Increased mortality in alternative bivalve prey during a period when the tidal flats of the Dutch Wadden Sea were devoid of mussels

In the course of 1990, stocks of mussels (Mytilus edulis) declined to unprecedentedly low levels in the Dutch Wadden Sea. Hardly a wild mussel bed was left on the tidal flats as a consequence of three years (1988, 1989, and 1990) with failing recruitment and intensive fishing for seed mussels. During these three years, recruitment of cockles (Cerastoderma edule) also failed, whereas fishing was continued. Bird species taking these bivalves as staple food, the oystercatcher (Haematopus ostralegus) and the eider (Somateria mollissima), experienced food shortage. Significant numbers of eiders left the Dutch Wadden Sea area or died, whereas oystercatchers remained abundant trhoughout the winter in most of the Dutch Wadden Sea.Alternative prey species of oystercatchers experienced unsually high mortality rates in the appropriate size classes. This was so in all other common species of bivalves, viz. first-year and older cockles, adult Macoma balthica, and juvenile Mya arenaria. This led to minimal stocks of food for oystercatchers in the late winter of 1991. In March 1991, cockles were depleted and the combined stocks Mya and Macoma would soon have run out of food supply to the overwintering oystercatcher population. Apparently, oystercatchers are able to reduce the stocks of their various bivalve prey species to very low levels.     

On the timing of foraging flights by oystercatchers,haematopus ostralegus,on tidal mudflats

The tidal movements of flocks of oystercatchers foraging on mudflats at low tide and roosting inland behind a dike at high tide were studied and the effects of day-to-day variations in the time of mudflat exposure by ebb analysed.High mean water levels and short low tides led to reduced intake during low water due to increased bird densities in addition to temporal constraints (Fig. 4). Increased feeding around the roost apparently compensated for some of the reduced intake (Figs 6 ad 7) although accurate intake measurements could be made for foraging on the tidal flats only. It is argued that optimal timing of foraging flights to coincide with exposure of the mussel banks would contribute to exploitation of this tidal food source.The median departure time from the roosts relative to the time of mudflat exposure was early on days when the tide went out late and late when the tide was early (Figs 8 and 9). Daily variations in departure time were predicted by the daily variations in tabulated high water times, but not by variations in mudflat exposure or coverage (Fig. 10). The conclusion is drawn that the birds employ a timing mechanism not directly associated with the tidal water movements.In some pilot experiments in caged oystercatchers, feeding schedules elicitated feeling attempts in anticipation of expected food. The anticipatory patterns were different for fixed and tidally shifting daily food schedules, and moreover differed between the two feeding times per day (Figs 12 and 13).Five possible mechanisms for tidal anticipation are discussed, making use either of unknown exogenous cues, or of—likewise unknown—endogenous timers of hourglass type of rhythmic with circatidal, circalunadian or circadian period. Experimental tests for these possibilities are outlined.     

The availability and quality of the mussel prey (Mytilus edulis) of oystercatchers (Haematopus ostralegus)

Spatial variations in the availability and quality of the mussel Mytilus edulis food supply of Oystercatchers Haematopus ostralegus on the Exe estuary, England, are described. Oystercatchers open mussels by stabbing into gaping mussels (or prising open closed ones) or by hammering a hole in either the dorsal or ventral shells. Spatial variations in the food supply are considered at four scales. In decreasing order of size, these are (i) whole mussel beds, (ii) zones within a mussel bed, (iii) different places within one zone, and (iv) different places within one clump of mussels. The first two scales are clearly related to exposure time. Both between and within the 12 main mussel beds of the estuary, most upshore mussels are up to 10% less likely than downshore mussels to be hidden under mud. However, upshore mussels of a given length contain less flesh, have thicker ventral shells and, except on high-level beds subject to wave erosion, have thicker dorsal shells than downshore mussels. Mussels at the top of the shore also contain the highest infestations of the helminth parasite of Oystercatchers, Psilostomum brevicolle. At a particular shore level, mussels of a given length have less flesh and thicker shells, though only on the dorsal side, in areas of high mussel density. Within one clump, mussels of a given length with thick dorsal shells have more flesh than those with thin shells. In contrast, flesh content is slightly higher in mussels that are thin on the ventral side. Flesh content and shell thickness on both sides are unaffected by whether a mussel is visible at the edge of a clump or hidden inside. Simulations with a model of foraging Oystercatchers suggested that variations in mussel availability (visibility and shell thickness) and flesh content at all four scales could sometimes have an important influence on intake rate. Most published values of intake rates of Oystercatchers eating surface-dwelling mussels may be biassed (and often considerably over-estimated) by the erroneous assumption that prey of average flesh content are taken.     

Helminth parasites of the digestive tract of the oystercatcher,Haematopus ostralegus,in the Wadden Sea,The Netherlands

The digestive tracts of 90 oystercatchers (equal numbers of males and females and of juveniles, subadults and adults) wintering in the Dutch Wadden Sea were examined for helminth parasites. The nematodes Capillaria sp. (36.7%) and Streptocara crassicauda (7.8%) were found in the stomach. Unidentified cestodes (76.7%) and the trematodes Psilostomum brevicolle (42.2%), Notocotylus sp. (81.1%), and unidentified gymnophallids (100%) were found in the intestine and caeca. Two birds were infected with Gymnophallidae only, while all other birds contained additional helminth species. Compared with subadult and adult birds, the juveniles had significantly more infections with Capillaria sp. and cestodes. Moreover, the juveniles were infected with a greater variety of species. No further relation was found between the presence of helminths or worm numbers and age groups or sexes of birds.     

Scale and intensity of intertidal habitat use by knots Calidris canutus in the Western Wadden Sea in relation to food,friends and foes

In August–October 1988–1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10 000 to 25 000 birds. Over 100 000 knots were recorded on three occassions. Knots feed in large flocks, individual birds usually experiencing 4 000 to 15 000 flock-mates. The Siberian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964–1992 there were no clear trends in the number of knots, but canutus-knots were particularly abundant in July–August 1991, whereas in 1992 both subspecies were absent.Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distributon of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year and season, i.e. Macoma (main prey when their harvestable biomass densities were greater than ca 0.8 g AFDM per m²) and Cerastoderma, and by the avoidance of situations where they run the risk of attack by bird-eating birds. Flocks of knots covered most of the intertidal flats in the Western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km², much larger than the equivalent area used by knots on their wintering grounds in Mauritania (10–15 km²), a difference that is correlated with prey spectrum, prey availability and predictability.     

How the food supply harvestable by waders in the Wadden Sea depends on the variation in energy density,body weight,biomass, burying depth and behaviour of tidal-flat invertebrates

For several reasons, waders in the Wadden Sea face a large seasonal and annual variation in their food supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that:

• 1.- (1) The average energy density of ten invertebrate prey species varies between 21 and 23 kJ·g⁻¹ AFDW. In Scrobicularia plana and Mya arenaria, but not in Macoma balthica, the energy density is 10% lower in winter than in summer.
• 2.- (2) Depending on the species, body weights of prey of similar size are 30 to 60% lower in winter than in summer.
• 3.- (3) The year-to-year fluctuation in standing-crop biomass is larger in some species than in others, the difference depending mainly on the frequency of successful recruitment. The overall biomass of the macrobenthos in winter is half of that in summer, but the timing of the peak biomass differs per species.
• 4.- (4) The burying depth varies per species: Cerastoderma edule live just beneath the surface, while M. balthica, S. plana, M. arenaria, Arenicola marina and Nereis diversicolor bury more deeply and the majority of these prey live out of reach of the bird's bill. In all six species, burying depth increases with size. There is no seasonal variation in depth of C. edule and M. arenaria, but the four other species live at most shallow depth in early summer and most deeply in midwinter. Burying depths in winter vary from year to year, but are unrelated to temperature. Neither has temperature any effect on depth within months. For knot Calidris canutus feeding on M. balthica, the fluctuation in the accessible fraction was the main source of variation in the biomass of prey that is actually harvestable, i.e. the biomass of prey of suitable size that is accessible.Accordingly, the paper reviews the available data on the temporal variations in accessibility, detectability, ingestibility, digestibility and profitability of prey for waders. Only a small part of the prey is harvestable since many accessible prey are ignored because of their low profitability, while many profitable prey are inaccessible. The profitability of prey depends on their size and weight but also on their depth in the mud, since handling time increases with burying depth. A simple biomechanical rule explains why the handling time of small prey increases with bill length and why large, long-billed waders ignore a disproportionately larger part of the small prey. The fraction detectable for visually feeding waders is usually very low, especially when the temperature of the substrate is below 3–6°C. Waders vary their prey choice over the year in response to the changes in the availability and profitability of their different prey species. The food supply harvestable by waders is much lower in winter than in summer. For waders wintering in the Wadden Sea, the food supply may be characterized as unpredictable and usually meagre. Waders wintering in NW Europe are concentrated in coastal sites where the average surface temperature is above 3°C. This probably cannot be explained by a greater burying depth, and only partly by a lower body condition, of prey in colder areas. Yet the harvestable fraction is lower in colder sites, especially for sight-feeding waders, as invertebrates are less active at low temperatures. However, the lower energetic cost of living and reduced chances of the prey being covered by ice may also contribute to the waders' preference for warmer sites.

     

Wind effects on prey availability: How northward migrating waders use brackish and hypersaline lagoons in the sivash,Ukraine

Large numbers of waders migrating northward in spring use the Sivash, a large system of shallow, brackish and hypersaline lagoons in the Black Sea and Azov Sea region (Ukraine). The bottoms of these lagoons are often uncovered by the wind. Hence, for waders the time and space available for feeding depend on wind conditions. In hypersaline lagoons the benthic and pelagic fauna was very poor, consisting mainly of chironomid larvae (0.19 g AFDM·m⁻²) and brine shrimps Artemia salina, respectively. Brine shrimp abundance was correlated with salinity, wind force, wind direction and water depth. Dunlin Calidris alpina and curlew sandpiper Calidris ferruginea were the only species feeding on brine shrimp. As brine shrimp densities are higher in deeper water, smaller waders such as broad-billed sandpipers Limicola falcinellus are too short-legged to reach exploitable densities of brine shrimp. In brackish lagoons the benthic and pelagic fauna was rich, consisting of polychaetes, bivalves, gastropods, chironomid larvae, isopods and amphipods (8.9 to 30.5 g AFDM·m⁻²), but there were no brine shrimps. Prey biomass increased with the distance from the coast, being highest on the site that was most frequently inundated. Dunlin, broad-billed sandpiper and grey plover Pluvialis squatarola were the most abundant birds in the brackish lagoon. Due to the effects of wind-tides only a small area was usually available as a feeding site. Gammarus insensibilis was the alternative prey resource in the water layer, and their density varied with wind direction in the same way as brine shrimp. Curlew sandpipers and dunlins in the hypersaline lagoons and broad-billed sandpipers in the brackish lagoons often changed feeding sites, probably following the variation in prey availability. Only because of the large size and variety of lagoons are waders in the Sivash always able to find good feeding sites.     

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.     

Changes in predation by the juvenile goby deltentosteus quadrimaculatus (teleostei,gobiidae)

Juveniles of the goby Deltentosteus quadrimaculatus hunt at sight lying in wait for their prey. They feed mainly on benthic organisms which are swallowed alive.The stomachs were analysed. The preys counted belong to 11 zoological groups of which seven are crustacean orders. The crustaceans comprise on average 92% of the total number of preys or 85% of the weight of preys.The species composition of the food changes during early growth. The postlarvae feed on zooplankton. Once the gobies reach 15 mm total length, the contribution of zoobenthos to their diet increases, to become the almost exclusive food source above 20 mm length. They then pass through two stages according to the size of their prey, feeding firstly on meiobbenthos and passing over progressively on macrobenthos. This development in the choice of food is related to the goby's behaviour.Food composition also appeared to depend on water depth. For youngs the frequency of harpacticoid copepods in the stomach contents decreased with depth whereas the occurrence of amphipods increased.The selective character of predation was also evident from the fact that the mean size of preys increased regularly with that of the gobies. This increase, even as that of the total weight of the stomach contents, followed exponential trends.     

Flocking and feeding in the fiddler crab (UCA tangeri): Prey availability as risk-taking behaviour

For a full understanding of prey availability, it is necessary to study risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on the surface of the intertidal flats during low tide, thereby exposing themselves to avian predators. A study in an intertidal area along the coast of Mauritania showed that small crabs always stayed in the vicinity of their burrow, but large crabs wandered in large flocks (also referred to as droves) to feed on sea-grass beds downshore. Transplanting downshore feeding substrate to the burrowing zone of the small crabs proved that they too preferred to feed on it. Since small crabs can be preyed upon by more species of birds, this suggests that the decision not to leave the burrowing zone might be related to the risk of being fed upon by birds. We calculated predation risk from measurements on the density and feeding activity of the crabs, as well as the feeding density, the intake rate and the size selection of the avian predators. Per hour on the surface, crabs in a flock were more at risk than crabs feeding near their burrow. Thus, though flocking crabs may have benefited from ‘swamping the predator’ by emerging in maximum numbers during some tides only, this did not reduce their risk of predation below that of non-flocking crabs. Furthermore we found that irrespective of activity, large crabs suffered a higher mortality per tide from avian predators than small crabs. This suggests that large crabs could not sufficiently reduce their foraging time to compensate for the increased risk while foraging in a flock, even though they probably experienced better feeding conditions than small crabs staying near their burrow. The greater energy demands of large crabs were reflected in a greater surface area grazed. Thus, with increasing size a fiddler crab has to feed further away from its burrow and so may derive less protection from staying near to it. It seems that growing big does not reduce the risk of predation for fiddler crabs, as it does in many other species with indeterminate growth. As in such species, the most probable advantage of growing big is increased mating success. Ultimately, therefore, prey availability must be understood from the life-history decisions of the prey species.     

西南桦自由授粉子代适应性与斑块大小、遗传多样性间的相关性

以位于广西靖西土石山生态交错区的9个西南桦斑块为研究对象,通过种子收集、种子萌发与幼苗培育试验,揭示西南桦子代适应性与斑块大小、遗传多样性间的关系.结果表明,种子重量与种子萌发率、胚根长、芽苗长呈极显著正相关(p＜0.01),而仅与移苗后2个月时的苗高及移苗后5个月时的地径呈显著正相关(p＜0.05),说明种子萌发阶段母本效应很明显,而在幼苗生长阶段母本效应逐渐减弱;种子千粒重、萌发特征、幼苗生长表现指标与斑块大小、遗传多样性指标大多呈不显著相关(p＞0.05)或呈显著负相关,而且各石山斑块跟土山大斑块间的地理距离与相应斑块的子代适应性指标大多呈负相关,说明斑块间存在强烈的花粉流,这种相关性规律是斑块大小以及斑块间地理距离等因素共同作用的结果.     

Diving speed and food-size selection in common guillemots,Uria aalgae

Results of choice experiments showed that guillemots Uria aalge prefer sprats Sprattus sprattus of a body depth of 16 to 17 mm. This is notably smaller than the maximum size that guillemots are able to swallow (body depth about 44 mm). A small proportion of the selected fish was used for display on the breeding ledge; these were significantly bigger than those preferred for consumption.Mean velocity during horizontal underwater flight was 2.18 m·s⁻¹, with the maximum recorded being 2.57 m·s⁻¹. This is considerably faster than the speed of any prey.     

Elimination of sand in the blue mussel,Mytilus edulis

Sand elimination during the first days after fishing was studied quantitatively in commercially fished Mytilus edulis. Three phases can be distinguished in the elimination process: a first stage with a large and rapid discharge (15 minutes), a second stage of an exponentially decreasing rate (4 hours) and a last stage procedding slowly (4 to 48 hours). The results of the experiments are compared with published reports on the effect of activities of ciliary systems on feeding and internal transport in mussels.     

Large-scale distribution patterns of the mussel Mytilus edulis in the Wadden Sea of Schleswig-Holstein: Do storms structure the ecosystem?

The distribution of mussel beds in the Wadden Sea of Schleswig-Holstein was mapped by aerial surveys from 1989 to 1991. The number of mussel beds decreased from 94 in 1989 to 49 in 1990, as a result of severe storms in early 1990. Thereafter only small changes were observed. The mussel beds that remained in 1990 were found only in the shelter of islands; all beds in exposed areas had disappeared between the surveys of 1989 and 1990, leaving large areas without mussel beds. Storms are thus identified as a major factor limiting the distribution of mussel beds to the sheltered parts of the Wadden Sea. Beds in the exposed parts of the Wadden Sea are highly dynamic, whereas beds in sheltered areas may persist over long times. A comparison with distribution patterns of older surveys (from 1937, 1968 and 1978) revealed great similarities with the results of recent investigations, indicating a constant distribution pattern over a long period.The results are discussed in relation to eutrophication and the structure of the benthic communities of the Wadden Sea. It is concluded that any eutrophication-induced increase of the mussel population would be restricted to the sheltered parts of the Wadden Sea. Storms will largely determine whether the communities of a given area have to compete with mussels, which are most important filter feeders of the ecosystem. As competition for food is a major factor structuring the benthic communities of the Wadden Sea, it is assumed that storms indirectly affect all other communities, giving deeper-burying, storm-tolerant species a competitive advantage in exposed areas where epibenthic mussels are excluded. The impact of mussel fisheries will be different for persisting and dynamic beds: fishing on persisting beds in sheltered areas may remove the crucial reserve which mussel-feeding birds such as eiders or oystercatchers need in times of low mussel populations.     

Trophic resource partitioning between two coexisting flatfish species off the North coast of Anglesey,North Wales

Interspecific and intraspecific trophic resource partitioning between coexisting plaice, Pleuronectes platessa L. and dab, Limanda limanda (L.) populations were investigated at three subtidal locations in the Irish Sea: Two inshore locations off the coast of Anglesey, North Wales and one off-shore location. In shallow inshore waters, the two species had similar diets dominated by the polychaete Pectinaria koreni. Offshore dab appeared to concentrate on Ensis ensis whilst the plaice took a wider variety of prey including bivalves, polychaetes, crustaceans and sand eels. Trawls made over a 24-hour period indicated some crepuscular and nocturnal feeding by individuals of both species. The observed patterns of resource partitioning were due to the abundance of prey species which vary spatially and seasonally and are discussed in relation to current work on resource partitioning between fishes.     

Size and phytoplankton selection by oosterschelde zooplankton

Grazing of adults of the copepods Acartia spp., Temora longicornis, Centropages hamatus and nauplii of Balanus spp. on natural particulate matter from the Oosterschelde was studied using a Coulter counter. Two types of particulate matter distributions were found to occur in the Oosterschelde: 1. distributions with distinct peaks in the > 20 μm size range (A) and 2. more flattened distributions which were bell-shaped or contained multiple small peaks spread over the entire 4–100 μm size range (B). In A-type distributions, peak tracking was performed in all species studied, especially in Acartia spp. and T. longicornis. In B-type distributions, the animals tended to spread their grazing activity towards the smaller particles.Quantitative microscopical analysis of the phytoplankton distributions shows that A- and B-type distributions were caused by the presence (A) or absence (B) of blooms of phytoplankton species with an SED of > 20 μm, reaching concentrations of >1.40·10⁶ μm³·cm⁻³.The demonstrated clearance rate distributions are shown to result in a concentration of phytoplankton in the ingested material as compared to the medium for all copepod species studied. Balanus spp. nauplii did not demonstrate this systematic selection of phytoplankton. When B-distributions occurred, only Acartia spp. switched its feeding activity towards smaller particles to such a degree that a substantial contribution of the <20 μm size range to the ingested material resulted. Whether this behaviour represents a higher selection capacity for small phytoplankton species or a detritivorous behaviour remains an open question.     

The role of epibenthic predators in structuring the marine invertebrate community of a British coastal salt marsh

The marine fauna of salt marshes are subjected to predation by birds, tidally feeding flatfish, crabs, prawns and small gobiid fish. The role of these epibenthic predators in structuring the community was investigated using cages to exclude predators. A range of designs of cages and partial cages was employed to control for artefacts due to caging, and sufficient cages were employed so that each cage was only sampled once to prevent the compounding of disturbance due to predation and sampling. Two mesh sizes were employed, a fine mesh excluding epibenthic predators and a coarse mesh allowing access by small crabs, prawns and gobiid fish but excluding birds and larger fish. The exclusion was maintained for 2 years. The presence of any experimental structure had a significant effect on the sedimentary regime within the cage. Epibentic predator exclusion let to an increase in infaunal predator density, but had no significant effect on the infaunal deposit feeders. There was some evidence that predators limit the surface deposit feeding gastropood Hydrobia ulvae during the winter. The gastropod Littorina littorea responded positively to the presence of any caging structure; this may be the result of changes in the availability of food, as the sides of a cage support a diatom flora which this species can exploit. The lack of a response from the infaunal deposit feeders is attributed to their horizontal mobility within the sediment. The possible interactions between epibenthic and infaunal predators are discussed.     

扰动因子对龙凤湿地鸟类影响的研究

为了研究湿地生态系统的扰动因子对生活其中的鸟类种类多样性的影响,进而保护湿地生态平衡和保护湿地鸟类多样性,于2009年3月—2012年6月,采用样线法和样点法对龙凤湿地鸟类进行了研究。结果显示,龙凤湿地的鸟类种类由16目33科减少到13目20科,种数由142种减少到46种。随着城市湿地自然因子和湿地污染、湿地水资源利用过度、湿地景观破碎化、噪声和电磁辐射污染等人为扰动因子的变化加剧,龙凤湿地的鸟类种类明显减少。扰动因子的变化,是湿地鸟类减少的重要诱因。减少扰动因子对龙凤湿地鸟类的影响,保护湿地鸟类,任务非常紧迫。     

Consumption of eelgrass,zostera marina,by birds and invertebrates during the growing season in lake grevelingen (SW Netherlands)

During the growing season May through August 1976 living eelgrass (Zostera marina L.) in brackish Lake Grevelingen (108 km²) provided quantifiable amounts of food to 6 species of animals, viz. 5 bird species (Cygnus olor, Branta bernicla, Fulica atra, Aythya ferina and Anas platyrhynchos) and 1 invertebrate species, the isopod Idotea chelipes. Consumption of eelgrass amounted to 0.839 g C·m⁻² for the entire Lake Grevelingen, 23% for the birds and 77% for Idotea respectively. In a realistic estimate this means that only 3.7% of the living eelgrass production, estimated at 23 g C·m⁻²·y⁻¹ for the entire Lake Grevelingen, was consumed by macroconsumers during the growing season. A maximum estimate of eelgrass consumption leads to 1.486 g C·m⁻², i.e. 6.5% of the eelgrass production during the period May through August.     

The effect of organic pollution on local distribution of Nereis diversicolor and Corophium volutator

Distribution patterns of Nereis diversicolor and Corophium volutator in the SE-Dollard, a 92 km² tidal flat area in the NE Netherlands are described. Body condition as well as the maximal size of juvenile worms correlated positively with elevation of the mudflat, probably caused by a better food supply on the higher mudflats. No spatial variation in size was found for C. volutator.N. diversicolor disappeared from ∼1 km² of the intertidal mudflats in the autumn of 1985 as a consequence of the seasonal waste discharges, whereas C. volutator disappeared from an area of ∼4 km². Though the discharges decreased by almost 50% from 1982 to 1985, N. diversicolor disappeared from about the same area in the 2 years.N. diversicolor is the main prey species of the avocet Recurvirostra avosetta, large flocks of which stay in the Dollard during autumn migration. Though the avocets lost ∼30% of their potential feeding area in the Dollard in the 1970s because of the high waste discharges, against only 5 to 10% in 1982 and thereafter (at significant lower discharges), the numbers of the avocets did not respond to the decrease of the discharges.