Pioneer Assemblage Research Articles

Rhizosphere microbiome functional diversity and pathogen invasion resistance build up during plant development.

The rhizosphere microbiome is essential for plant growth and health, and numerous studies have attempted to link microbiome functionality to species and trait composition. However, to date little is known about the actual ecological processes shaping community composition, complicating attempts to steer microbiome functionality. Here, we assess the development of microbial life history and community-level species interaction patterns that emerge during plant development. We use microbial phenotyping to experimentally test the development of niche complementarity and life history traits linked to microbiome performance. We show that the rhizosphere microbiome assembles from pioneer assemblages of species with random resource overlap into high-density, functionally complementary climax communities at later stages. During plant growth, fast-growing species were further replaced by antagonistic and stress-tolerant ones. Using synthetic consortia isolated from different plant growth stages, we demonstrate that the high functional diversity of 'climax' microbiomes leads to a better resistance to bacterial pathogen invasion. By demonstrating that different life-history strategies prevail at different plant growth stages and that community-level processes may supersede the importance of single species, we provide a new toolbox to understand microbiome assembly and steer its functionality at a community level.

Floristic differentiation between mid-seral communities of abandonedcharcoal kilns in the forests of the Gargano Promontory (south Italy)

We searched for patterns in species composition and richness of mid-seral communities developed on abandoned charcoal hearths that might be induced by differences in succession progress and forest context. One phytosociological releve was performed on each of the 22 charcoal platforms located in beech (Anemono apenninae-Fagetum) and oak-hornbeam forests (Doronico-Carpinetum). The ratio of forest understorey to non-forest species richness (F/nF) at plot level was used as a proxy for the succession progress. There was a sharp differentiation of plant communities due to their floristic resemblance with the understory vegetation of the forest type in which they were integrated. Nevertheless, these mid-seral communities still contained a number of nitrophilous weeds that were remnants of the preceding pioneer assemblages. A subgroup of communities (integrated in the beech forest) that lacked the non-forest species mentioned above (high F/nF ratio) could be ascribed to a more advanced seral stage. Species richness on charcoal platforms was not related to forest type, but that might be the consequence of the small sample size. Instead, the number of species decreased steadily with increasing F/nF ratio, which is consistent with the predictions emerging from the paradigm of ecological succession. The forest context accounted for most of the observed floristic dissimilarity between these mid-seral communities. Differences in succession progress seem to be traceable through the number of species, which declines towards more mature stages. The observed communities shared probably the same type of pioneer plant assemblage (Alliario-Chaerophylletum temuli) in the preceding seral stage.

Stable seafloor conditions, sea level and food supply during the latest Maastrichtian at Brazos River, Texas

Abstract The Cretaceous/Paleogene (K/Pg) boundary extinction event is inextricably linked to the Chicxulub impact. Environmental changes like sea level changes and changes in food supply prior to or after the impact could however have contributed to the biotic turnover. In this study, benthic foraminifera from two cores from the Brazos River area in Texas are quantitatively analyzed in order to reconstruct environmental changes across the K/Pg boundary. Cluster analysis as well as non-metric multidimensional scaling reveals three distinct benthic foraminiferal assemblages: an upper Maastrichtian assemblage, a post-impact assemblage, and a lower Danian assemblage. The rather uniform upper Maastrichtian assemblage at Brazos River, composed of e.g. Clavulinoides midwayensis, Gavelinella dumblei, Cibicidoides harperi, Gyroidinoides aequilateralis and Bulimina kickapooensis, indicates a stable paleodepth of about 75–100 m up to the K/Pg boundary. Food supply indicators such as percentage Buliminaceae and foraminiferal numbers suggest a relatively low and stable food supply during this interval. Environmental disruption caused by the Chicxulub impact led to the settlement of a typical pioneer assemblage, characterized by endobenthic morphotypes such as Ammobaculites spp. and Pseudouvigerina naheolensis. During the early Danian, the typical Midway-type benthic foraminiferal shelf fauna (e.g. Alabamina midwayensis, Anomalinoides acutus, Gyroidinoides subangulatus, Praebulimina carseyae and Cibicidoides alleni) developed, indicating a paleodepth of 75–100 m. No clear water depth changes can be derived from the benthic foraminiferal record during this interval, although the sedimentary record suggests a shallowing followed by a transgressive pulse in Zone P1a. Food supply indicators suggest a lower food supply in the early Danian than during the upper Maastrichtian. Because of the stability of the late Maastrichtian environment we conclude that environmental disruption caused by the impact was most likely the principal cause of the benthic foraminiferal turnover across the K/Pg boundary observed at Brazos River.

Ostracoda (Crustacea) as indicators of subaqueous mass movements: An example from the large brackish lake Tangra Yumco on the southern Tibetan Plateau, China

A conceptual model of subaquaeous mass movements and ostracod distribution in lacustrine sediment event layers was tested. Integrated methods (geophysical, sedimentological and microfossil analyses) were performed on a short sediment core retrieved from 220m water depth in the large brackish lake Tangra Yumco on the southern Tibetan Plateau, central Asia. The event layers of the core and their underlying and overlying sediments were investigated. Four major event layers composed of sandy silt with graded bedding are interpreted as turbidites. The fifth layer consisting of fine sand and silt and without graded bedding is characterized as a debrite.The ostracods, small bivalved aquatic crustaceans, identified are Leucocytherella sinensis Huang, 1982, Leucocythere? dorsotuberosa Huang, 1982, Limnocythere inopinata (Baird, 1843) and Fabaeformiscandona gyirongensis (Huang, 1982).Ostracod evidence is a good proxy for the evaluation of massive sediment event layers formed by subaqueous mass movements. Four assumptions of a conceptual model were confirmed: (i) fine grained sediments of event layers (turbidite deposits) contain very low numbers of ostracods compared to the underlying and overlying sediments; (ii) ostracods are sorted according to size and display high abundance and high proportion of adult valves of Leucocytherella sinensis at the base of event layers; (iii) a relatively low number of carapaces is present within the event layers; and (iv) the recolonization of newly event-generated habitats by pioneer assemblages transforming into pre-event association gradually.

Recolonisation patterns of benthic invertebrates: a field investigation of restored former sewage channels

Summary The response of benthic invertebrate assemblages to stream restoration is the subject of numerous recent studies, but the process of recolonisation of restored river sections is still poorly understood. Field studies are often flawed by uncertainties about whether the observed species colonised the restored sections after restoration or whether they were present before restoration. We investigated restored streams in the Boye catchment in an urbanised area in Germany. The streams were formerly open sewers and thus were not inhabitable by benthic invertebrates, except for Oligochaeta. Restoration included the construction of underground sewers for wastewater and morphological remodelling of the stream bed. In the spring of 2012, we sampled seven restored former channels connected with near‐natural upstream sections, which were never used as sewage channels and are in good status morphologically, and six unconnected restored former channels. Restoration measures had been conducted between one and 19 years before sampling. Additionally, we sampled 21 near‐natural sites within the catchment and eleven near‐natural sites in neighbouring catchments. Near‐natural sites were considered to be potential source sites from which benthic invertebrates might colonise the restored sites. We recorded 128 taxa and categorised them into five dispersal classes that reflect dispersal capabilities and degree of ecological specialisation. Assemblages at restored sites were characterised by lower numbers of taxa and/or high abundance of hololimnic taxa and poorly dispersing winged species and by higher species numbers and abundance of strongly dispersing generalists. A distinct recolonisation sequence was derived from the observed patterns, in which winged, strongly dispersing generalists colonised most rapidly and were followed by hololimnic species, weakly dispersing generalists and habitat specialists. Restored sites connected with near‐natural upstream sections were colonised more rapidly than unconnected restored sites, particularly by habitat specialists. Almost 90% of the recolonisation events originated from sources within a distance of 5 km. We observed a succession from pioneer assemblages to more mature communities, which resembles that of the surrounding near‐natural sites. In conclusion, assemblages in connected, restored sites needed nine to 19 years to reach maturation, while the settlement of assemblages in unconnected sites is expected to require more time.

Benthic foraminiferal response to the removal of aquaculture fish cages in the Gulf of Aqaba-Eilat, Red Sea

Abstract For about 20 years, finfish were reared in floating cages at the northern end of the Gulf of Aqaba-Eilat, Red Sea. The benthic ecosystem at the fish cages area was severely impacted by organic enrichment, resulting in an environment with no living foraminifera. A government decision led to the fish cages' removal in June 2008, creating a unique opportunity to monitor and assess post-removal changes in the benthos. Three years of benthic foraminiferal assemblage monitoring, beginning in July 2008 and ending in July 2011, are summarized here. Monitoring was carried out monthly by collecting sediment samples from stations of varying distances from the fish farm location, and, after its appearance in the summer of 2009, sampling the native seagrass Halophila stipulacea. Living foraminifera first appeared in the sediment in January 2009, progressively increasing in abundance thereafter. A clear difference in the rate of the rehabilitation process was observed on a spatial scale, related to distance from the point source of the organic enrichment. Recovery began with the first appearances of a few living individuals of Ammonia spp., Amphistegina lessonii and Nonion spp. By July 2009, a significant increase in overall abundance was recorded in the stations furthest from the fish cages, with Operculina ammonoides strongly dominating the assemblages. Populations of O. ammonoides revealed polymorphism in the coiling mode of their shells. Inflated involute and semi-involute forms dominate the living assemblages, whereas flattened evolute tests are more common in the dead assemblages, representing the period that preceded the fish farms. Unlike previous interpretations in the literature, in which such morphological variation was attributed to hydrodynamic energy or depth habitats, here it is hypothesized that the inflated involute and semi-involute forms are a morphological trait characteristic of the pioneer assemblages colonizing the area after its recovery from fish cages eutrophication. The reestablishment of the native H. stipulacea seagrass community was an important factor enabling epiphytic foraminifera to colonize the previously impacted sediments. All living foraminiferal species found on the seagrasses were also found in the former assemblages, suggesting that seagrass meadows existed before eutrophication and were the main habitats of the dead assemblages.

Nestedness and successional trajectories of macroinvertebrate assemblages in man-made wetlands

Current successional models, primarily those based on floral succession, propose several distinct trajectories based on the integration of two key hypotheses from succession theory: convergence versus divergence in species composition among successional sites, and progression towards versus deviation from a desired reference state. We applied this framework to faunal succession, including differential colonization between active and passive dispersers, and the nested patterns generated as a consequence of this peculiarity. Nine man-made wetlands located in three different areas, from 0-3 years from wetland creation, were assessed. In addition, 91 wetlands distributed throughout the region were used as references for natural macroinvertebrate communities. We predicted the following: (1) highly nested structures in pioneering assemblages will decrease to lower mid-term values due to a shift from active pioneering taxa to passive disperser ones; (2) passive idiosyncratic taxa will elicit divergent successional trajectories among areas; (3) the divergent trajectories will provoke lower local and higher regional diversity values in the mid-term assemblages than in pioneer assemblages. Our results were largely congruent with hypotheses (1) and (2), diverging from the anticipated patterns only in the case of the temporary wetlands area. However, overall diversity trends based on hypothesis (3) did not follow the expected pattern. The divergent successional trajectories did not compensate for regional biodiversity losses that occurred as a consequence of pioneering colonizer decline over time. Consequently, we suggest reconsidering wetland construction for mitigation purposes within mid-term time frames (≤ 3 years). Wetlands may not offset, within this temporal scenario, regional biodiversity loss because the ecosystem may not support idiosyncratic taxa from natural wetlands.

Responses of Pioneer and Later‐Successional Plant Assemblages to Created Microtopographic Variation and Soil Treatments in Riparian Forest Restoration

Riparian forest restoration generally involves introduction of later‐successional tree species, but poor species suitability to severely altered or degraded site conditions results in high mortality and poor community development. Additionally, while microtopographic heterogeneity plays a crucial role in the development of natural riparian forests, little is known regarding effects of restored or created microtopography on the development of introduced plant communities. The objective of this study was to determine the influence of created microtopography and soil treatments on early development of introduced pioneer and later‐successional plant communities in riparian forest restoration. Ridges, flats, and a mound‐and‐pool complex were created, and pioneer and later‐successional tree assemblages were planted within plots in each of these three microtopographic positions. Straw‐based erosion control mats were placed on half the plots as a source of mulch. After two growing seasons, growth and survival of the pioneer assemblage were equal among microtopographic positions, but survival of the later‐successional assemblage was significantly higher on ridges (59%) than on mounds and pools (22%) and flats (26%). A suitability index indicated that performance of the later‐successional assemblage on ridges was higher than that of the pioneer assemblage for all microtopographic positions. Flood duration explained much of the variation in plant assemblage survival, and erosion control mats had little influence on seedling survival. Restoring microtopographic features has the potential to enhance species survival and promote community development. Microtopographic restoration may be as important in riparian forest restoration as proper species selection and hydrologic reestablishment, especially at severely disturbed sites.

Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism

One of the five largest mass extinctions of the past 600 million years occurred at the boundary of the Triassic and Jurassic periods, 201.6 million years ago. The loss of marine biodiversity at the time has been linked to extreme greenhouse warming, triggered by the release of carbon dioxide from flood basalt volcanism in the central Atlantic Ocean. In contrast, the biotic turnover in terrestrial ecosystems is not well understood, and cannot be readily reconciled with the effects of massive volcanism. Here we present pollen, spore and geochemical analyses across the Triassic/Jurassic boundary from three drill cores from Germany and Sweden. We show that gymnosperm forests in northwest Europe were transiently replaced by fern and fern-associated vegetation, a pioneer assemblage commonly found in disturbed ecosystems. The Triassic/Jurassic boundary is also marked by an enrichment of polycyclic aromatic hydrocarbons, which, in the absence of charcoal peaks, we interpret as an indication of incomplete combustion of organic matter by ascending flood basalt lava. We conclude that the terrestrial vegetation shift is so severe and wide ranging that it is unlikely to have been triggered by greenhouse warming alone. Instead, we suggest that the release of pollutants such as sulphur dioxide and toxic compounds such as the polycyclic aromatic hydrocarbons may have contributed to the extinction.

15,000-yr pollen record of vegetation change in the high altitude tropical Andes at Laguna Verde Alta, Venezuela

Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9°N) Andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation. Although deglaciation commenced ∼15,500 cal yr B.P., the area around the Laguna Verde Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no modern analog colonized the superpáramo. Although this community persisted until ∼6000 cal yr B.P., it began to decline somewhat earlier, in synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was replaced by a low-diversity superpáramo community that became established ∼9000 cal yr B.P. This replacement coincides with regional declines in temperature and/or available moisture. Modern, more diverse superpáramo assemblages were not established until ∼4600 cal yr B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between 14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superpáramo, causing the apparent increase in tree pollen at high altitude. The association between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this relationship.

Tubeworm succession at hydrothermal vents: use of biogenic cues to reduce habitat selection error?

Species colonizing new deep-sea hydrothermal vents along the East Pacific Rise show a distinct successional sequence: pioneer assemblages dominated by the vestimentiferan tubeworm Tevnia jerichonana being subsequently invaded by another vestimentiferan Riftia pachyptila, and eventually the mussel Bathymodiolus thermophilus. Using a manipulative approach modified from shallow-water ecological studies, we test three alternative hypotheses to explain the initial colonization by T. jerichonana and its subsequent replacement by R. pachyptila. We show that R. pachyptila and another vestimentiferan, Oasisia alvinae, colonized new surfaces only if the surfaces also were colonized by T. jerichonana. This pattern does not appear to be due to restricted habitat tolerances or inferior dispersal capabilities of R. pachyptila and O. alvinae, and we argue the alternative explanation that T. jerichonana facilitates the settlement of the other two species and is eventually outcompeted by R. pachyptila. Unlike the classic model of community succession, in which facilitating species promote their own demise by modifying the environment to make it more hospitable for competitors, we suggest that T. jerichonana may produce a chemical substance that induces settlement of these competitors. This process of selecting habitat based on biogenic cues may be especially adaptive and widespread among later-successional species that occupy a physically variable and unpredictable environment. In these cases, the presence of weedy species implies some integrated period of environmental suitability, whereas an instantaneous assessment of physical habitat conditions, such as water temperature for vent tubeworms, provides a poorer predictor of long-term habitat suitability.

Physical and Biological Influences on the Formation of Sedimentary Fabric in an Oxygen-Restricted Depositional Environment: Eckernforde Bay, Southwestern Baltic Sea

Sedimentological and radiochemical investigations of sediments in Eckernforde Bay were coordinated with observations of benthic biology and benthic-boundary-layer dynamics to identify processes influencing the formation of preserved sedimentary fabric. The bay is a trap for fine sediments because of its fjord-like basin geometry and hydrodynamic regime. Sediments are derived from both local sources (transported by winter storms) and sources in adjacent Kiel Bight (fair-weather transport of suspended sediment by internal waves). In the central basin of the bay, strata consist of pelletized, clay-rich beds and silty laminations, and reflect the interaction of bioturbation with alternating fair-weather and storm-associated sediment transport and deposition. Sediment-accumulation rates in the central basin of the bay are approximately 0.39 cm/yr (from 210 Pb geochronology). Results from a numerical model, as well as study of X-radiographs and accumulation rates, indicate that fair-weather processes contribute 0.15 cm/yr of time-averaged sediment accumulation, and the remaining 0.24 cm/yr is contributed by storm-generated deposition. Seasonal oxygen depletion restricts the benthic community to a pioneer assemblage of small-bodied, deposit-feeding polychaetes and bivalves. Excess 234 Th analyses indicate that bioturbation is restricted to the upper 1 cm of the seabed. Macrofaunal feeding activity produces abundant ovoid fecal pellets and biogenic microfabric in beds deposited under fair-weather conditions. In contrast, primary depositional fabric is preserved as laminations when storm deposits thicker than the depth of bioturbation disrupt biological communities.

Colonization of new habitats by benthic foraminifera: a review

Colonization of new habitats, which have been established as a result of a catastrophic disturbance of the environment, is one of the characteristic repetitive events throughout the Phanerozoic. In recent years, much attention has been paid to investigations focusing on biological recovery of benthic habitats severely disturbed by human activity. In order to improve our environmental and stratigraphical interpretations of such events, we need a more thorough understanding of the processes involved in colonization by one of the most abundant and useful fossil groups, the benthic foraminifera. The present review focuses on processes governing benthic foraminiferal dispersion and colonization patterns in modern environments. For benthic foraminifera, the only active dispersal mechanism is through self-locomotion on or within the sediment and this is considered to be efficient over short distances only. Several passive dispersal methods have been suggested but two seem to be of more general importance. These are dispersal through release and transport of embryonic juveniles and passive suspension and transport of various growth stages. Both are probably important for most benthic foraminifera but the former is likely to be the main mechanism for attached, tubular and larger foraminifera, which are not easily entrained at a later life stage. The latter seems to be a more important dispersion mechanism for benthic foraminifera than previously realized. The colonization rate of soft-bottom substrates is closely related to the hydraulic regime in, and the transit time from, the source area inhabited by species capable of colonizing the new habitat (as long as food and other environmental characteristics are not limiting factors). The transit time depends on the speed of the transporting medium and the distance from the source area. There seems to be two end-processes which can operate during the colonization, depending on whether physically induced or biological processes are allowed to dominate. They are characterized by different colonization patterns. In high energy environments (bottom current velocities often >20 cm/s), a short transit time may cause the major components of the nearest ambient seafloor assemblages to colonize the new habitat within days. In this case the colonization is simply through a physical transfer of parts of the source community to the new habitat, allowing no time for pioneer, opportunistic assemblages to develop. In low energy environments (bottom current velocities generally <10 cm/s), the transit time is long for most species. Here, colonization follows the classic metazoan successional pattern with an initial, high abundance pioneer assemblage strongly dominated by small opportunists followed by development of assemblages with increasing numbers of specialized species and recovery can take from one to several years. Initial lack of food (e.g., volcanic ash) or `hostile' substrate properties (e.g., recently reoxygenated or severely contaminated sediments) may delay colonization by months or even years. Small, infaunal species (both calcareous and agglutinated) are among the first and most successful colonizers of soft bottom habitats from shallow waters to the deep sea. Throchospiral agglutinated taxa are among the most abundant colonizers on deep sea hard substrates.

The impact of eucalypt on the leaf-associated aquatic hyphomycetes in Spanish streams

Aquatic hyphomycete assemblages on decomposing leaf litter of the exotic species Eucalyptus globulus Labill. were compared with those on the native riparian species Alnus glutinosa (L.) Gaertn. in three low-order streams of the Agüera watershed (Basque Country – Cantabria, Spain). These streams contrast by the importance of eucalypt in the riparian vegetation and by the nutrient contents in the water. Neither the total conidial production nor the number of fungal species differed between the two leaf species in any of the streams. Fungal colonization of eucalypt leaves appeared to be delayed by about 2 weeks, probably owing to their high content in inhibitory constituents. While Flagellospora curvula Ingold largely dominated pioneer assemblages on both leaf species, the second most important species, Lunulospora curvula Ingold, exhibited a preference for eucalypt. In the stream surrounded by eucalypt, the fungal diversity was surprisingly lower on eucalypt than on alder. In the stream with higher orthophosphate concentrations, the conidial production was similarly increased on both leaf species. The present data together with recent results from a similar investigation in Portugal suggest a minor impact of eucalypt on the activity and diversity of aquatic hyphomycete leaf-associated assemblages in southern European streams. Key words: aquatic hyphomycetes, eucalypt, alder, leaf litter, stream.

Late Cretaceous and Paleocene Decapod Crustaceans from James Ross Basin, Antarctic Peninsula

Seventeen species of decapod crustaceans have been described from Campanian through Paleocene rocks in the Santa Marta, López de Bertodano, and Sobral Formations of the James Ross Basin, Antarctica. Of these, nine are new species:Metanephrops rossensis, Glyphea australensis, Paguristes santamartaensis, Munidopsis foersteri, Retrorsichela laevis, Plagiophthalmous collinsi, Rhinopoupinia bicornis, Cristafrons praescientis, andTorynomma (Torynomma) australis.One new family, Retrorsichelidae, and three new genera,Retrorsichela, Rhinopoupinia, andCristafrons, were also named. This assemblage includes the first notice of brachyurans from the Cretaceous of Antarctica; six species are described. The nephropid lobsterHoploparia stokesi(Weller), the most common decapod throughout the section, exhibits significant morphological change throughout its range from late Santonian or earliest Campanian to Paleocene; however, variation of key features is asynchronous. The raninid brachyuran,Cristafrons praescientis, is second in abundance toH. stokesi.The occurrence ofMetanephrops rossensisandMunidopsis foersterirepresents the oldest geological records for these genera and the recognition of species ofPaguristes, Plagiophthalmous, Torynomma, andNecrocarcinusconstitutes the first notice of these genera in Antarctica. Of those taxa that have living congenors, the species ofMetanephrops, Linuparus, andMunidopsisoccupied habitats at inner shelf depths in the Cretaceous whereas their extant descendants are restricted to outer shelf and bathyal depths. This diverse decapod fauna is dominated by genera that range into the Cenozoic and appears to be a pioneer assemblage.

Disturbance and Ecologic Succession in an Upper Ordovician Cobble-Dwelling Hardground Fauna

An Ordovician hardground fauna in northern Kentucky provides an example from the fossil record of the maintenance of species diversity by periodic disturbance of an autogenic ecologic succession. The marine invirtebrates found encrusting limestone cobbles show an order of community development from a low-diversity pioneer assemblage through a high-diversity association to a monospecific stable fauna. All species, including the late successional dominants, were present in the early stages of colonization. Periodic overturning of the cobbles killed the encrusters and opened up new spaces on which succession was reinitiated. These disturbances maintained high diversity within the encrusting community by limiting the spatial distribution of the most efficient space competitors.

Effects of hydrodynamic and biological processes on sediment geoacoustic properties in Long Island Sound, U.S.A.

Concurrent acoustical, physical, and biological properties were measured from replicate core liner and box-core samples collected by SCUBA divers from each of two locations in Long Island Sound, 27–28 August, 1980. A very low diversity pioneer assemblage dominated by the filter-feeding bivalve Mulinia lateralis was found at the shallower (10 m water depth) FOAM site. Nearby sediments were dominated by surface-dwelling tubiculous polychaetes and amphipods. Sediment laminations produced by storm-induced erosional and depositional events were preserved at the FOAM site because sediment mixing by macrofauna was uncommon below the upper few centimeters of sediment. Horizontal patchiness of macrofauna and preservation of sediment laminations resulted in vertical and horizontal variability of sediment physical and acoustical properties. Spatial and temporal changes of dominant species at the FOAM site resulted in considerable large scale (10–100 m) variability of physical and acoustical properties of surficial sediment. A low-diversity equilibrium assemblage dominated by surface deposit-feeding bivalves and deeper-dwelling errant and tubiculous polychaetes was found at the deeper (16 m water depth) NWC site. Intense bioturbation by surface deposit-feeding bivalves precluded preservation of primary laminations created by storm-induced erosional and depositional events. Bioturbation was responsible for spatial and temporal large-scale homogeneity of physical and acoustical properties of NWC sediments. Deeper-dwelling polychaetes mixed sediment to depths of 15 cm, creating random variability of fine-scale physical and acoustical structure by production of burrows, tubes and feeding voids, and by mixing shell remains throughout the upper 15 cm. Physical and acoustical properties of many coastal marine sediments are controlled by the interaction of biological and hydrodynamic processes. Study of relationships between these two processes and resultant sedimentary properties should lead to improvement of predictive geoacoustic models for coastal environments, especially where high levels of biological activity are expected.

Minimum age of deglaciation of upper Elk Valley, British Columbia

Bog sediments at an elevation of 1582 m in the upper Elk Valley of British Columbia were dated at three horizons. The lowermost of these, radiocarbon dated at 13 430 ± 450 years BP, provides a minimum age for deglaciation of Elk Valley. This date is probably also a minimum for the Canmore advance in the Bow Valley, Alberta, and suggests that the Canmore was not related to deposition of the Bighill Creek Formation at Cochrane. Pollen samples from the bog indicate a shrub–herb pioneer assemblage following deglaciation of that portion of the valley.