Alba Community Research Articles

Long-term changes of four macrobenthic assemblages from 1978 to 1992

Changes in four macrobenthic assemblages of the muddy-fine sandAbra albacommunity along the north-west French coast were compared using several numerical analyses (distogram, Eigen vector filtering, clustering and multi-dimensional scaling). The faunistic composition of these four assemblages continuously changed during the 1978–1992 period. These modifications are characterized by more or less rapid and abrupt successions of distinct groups of species, but the amplitude and the frequency of these changes are different between sites. Comparison of the results provided by different analyses also reveal that these four macrobenthic assemblages did not fluctuate in parallel, and that local conditions might play a key role on their temporal variations. However, it is shown that a major mesoscale climatic event, the alternation of mild and cold periods, probably also influenced the temporal variations of these four macrobenthic assemblages. Thus it is likely that there is a combination of both local and mesoscale events influencing these communities. In certain cases, changes in local conditions were the major source of variations. Sometimes the impact of the mesoscale conditions was more apparent, especially at the northern sites where the amplitude of the variations of temperature was greater.

Habitat selection and adult-recruit interactions in Pectinaria koreni (Malmgren) (Annelida: Polychaeta) post-larval populations: Results of flume experiments

The fate of recently settled populations of soft-bottom invertebrates depends not only on dispersal of pelagic larvae by tidal currents but also on other physical ( e. g. resuspension) and biological mechanisms ( e.g. habitat selection and adult-recruit interactions) acting at the water-substratum interface. To assess the relative importance of such processes under megatidal conditions in the Abra alba community of the eastern Baie de Seine (English Channel), flume experiments were conducted on post-larvae of the dominant polychaete species, Pectinaria koreni. Habitat selection by post-larvae of P. koreni was determined in a first set of experiments, where individuals were sowed either on a suitable or on an unsuitable substratum. Once resuspended, post-larvae were given a choice between two highly contrasting treatments with a natural organic-rich muddy sand and a bare flat PVC surface. P. koreni post-larvae were able to leave an unfavourable substratum into which they had initially burrowed and reach a more suitable substrate by drifting (induced by the secretion of mucus) before final settlement. The influence of adults on habitat selection and survival of P. koreni post-larvae was analysed in a second set of experiments, where individuals were sowed onto a suitable sediment with adults (test treatment) or without (control treatment). The presence of conspecific adults induced a high resuspension rate of the post-larvae. Drifting occurred mainly just after the introduction of the current and affected the whole experimental population, regardless of size. Such a response seems to be related to the intense bioturbation caused by the sub-surface deposit-feeding habit of the adults, which alters the boundary-layer flow. In contrast, the presence of adults of Owenia fusiformis, another dominant polychaete of the Abra alba community, led to an enhanced recruitment by a reduction in post-larvae resuspension. In fact, at low shear velocities, dense aggregates of tubes stabilize the sediment and so act as particle traps. Thus, in the megatidal environment of the Baie de Seine, physical conditions and behavioural responses interact and contribute to structuring the benthic community.

Drifting in post-larvae and juveniles in an Abra alba (wood) community of the eastern part of the Bay of Seine (English Channel)

Recent evidence has shown that some intertidal benthic species may re-enter the water column after larval settlement and undergo active and/or passive redistribution but there are no comparable studies of subtidal benthoplanktonic species with an isolated distribution in a megatidal sea. The present study was therefore conducted in June 1992 at a site located in the Abra alba (Wood) community of the eastern part of the Bay of Seine. The purpose of this work was to determine the species involved in post-settlement transport, to assess the number and the developmental stages of the resuspended organisms and to investigate the processes controlling the abundance of drifting post-larvae or juveniles. Under megatidal conditions drifting was quantitatively important in at least eight fine-sediment dominant species. Their abundance fluctuations in the near-bottom water stratum correlated with the bed shear stress (τ) caused by the tidal current. Swell may induce a doubling of the intensity of τ. However, species differed in their responses to hydrodynamic variables. Some species were most abundant at flood tide ( Abra alba, Tellina fabula Gamelin and Natica alderi Forbes), ebb tide ( Cultellus pellucidus Pennant, Nassarius reticulatus L.) or both ( Mysella bidentata Montagu, Owenia fusiformis delle Chiaje, Pectinaria koreni Malmgren). The vertical distribution of the resuspended benthos varied with species, individual size and hydrodynamic conditions. The results are discussed in relation to the eco-ethological characteristics of the species and the consequences of such drifting mechanisms on population dynamics.

Diopatra Marocensis, a New Brooding Species of Onuphidae (Annelida: Polychaeta)

A new species, Diopatra marocensis, is described from the fine sand Abra alba community, Sidi Boulbra, south Moroccan Atlantic coast. The new species is a member of the D. cuprea complex and is characterized by having crescentic nuchal grooves, short ceratophores, a short branchiate region, four pairs of modified parapodia with bidentate pseudocompound hooks, pectinate setae with 11–20 teeth, limbate setae with conspicuously serrated shelf, and subacicular hooks from setiger 13–15. Eggs of Diopatra marocensis sp. nov. are deposited in the parental tube, where they undergo direct development and remain until the young consist of 32–34 setigers.

Experimental Re-establishment of a Soft Bottom Community: Utilization of Multivariate Analyses to Characterize Different Benthic Recruitments

In situ community development was studied in the southern North Sea, using containers of defaunated sediments placed at the entrance of Dunkerque west harbour in order to simulate a possible faunal recovery.After a lag phase during which the pollution load (essentially heavy metals) was partially abated, development was characterized by a progressive increase in number of taxa, density and biomass. Diversity increased with the increasing colonization periods for samples collected during the same season. The dominant taxon was Polychaeta.In the experimental environment, larval recruitment accounted for the greatest proportion of all colonizing individuals (approximately 70%).With the help of multivariate analyses different recolonization scenarios were observed, all of which can be described in terms of successive recruitments. We distinguished four principal recruitments recorded as α, β, γ, δ and several secondary ones, characteristic of the season of immersion. Almost all the species concerned belong to the Abra alba community which predominates in the natural surrounding area.

Is the ‘muddy heterogeneous sediment assemblage’ an ecotone between the pebbles community and the Abra alba community in the southern bight of the North Sea?

Three assemblages of the Southern Bight of the North Sea seem to constitute a special system of two adjoining communities separated by an ecotone. The specific trait of the middle assemblage, established on a muddy heterogenous sediment, resembles the pebbles community in its western part and the Abra alba community in its eastern part. The pebbles community is dominated by epifauna (sessile and mobile species) and the Abra alba community is dominated by infauna. The bionomic and trophic structures of the ‘muddy heterogenous sediment assemblage’ are a combination. This assemblage is composed of suspension feeders and deposits feeders, whereas the pebbles community is dominated by suspension feeders and the Abra alba community by deposit feeders. A comparison of the development of the Abra alba and the muddy heterogenous assemblages during an eleven-year period shows that both assemblages have evolved in the same way. They show a bionomical stability during the same period, simultaneous bionomical changes, and recruitments of the same species at the same time. These parallel evolvements indicate that the muddy heterogenous sediment assemblage can be considered an ecotone between the Abra alba and the pebbles communities.

The soft bottom fauna near the River Elbe Estuary under the influence of sewage sludge. II. Seasonal changes in the community structure

Macrobenthic fauna and sediment parameters were studied at 40 stations representing a 494 km2 survey area in the eastern part of the German Bight and encompassing the sewage sludge dumping site. Data were taken in July 1980, one month after dumping ended, and in April 1981. pH-value, redox potential of the interstitial water, amount of sand in the sediment as well as total number of species and diversity were lower in the dumping area than outside, whereas alkalinity and water content of the sediment were higher. Due to mass development of a few species around the dumping site, biomass and number of individuals were high in July. Abundance and biomass values were lower in April. 2 communities are described and related to abiotic parameters such as sediment structure. Due to disturbance effects of sewage sludge, 2 stages of the second successional phase (Pearson & Rosenberg, 1978) were ascertained, described by one of these communities each time. The first faunal community, characterized by anAbra alba assemblage, describes an early stage of the second successional phase. In summer (July) theAbra alba community showed a wider spatial range than in spring. The other, dominated byNucula nitida, indicated a later stage of the second succession phase that was passing to the third phase.

Contribution à l'étude de la dynamique des populations de Pectinaria koreni Malmgren (Polychète) en Baie de Seine Orientale

Continuous observations, over 2 yr (1982–1984) of population changes in the benthic Abra alba (Wood) — Pectinaria (Lagis) koreni Malmgren community have been made in two fine sediments areas in the eastern Seine Bay (estuarine station and marine station). This analysis clearly demonstrated the rôle of some species in the community dynamics. The reproduction and the demographic processes associated with production of P. koreni, a major species showing great fluctuations, provided a good representation (combined with computer-aided modelling) of the regular dynamism of the Abra alba community. Each step of the biological activity (spawning, recruitment and development) was correlated with an optimal hydro-sedimentary process. The reproductive characteristics in the two areas (early maturity, high fecundity) associated with an annual ratio P B ̄ = 3 , required a high energetic investment and allowed a constant population level; yet this species has a pelagic larval period, high mortality, and short longevity. Similar results have been observed in these populations in spite of different environmental conditions.

Temporal changes of community structure and biomass in two subtidal macroinfaunal assemblages in La Coruña bay, NW Spain

Benthic samples were collected from 1982 to 1985 in two stations in La Coruna Bay in order to study the temporal changes of two subtidal benthic assemblages: (1) a Tellina fabula — Paradoneis armata community, and (2) a Thyasira fluxuosa facies of an Abra alba community. The Tellina station is located in an area with little human disturbance, whereas the Thyasira station is located inside the organically polluted harbour area, where dredging operations took place at the beginning of this study. Temporal changes in both communities follow quite different patterns. The sedimentary environment of the Tellina station remains fairly constant through time: sediment size ranges from 2.80 to 3.85 Φ, and organic matter content (loss by ignition) varies between 3.15 and 4.53%. In the Thyasira station these sedimentary variables cover a much wider range: 3.10 to 5.75 Φ mean diameter and 5.19 to 17.05% organic matter.

Spatial and temporal distribution patterns within an Amphiura filiformis — Abra alba community

Spatial and temporal distribution patterns within an Amphiura filiformis — Abra alba community

Suivi pluriannuel du peuplement à Abra alba dans la partie méridionale de la Mer du Nord (région de Dunkerque-France)

The Abra alba community is considered as a faunistical unity well established on coastal areas of North Sea. This community shows, since several years, very important recruitments of certain species. Since 1978, recruitments of Abra alba, Tellina fabula, Phyllodoce (Anaitides) mucosa, Lagis koreni or Lanice conchilega have been observed. However these demographic phenomena does not affect the structure of the community, which shows, through rank-frequency diagrams, the characteristic of a mature population, principally constituted of 7 species.

Benthic communities in the Southern Bight of the North Sea and their use in ecological monitoring

Macrobenthic and meiobenthic communities of an area off the Belgian coast of the North Sea were studied from 1970 until 1975 at 74 stations. On the basis of both macro- and meiobenthos, three zones can be distinguished in the area. The coastal zone is characterized by the macrobenthicAbra alba community, corresponding to the meiobenthicMicroarthridion littorale — Halectinosoma herdmani community, and the open sea zone by the macrobenthicVenus gallina community and the meiobenthicLeptastacus laticaudatus — Paramesochra helgolandica community. In between is a transient zone where elements of both other zones mix. The distribution of these zones is governed by the hydrodynamical regime of the region, especially by the residual and tidal current system of the Southern Bight. Within the coastal zone, the composition of the community is influenced by pollution which especially affects the epibenthic detritus-feeders of the meiobenthos. The spatial stability of parameters describing community structure can be used for monitoring changes. Temporal characteristics of these parameters could not be investigated properly, but diversity seems to be much stabler than biomass.

Faunenverarmung in einem Schlickgebiet in der Nähe Helgolands

Variations in abundance and species composition of macro-zoobenthos were studied at a sublittoral muddy-bottom station in the inner part of the German Bight from 1969 to 1976. The fauna of this area can be included in the classicalAbra alba community. A total of 98 species were found in 62 samples. Of these, only 4 species occurred in all samples:Nucula nitidosa, Diastylic rathkei, Ophiura texturata andNephtys hombergii. Species richness has declined since 1969. Populations of several species disappeared in the investigated area and its surroundings. Other populations are shown to exhibit remarkable fluctuations in abundance with sometimes heavy spatfalls or sudden breakdowns in population density, mainly during late summer. The long-term trend of impoverishment of the fauna is attributed to the development of anaerobic conditions in the muddy sediment. Such conditions are favoured by stable, thermohaline strafications of the water during summer, resulting in low oxygen levels and, probably, enrichment of hydrogen sulfide even in near-bottom water. Moreover, both continuou influx into the area of Elbe water, rich in suspended matter and nutrients and dumping of sewage sludge about 8 km east of the investigated area are considered to have increased the strees on the bottom fauna.