Seasonal occurrence of the ctenophore Pleurobrachia pileus in the western Dutch Wadden Sea

Due to the short residence time of the water in the western Dutch Wadden Sea the ctenophore Pleurobrachia pileus has to be considered as an allochthonous species, which enters the estuary passively from the North Sea. The seasonal occurrence of this species in the Dutch Wadden Sea is a reflection of its abundance in the coatal zone of the North Sea and is characterized by a pronounced spring peak of up to 17000 individuals per 1000 m³ and a lower autumn peak with numbers up to 400 individuals per 1000 m³. Similar to other areas, predation by Beroë gracilis, another ctenophore species, may be a factor in controlling the P. pileus population in this region, although B. gracilis abundance is low. P. pileus is only important as a predator for a short time, i.e. the month of May, when it reaches maximum biomass values of up to 12 00 mgC per 1000 m³. Both the estimated potential consumption rates in comparison with the available food as well as the observed suboptimal growth rate of the population suggest that in the coastal zone of the North Sea area food conditions may not be optimal for P. pileus.     

Short-term and long-term facilitation of goose grazing by livestock in the Dutch Wadden Sea area

We studied the impact of livestock grazing on the distribution ofBranta bernicla bernicla (Dark-bellied Brent goose) in the Dutch Wadden Sea during spring. It was hypothesized that livestock facilitate short-term (within-season) grazing for geese as well as long-term (over years). Therefore we measured grazing pressure by geese in salt marsh and polder areas that were either grazed (spring-grazed) or ungrazed during spring (summer-grazed). Additionally, we carried out a preference experiment with captive geese to test the preference between spring-grazed and summer-grazed polder swards. We furthermore compared patterns of use by geese between long-term ungrazed and grazed salt marshes. In May, there is a difference in grazing pressure by geese between polder pastures that are grazed or ungrazed during spring. In this month, the ungrazed polder pastures are abandoned and the geese shift to either the grazed polder pastures or to the salt marsh. Vegetation in the polder that had been spring-grazed had a lower canopy height and a higher tiller density than summer-grazed vegetation. The captive geese in the preference experiment showed a clear preference for vegetation that had been spring-grazed by sheep over ungrazed vegetation. Goose grazing pressure was negatively correlated to canopy height, both on the polder and on the salt marsh. Within the plant communities dominated byFestuca rubra andPuccinellia maritima, marshes that were intensively grazed by livestock generally had higher grazing pressure by geese than long-term ungrazed or lightly grazed salt marshes.     

Salt-marsh erosion and restoration in relation to flood protection on the Wadden Sea barrier island Terschelling

This paper explores the impact of erosion and restoration measures on habitat development and on wave damping by a small salt marsh nestled alongside a dike on the Wadden island of Terschelling. The aim is to advance knowledge about the benefits and possible side-effects of salt-marsh restoration. Analysis of a time series of aerial photographs from 1944 to 2010 indicates that the salt marsh decreased steadily in size after maintenance of accretion works was terminated. In the western part of the marsh, which is accessible to sheep, vegetation is low (5–15 cm) and dominated by Salicornia europaea and by Spartina anglica. In the most intensively grazed parts, vegetation is very scarce. The eastern, inaccessible part of the salt marsh is covered by dense patches of the shrubby perennial Atriplex portulacoides and Spartina anglica (15–25 cm in height). SWAN wave models show that wave height at this location is significantly affected by the areal extent of the salt marsh as well as by the vegetation. High or dense vegetation are in the models nearly as effective in damping waves (with an initial height of 0.15 and 0.5 m) as widening the salt-marsh area by 350 m. A low density of low plants, as observed in the grazed part of the marsh, has almost no wave-damping effect. Even under conditions of sea level rise, a broader salt marsh vegetated with high plants significantly affects modelled wave height. Therefore, salt-marsh restoration is an adaptation measure worth exploring, though an array of effect types must be considered.     

Recurrent and changing seasonal patterns in phytoplankton of the westernmost inlet of the Dutch Wadden Sea from 1969 to 1985

Data for phytoplankton composition and abundance in the Marsdiep are presented for the period from 1969 to 1985 inclusive. Only a few species dominated the phytoplankton. A recurrent pattern was observed in the seasonal succession: in winter, total cell numbers were invariably low, but freshwater algae, sluiced into the Wadden Sea from IJssel Lake, showed highest densities in winter. A diatom spring peak was observed around mid-April, followed by a Phaeocystis pouchetii peak about three weeks later. Later in summer usually two more diatom peaks followed by non-diatom peaks were present. The exact timing of the spring peak varied from year to year, with the extremes being late March and early May. A relatively late spring peak usually coincided with a relatively high turbidity in the preceding winter. An increase in total cell numbers was found over the 17-year observation period. Diatoms decreased from 1969 to 1974 but have increased since then, reaching values above those of 1969 during recent years. Flagellates showed a consistent increase over the entire observation period.     

Feeding of calanoid copepods in relation to Phaeocystis pouchetii blooms in the German Wadden Sea area off Sylt

Surface sediments from the Svartnes basin (195 m deep) in Balsfjorden, northern Norway (ca. 70°N), were partially characterized to assess the nature and origin of the organic material present and its potential nutritive value for sediment-ingesting animals. Seasonal analyses were carried out on material collected between May 1979 and August 1980 for total organic matter, organic C and N, acid-extractable amino acids and lipids extractable with chloroform:methanol. Little or no seasonal variation was seen in any of the parameters analysed. The mean apparent organic content was 9.3% of sediment dry weight, the organic C content was 2.38%, the organic N content was 0.26% and the C:N ratio was 9.1:1. Acid hydrolysis of sediment yielded 575 mg of amino acids and 41 mg of NH₃ per 100 g sediment dry weight, the composition of the amino acids being similar to that of nutritionally highquality animal or microbial protein. Glucoseamine was not detected in acid hydrolysates of sediment, consistent with the absence of chitin. Chloroform:methanol extraction yielded 133 mg of material per 100 g sediment dry weight, 62% of which was accounted for by saponifiable lipids (fatty acids) and non-saponifiable lipids present in approximately equal amounts. Fatty alcohols accounted for 30% of the non-saponifiable lipids and phytol accounted for 40% of the fatty alcohols. Small amounts of very long-chain fatty alcohols characteristic of terrestrial plants were present, but long-chain monounsaturated fatty alcohols characteristic of marine zooplankton were essentially absent. Very small quantities of long-chain ketones characteristic of marine coccolithophores were detected. The major fatty acids present in sediments were 16:0, 14:0, 16:1 (n-7) and 18:1 (n-9), and 11% of the total fatty acids were comprised of a mixture of odd-numbered straight-chain and branched-chain moieties characteristic of micro-organisms. The data point to small amounts of material characteristic of marine and terrestrial photosynthetic organisms being present in sediments at any given time, whereas material characteristic of marine zooplankton is not present. The presence of material characteristic of micro-organisms is consistent with conversion of sedimenting material into a pool of sediment micro-organisms. Although the biological availability to sediment-ingesting organisms of the esterified fatty acids and the acid-extractable amino acids in sediments remains to be assessed, the maximum amounts available are equivalent to approximately 60 mg of polar lipid and 575 mg of protein per 100 g sediment dry weight. These amounts are small in relation to the production rates in the photic zone of the fjord, implying that the bulk of the energy flow in Balsfjorden occurs in its relatively short pelagic food chains.     

An evaluation of the ABC-method (abundance/biomass comparison) as applied to macrozoobenthic communities living on tidal flats in the Dutch Wadden Sea

The ABC-method proposed by Warwick (1986) for detecting pollution effects on marine zoobenthic communities, was tested on distribution patterns of numbers and biomass among species in macrozoobenthos samples taken annually for 13 yr in a uniform way at 15 tidal-flat stations in the western part of the Dutch Wadden Sea. Along the margins of the tidal-flat area studied, where exposition to either drainage or water movements is extreme, the k-dominance plots for numbers were generally situated above those for biomass. In these areas values for biomass and species richness were relatively low and a few small-sized species such asCorophium volutator andHydrobia ulvae were very numerous. In the central part of the area, where environmental conditions are less severe, values for biomass and species richness were higher. Biomass was dominated by large adults ofMya arenaria, Mytilus edulis and/orArenicola marina. However, in only a restricted part of this area were plots for numbers consistently below biomass plots. In most of the central area high numbers ofH. ulvae occasionally occurred, causing plots for numbers to be situated above those for biomass during such periods. There appears to be no reason to interprete a high abundance ofH. ulvae as a sign of pollution or any other kind of disturbance or stress. It is concluded that the ABC method cannot be applied to tidal flat communities without reference to long-term and spatial series of control samples. In areas where such smallsized and sometimes numerous species asH. ulvae occur in strongly fluctuating numbers, the method appears useless for assessing the pollution status of a benthic community.     

Reintroduction of eelgrass (Zostera marina) in the Dutch Wadden Sea; review of research and suggestions for management measures

Eelgrass (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal flats. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Effect of periphyton grazing by Hydrobia ulvae on the growth of Zostera noltii on a tidal flat in the Dutch Wadden Sea

The decrease of the intertidal seagrass Zostera noltii in the Dutch Wadden Sea may have been the result of enhanced periphyton load due to eutrophication. Decrease of this seagrass species coincided with an increase in the mudsnail Hydrobia ulvae. Feeding of this mudsnail on periphyton may have partly counteracted an increase of periphyton biomass. Exclosure experiments on seagrass stands in the Dutch Wadden Sea in 1987 showed that density of periphyton on leaves of Z. noltii decreased significantly with increasing density of grazers. An increased density of mudsnails significantly enhanced the density and biomass of seagrass, in particular of the below ground parts. Since this seagrass species survives winter in temperate climate zones mainly by means of rhizomes, grazing may also influence the seagrass dynamics in the long term. Results of the experiment were in agreement with field observations on coinciding low densities of mudsnails and high densities of fouling of seagrass stands on the tidal flats of western Wadden Sea in the late 1970s.     

Decadal changes in a NW Mediterranean Sea food web in relation to fishing exploitation

We analysed changes in the ecological roles of species, trophic structure and ecosystem functioning using four standardized mass-balance models of the South Catalan Sea (North-western Mediterranean). Models represented the ecosystem during the late 1970s, mid 1990s, early 2000s, and a simulated no-fishing scenario. The underlying hypothesis was that ecosystem models should quantitatively capture the increasing exploitation in the ecosystem from the 1970s to 2000s, as well as differences between the exploited and non-exploited scenarios. Biomass showed a general decrease, while there was an increase in biomass at lower trophic levels (TL) from the 1970s to 2000s. The efficiency of energy transfer (TE) from lower to higher TLs significantly increased with time. The ecosystem during the 1990s showed higher biomass and flows than during the 1970s and 2000s due to an increase in small pelagic fish biomass (especially sardines). Exploited food webs also showed similarities in terms of general structure and functioning due to high intensity of fishing already in the 1970s. This intensity was highlighted with low trophic levels in the catch, high consumption of production by fisheries, medium to high primary production required to sustain the catches and high losses in secondary production due to fishing. Significant differences on ecosystem structure and functioning were highlighted between the exploited and no-fishing scenarios. Biomass of higher TLs increased under the no-fishing scenario and the mean trophic level of the community and the fish/invertebrate biomass ratios were substantially lower in exploited food webs. The efficiency of energy transfer (TE) from lower to higher TLs was lower under the no-fishing scenario, and it showed a continuous decrease with increasing TL. Marine mammals, large hake, anglerfish and large pelagic fish were identified as keystone species of the ecosystem when there was no fishing, while their ecological importance notably decreased under the exploited periods. On the contrary, the importance of small-sized organisms such as benthic invertebrates and small pelagic fish was higher in exploited food webs.     

Effects of predation by common eiders (Somateria mollissima) in an intertidal rockweed bed relative to an adjacent mussel bed

Marine Biology - Little is known about the effect of commercial rockweed (Ascophyllum nodosum) harvest on intertidal community structure, and on interactions among species. Using predator-exclusion...     

Reintroduction of eelgrass ( Zostera marina ) in the Dutch Wadden Sea; Review of research and suggestions for management measures

Eelgras (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal falts. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Causes of Extirpations in the Wadden Sea, an Estuarine Area in The Netherlands

Abstract: The various causes of extirpation of marine and estuarine species were investigated for the Dutch Wadden Sea to gain an impression of their relative importance. I obtained data from geological, archaeological, historical, and biological publications. At least 10 species of algae, 10 invertebrates, 13 fishes, 5 birds, and 4 marine mammals became extinct during the past 2000 years. Habitat destruction played a part in at least 26 cases, overexploitation in at least 17, and pollution in at least 3. Causes of the disappearance of two species are unknown. Introduction of exotic marine species was not involved in any of the extirpations. I compared this pattern of extinction to that of worldwide extinctions of marine and estuarine species, which have been attributed to overexploitation, habitat destruction, and in two cases to the introduction of terrestrial predators to the breeding habitats of marine birds. The results imply that conservation of marine species should address overexploitation and habitat destruction as the main causes of extinction.     

Possible causes of temporal fluctuations in primary production of the microphytobenthos in the Danish Wadden Sea

The effects of prior exposure to unlabelled naphthalene on the processes of uptake, tissue distribution and elimination of [1-¹⁴C] naphthalene by mussels (Mytilus edulis) were examined. Mussels collected from Whitsand Bay (S. W. England) in October 1980 were either held unexposed or exposed to unlabelled naphthalene (0.5 μg 1^?1) for 4 wk, prior to receiving a 4 h pulse of [1-¹⁴C] naphthalene in the medium (7 μg 1^?1). After 4 h exposure to [1-¹⁴C] naphthalene, the major tissues (digestive gland, gills, kidneys, mantle and remaining tissue) showed exponential depuration curves which could be resolved into two statistically significant components. The kinetics of elimination and the biological half-times of [1-¹⁴C] naphthalene in the tissues were determined. Mussels pre-exposed to unlabelled naphthalene had a significantly higher rate of elimination of [1-¹⁴C] naphthalene from the gills and kidneys. These results indicate that the toxicant, naphthalene, is actively excreted from the body by the gills and kidneys and the process is enhanced by prior exposure to the toxicant.     

Climate change adaptation in the Schleswig-Holstein sector of the Wadden Sea: an integrated state governmental strategy

Anthropogenic climate change constitutes a main challenge for the Wadden Sea. Accelerated sea level rise, increasing temperatures and changing wind climate may strongly alter present structures and functions of the ecosystem with negative consequences both for nature conservation and for coastal risk management. Being aware of these challenges, Schleswig-Holstein State Government decided to establish an integrated climate change adaptation strategy for the Schleswig-Holstein sector of the Wadden Sea. The strategy was adopted in June 2015. It aims at the long-term maintenance of present functions and structures as well as the integrity of the Wadden Sea ecosystem in a changing climate. The strategy was prepared by a project group consisting of representatives from State authorities as well as from nature conservation organisations and local institutions. First outcome of the strategy is that extra adaptation measures will not be necessary in the coming decades. However, pending on the future rate of sea level rise, shoreline erosion and sediment deficits in the Wadden Sea will increase and sooner or later drowning of tidal flats and terrestrial habitats like beaches, primary dunes and salt marshes will start. At the time when management measures to counteract the negative developments become expedient from a nature conservation viewpoint as well as for coastal risk management, adequate actions with minimized ecological interferences are possible. It is assumed that balancing the sediment deficits as the main adaptation measure may be implemented most efficiently by concentrating sediment suppletion at locations where natural forces organize redistribution in the Wadden Sea. Local technical coastal risk management measures like the strengthening of sea defences will, nevertheless, remain necessary as well.     

Distribution and abundance of moon jellyfish (Aurelia aurita) and its zooplankton food in the Black Sea

The distribution of moon jellyfish (Aurelia aurita Linnaeus, 1758) in the Black Sea was determined from plankton samples collected above the anoxic zone (maximum depth 200 m) in the summer, winter and spring during 1991–1995. Distribution was patchy. Average biomass ranged from 98 to 380 g m⁻², and abundance varied from 2 to 14 individuals m⁻². Biomass and abundance peaked in late spring and summer. The distribution was correlated with hydrographic features in the Black Sea, with higher concentrations occurring at the peripheries of anticyclonic eddies. Centers of the two main cyclonic gyres generally had a low biomass of A. aurita. From July 1992 to March 1995, the populations were largely concentrated in offshore regions. A. aurita were confined to the upper part of the mixed layer. Smaller A. aurita (≤1 cm) were present in early spring (March), and individuals reached maximum size in the summer. Release of the epyhrae occurred in spring on the northwestern shelf of the sea when the seawater temperature was 11–12 °C. Microscopic analysis of stomach contents showed that copepods and mollusks form their main diet. Received: 3 September 2000 / Accepted: 29 September 2000     

Distribution and abundance of ctenophores and their zooplankton food in the Black Sea. II. Mnemiopsis leidyi

The distribution of Mnemiopsis leidyi Agassiz, 1865 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum depth 200 m) in the summer, winter, and spring from 1991 to 1995. Distribution was patchy. Average biomasses of 15 to 500 g m⁻² were measured, and abundances varied from 10 to 180 ind m⁻². Biomass and abundance peaked in winter, and there was a secondary peak in the summer. The distribution of M. leidyi was correlated with hydrographic features in the Black Sea with higher concentrations in anticyclonic gyres. The centers of the two main cyclonic gyres generally had a low biomass of M. leidyi. From July 1992 to March 1995, the populations were largely offshore. M. leidyi were confined to the upper part of the mixed layer both day and night. Some individuals displayed a negative taxis to daylight and were concentrated below the thermocline at night. Smaller M. leidyi (1.5 to 2 cm) were present in the winter, and individuals reached maximum size in the summer. Although reproduction was continuous throughout the year, there were two distinct peaks: the larger peak in the summer and the smaller peak in the winter. Microscopic analysis of stomach contents showed that copepods and molluscs form their main diet. Received: 1 November 1997 / Accepted: 30 August 1999     

Analysis of the small-scale spatial patterns of free-living marine nematodes from tidal flats in the East Frisian Wadden Sea

Serial samples with a multiple corer were taken in order to understand the small-scale aggregation patterns in the nematode community. A total of 77 species was found in the sample. Fifteen dominant nematode species were examined by means of a dispersion analysis. Only one species (Metachromadora scotlandica) may be considered randomly distributed. A variogram analysis gave no proof of aggregation in that species, but all the other (aggregated) species displayed different spatial patterns: (1) a maximum/aggregation at the sample's margin, (2) two maxima/aggregations in the centre of the sample, (3) aggregation of individuals distributed at random over the entire sample (Metalinhomoeus typicus) and (4) aggregation of individuals distributed at random only in the centre of the sample (Paralinhomoeus ilenensis). These results allow the prognosis that distribution patterns are repetitive in five species (Microlaimus conothelis, M. marinus, Odontophora rectangula, Oncholaimellus calvadosicus, Viscosia rustica). Finally, by means of MDS-ordination (based on cosine similarity) two nematode associations were distinguished: one group represented by diatom-feeders (sensu lato), the other one comprised of DOM-users (sensu lato). Received: 13 May 1998 / Accepted: 15 January 1999     

Distribution and abundance of ctenophores, and their zooplankton food in the Black Sea. I. Pleurobrachia pileus

The distribution of Pleurobrachia pileus Müller, 1776 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum of 200 m) in the winter, spring, and summer of 1991 to 1995. The summer samples were collected in 1991 to 1993 (for a previous) and are included in this paper for comparative purposes. High concentrations of P. pileus were found at the northern edges of anticyclonic eddies along the southern coastal regions. The biomass and abundance of P. pileus increased from winter through spring to a peak in summer. The highest mean wet weight during a sampling period was 250 g m⁻², while the maximum wet weight was 1429 g m⁻². P. pileus was mostly found in a layer extending from the lower parts of the thermocline down to the anoxic zone, where the temperature was <8 °C. The vertical distribution of P. pileus biomass had two clear maxima at night: an upper maximum at 20 to 40 m was less pronounced than the lower maximum at 90 to 120 m depth. Mean body length of P. pileus did not exceed 12 mm. Smaller individuals (9 to 10 mm length) occurred in winter. P. pileus had two length classes in early spring (March 1995) and late summer (August 1993), indicating the presence of both newly hatched and larger individuals. Overall, the stomach contents of P. pileus consisted mainly of Copepoda (90%), Cladocera (1%), Mollusca (1%), fish eggs and larvae (1%), and other taxa (7%). The preferred food of P. pileus (frequency of occurrence) was: Calanus euxinus (39%), Pseudocalanus elongatus (30%), Acartia clausi (28%), Oithona similis (2%), and Paracalanus parvus (1%). The endoparasite Hysterothylacium aduncum was commonly found in P. pileus. Abundances of Mnemiopsis leidyi and P. pileus were either negatively correlated (r = −0.5 to −0.7) or positively correlated at a low significance level (r = 0.25 to 0.3) with abundance of A. clausi in different months of the year. Aurelia aurita abundance was correlated mainly with the abundance of C. euxinus from June 1991 to March/April 1995. Over the same period the abundance of P. pileus was significantly correlated with the abundance of P. elongatus, an important prey species. Received: 1 November 1997 / Accepted: 30 August 1999     

Distribution of euphausiid assemblages in the Mediterranean Sea

Eleven species of euphausiids from 24 Isaac-Kidd Midwater Trawl (IKMT) night collections taken at stations throughout the Mediterranean Sea were counted. The frequency of occurrence and dominance of individual species and percent similarity faunal analysis of the euphausiid community were used to describe changes in faunal composition between geographical areas and differences in vertical distribution. Although most species were widespread, three distinct patterns of abundance were apparent: Euphausia krohnii, Nematoscelis megalops, Meganyctiphanes norvegica, and Stylocheiron abbreviatum predominated in western basin areas west of the Tyrrhenian Sea; Euphausia hemigibba, Thysanopoda aequalis, and Stylocheiron longicorne predominated in the Tyrrhenian Sea and eastern Mediterranean Sea; Euphausia brevis and Stylocheiron suhmii predominated in the eastern Mediterranean Sea. Percent similarity analysis of data from the IKMT samples and data from Ruud (1936) indicates the Tyrrhenian Sea fauna at the time of the collections was more similar to eastern Mediterranean areas than to most other areas in the western basin, although the degree of similarity was dependent, to some extent, on the depth at which the samples were collected. The composition of euphausiids living above 150 m at night in this area was more similar to eastern basin areas, while the composition of deeper living forms was more similar to those of the rest of the western basin. Comparison of euphausiids collected at three points over a 60 year time-span in the Balearic Sea shows the similarity in composition to be greater within the area over time than between adjacent areas in the western Mediterranean Sea.Contribution No. 2732 from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA. This research was supported by the Atomic Energy Commission Contract AT (30-1)-3862, ref. NYO-3862-49, National Science Foundation Grant GA 29303 and Office of Naval Research Contract N00014-71-C-0284.