch dates of the larvae, and the hatch dates were compared to egg-production curves. Different cohorts of larvae invaded the area, originating from different spawning grounds. During a cold year, most larvae probably came from the Western Channel, but during the two warm years most larvae originated from more northern spawning grounds. Settlement on a tidal flat showed a delayed response to immigration in the first part of the season, but a direct response later in the season. Length-frequency distributions of settled juveniles were compared to expected frequencies based on a simple growth model. It is argued that early in the season growth-related mortality may have been too high to ensure successful settlement, and that timing could be of significant importance for larval immigration to be successful.