Silty Site Research Articles

Resuspension of benthic protists at subtidal coastal sites with differing sediment composition

Mechanisms and consequences of sediment resuspension are well studied in coastal environments, but the associated resuspension of micro-organisms is poorly understood. Using an in situ flume, and by sampling the benthic boundary-layer during tidal cycles, we measured resuspension of protists and sediment at a subtidal, sandy site in Buzzards Bay, Massachusetts, USA, and compared it to previous studies at a silty site in the bay with similar current magnitude. At the sandy site, where the bottom shear stress typically is greater on flooding than on ebbing tides, resuspension occurred only with the flooding current, whereas at the silty site, where the bottom stress is relatively similar on flooding and ebbing tides, resuspension occurred during both flood and ebb. For sediment and most protistan taxa at the sandy site, erosion thresholds were higher and the total material resuspended in strong flow was lower than at the silty site. At both sites, organic-rich, flocculent surficial sediment resuspended in weak flow, but the composition of the protistan community associated with it differed. At the sandy site it included heterotrophic nanoflagellates (HNan), euglenoid flagellates, oligotrich ciliates, and scuticociliates, whereas at the silty site it included HNan, oligotrich ciliates, and the diatom Navicula distans. Differences in the resuspension thresholds of ciliate groups between sites were due largely to differences in taxonomic composition, whereas differences for diatoms were linked to sediment dynamics. At both sites, protistan taxa resuspended in sequence as flow accelerated, implying that the assemblage of species exchanged between benthic and planktonic communities depends on the maximal bottom stress. At the silty site, however, the composition of this group should be more sensitive to variations in tidal strength (e.g. during the spring-neap cycle) than at the sandy site. Resuspension of protists may have strong impacts on microbial food-web ecology in the water column and sediments by linking the communities and by creating strong fluctuations of cell concentrations (measured up to 24x in the boundary-layer in the case of oligotrichs). Tidal resuspension of protists is common in the subtidal of Buzzards Bay, but its dynamics depend on local sediment properties, hydrography, and taxonomic composition.

Seafloor characterization from inversion of high-frequency backscatter

Geoacoustic inversion of high-frequency backscatter data is presented for characterization of the seafloor. The APL-UW (Applied Physics Laboratory at the University of Washington) backscattering model is used to model the grazing-angle dependence of the backscattering strength for a number of seafloor parameters. Backscattering strength versus grazing angle data sets (with frequencies 12–35 kHz) are provided for three sites along with sediment ground truth. Roughness measurements are also available for two of the sites. Inversion of these data sets is performed via simulated annealing, with some of the parameters constrained by empirical relationships with mean grain size. From the inversion, estimates are obtained for mean grain size, roughness, and volume interation. Inversion of data from a smooth, silty site in Arafura Sea yields estimates in good agreement with ground truth. Results also compare well with ground truth in a rough, sandy site (Quinault) and in Onslow Bay (sand). [Work supported by SPAWAR.]

Vertical rooting patterns of mature Quercus trees growing on different soil types in northern Germany

At two different sites in northern Germany, the vertical distribution patterns of roots with diameters of up to 10 mm were investigated with the trench profile wall technique beneath mature trees of Quercus petraea (Matt.) Liebl. growing on clayey (cambi-stagnic gleysol) or sandy soil (dystric cambisol). The rooting patterns were related to soil bulk density, maximum plant-available soil water and soil chemistry, and were compared with the rooting pattern of Q. petraea growing on a silty soil on limestone (luvisol) and with that of Q. robur L. growing on clayey loam (stagnic cambisol). All three Q. petraea sites differed in their vertical rooting pattern, as was indicated by significant differences in the parameter β which was calculated from the cumulative root fraction (Y) for the cumulative rooting depth (d; Y=1−βd). At the clayey site, a relatively large fraction of roots was found in deeper soil layers (highest β value); whereas, at the silty site, the rooting pattern was most superficial (lowest β value). No significant difference in the vertical rooting pattern was observed between Q. petraea and Q. robur growing on clayey soil. The soil-area related biomasses of living roots did not differ between the clayey and the sandy site of Q. petraea.Comparisons of the rooting patterns with soil water relations and soil chemistry lead to the hypothesis that under the climatic conditions of Central Europe, the vertical root distribution of Q. petraea is more influenced by the availability of nutrients, especially that of nitrogen, than by the amounts of plant-available soil water.

Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments:an annual study

The annual cycle of oxygen and nitrogen flux, denitrification and microphytobenthic variables (primary production, biomass, composition and calculated N demand) were studied for 2 shallow-water microtidal sediment sites in NE Kattegat, 1 sandy and 1 silty, by incubating undisturbed sediment in the laboratory in light and darkness. Both sites (2 stations within each) were characterised by low concentrations of inorganic nitrogen (IN) in the overlying water during summer (NO 3 generally < 1 μM), with winter concentrations of 5 and 30 pM for the sandy and silty site, respectively. Through the activity of microphytobenthos, the sediment systems appeared to be net autotrophic during most of the year. Net oxygen production varied between -400 and 6600 pmol m -2 h -1 , being highest during the warmest season. Although the composition of the microphytobenthos depended on the sediment type, it did not have a crucial effect on the magnitude of the microphytobenthic biomass or function. The temporal pattern of the function of the microalgal community, on the other hand, was significantly influenced by the sediment type; sandy sediment exhibited a smooth seasonality, controlled mainly by temperature and light, while the silty microtidal sediment was also controlled by stochastic events, such as sediment resuspension. Microphytobenthos had a significant effect on the IN flux, the clearest effect being found for NH 4 . Total denitrification (isotope-pairing technique) generally varied between <1 and 40 μmol m -2 h -1 , being dominated by nitrification-coupled denitrification (D n ), and being 1 order of magnitude higher at the silty site. Microphytobenthic activity generally inhibited denitrification in the low-N areas in this study. The results suggest that the microphytobenthos functions as a major control throughout the annual cycle, by forming communities that are net photoautotrophic throughout the year, and by significantly influencing both the IN flux and denitrification rates. Sandy sediment appeared to function as an IN sink during winter and early spring, while no clear seasonal pattern was found for silty sediment. Calculated N demand of the microphytobenthos far exceeded the measured sediment net uptake of N, supporting the idea that sandy systems in low-nitrogen areas can be highly productive through a closed recycling of N. The ratio between calculated microphytobenthic N demand and measured denitrification rates suggests that denitrification has a minor role as a N sink, particularly in sandy, cold-climate microtidal sediments.

Estimating primary production rates from photosynthetic electron transport in estuarine microphytobenthos

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 204:39-52 (2000) - doi:10.3354/meps204039 Estimating primary production rates from photosynthetic electron transport in estuarine microphytobenthos Christiane Barranguet1,2, Jacco Kromkamp1,* 1Netherlands Institute of Ecology, Centre for Coastal and Estuarine Ecology (NIOO-CEMO), PO Box 140, 4400 AC Yerseke, The Netherlands 2University of Amsterdam, Department of Aquatic Ecology and Ecotoxicology, Kruislaan 320, 1098 SM Amsterdam, The Netherlands *E-mail: kromkamp@cemo.nioo.knaw.nl ABSTRACT: Microphytobenthos primary production was measured using C fixation and compared with production estimates based on measurements on intact cores using variable chlorophyll fluorescence with a PAM (pulse-amplitude-modulated) fluorometer at a number of different sites on intertidal flats in the Westerschelde and Oosterschelde estuaries (The Netherlands). We observed a linear relationship between the minimal fluorescence, F0, and the chlorophyll a content of the sediments, but the relationship was quite noisy, which was partly due to a vertical scale mismatch between the depth of sampling (1 mm) and the measuring depth the of PAM fluorometer (100 to 200 ?m). On most sites, C fixation was a linear function of photosystem II electron transport (ETR, obtained from PAM measurements), although for Stn B, a relatively silty site on the Molenplaat, an intertidal flat in the Westerschelde, the agreement between C fixation and ETR was often non-linear at irradiances saturating C fixation. This was sometimes observed for some of the other stations as well. The differences at high irradiance might be related to vertical migration of epipelic diatoms, optical properties of the sediment or alternative electron acceptors. A conversion factor was calculated, allowing estimation of C fixation from ETR measurements. This conversion factor was not different for the different stations and no clear seasonal influence was observed, although the conversion factor could vary within hours. The seasonally averaged conversion factor for all stations and each site was used to calculate areal rates of production and compared with primary production estimates obtained from the C fixation measurements. It appeared that the PAM-based estimates gave a good prediction of the (potential) C fixation, with the exception of a number of dates for Molenplaat Stn B. This demonstrates that the variable fluorescence technique can become, after further calibration with 14C for other shallow sediments, a very useful tool in production studies, and can be instrumental in obtaining a good spatial coverage of primary production, which remains very difficult due to the patchiness in microphytobenthos biomass and production. KEY WORDS: Chlorophyll fluorescence · Photosynthetic electron transport · C fixation · Microphytobenthos · Primary production · Tidal flats Full text in pdf format PreviousNextCite this article as: Barranguet C, Kromkamp J (2000) Estimating primary production rates from photosynthetic electron transport in estuarine microphytobenthos. Mar Ecol Prog Ser 204:39-52. https://doi.org/10.3354/meps204039Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 204. Online publication date: October 05, 2000 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2000 Inter-Research.

Gradients in the Composition, Structure, and Diversity of Remnant Oak Savannas in Southern Wisconsin

Savannas with scattered oak canopies, once the most widespread communities in southern Wisconsin, now are among the most endangered. Surprisingly little is known about the composition, structure, and horizontal patterning of their species-rich ground layers. This study relates the distribution and ecological characteristics of 417 ground-layer species to local and regional gradients in soil composition and light regime, based on an analysis of 722 1-m2 quadrats in 12 remnant savannas. Our findings have important implications for efforts to restore/conserve midwestern oak savannas. Ground-layer composition was strongly related to among-site differences in soil texture and within-site differences in light availability, with variation in sand vs. silt content accounting for twice as much turnover in species composition as that accounted for by direct photon flux density (PFD) estimated from hemispherical photographs. Most species reached peak coverage under sunny or partly shaded conditions. Flowering/fruiting was often skewed toward sunnier microsites. Absolute forb cover increased with silt content and declined with PFD. Graminoid cover showed a curvilinear relationship to soil texture and light, being highest in well-lit, moderately sandy microsites. Total ground-layer cover increased with silt content at a given irradiance; it increased with PFD on silty sites and decreased with PFD on sandy sites. Forb cover increased regularly with PFD and sand content. When quadrats were stratified by sand content and PFD, species richness of forbs and graminoids increased linearly with coverage by each group, with far more forb species present at a given coverage. Among graminoids, C4 grasses were common only in bright, sandy microsites; C3 grasses and sedges had broader ecological distributions. Among forbs, leaf width increased and leaf inclination became more horizontal toward shadier and siltier microsites; tall herbs were generally found in silty areas with dense ground-layer cover. Plants with N-fixing symbioses were found mostly on sandy, well-lit microsites, although climbing species occured on shadier and/or siltier microsites, where N-fixation might be expected to be less advantageous. Most ground-layer species were perennial (88%), with few annuals (6%) or biennials (6%). For the 85 most common species, breadth of distribution across savanna microhabitats (4 soil × 4 light) was significantly correlated with presence across 34 Wisconsin community types, suggesting that similar factors help to constrain distributions at local and regional scales. Species in the two largest genera (Aster and Solidago) differed significantly in distribution according to the Syrjala test, supporting habitat partitioning as a mechanism of coexistence. Oak savannas are unusually diverse. At small spatial scales, there were 16.1 ± 1.3 species/m2 (mean ± 1 sd), compared with 11.4 ± 2.7 for prairies and 8.2 ± 2.5 for forests. At slightly larger scales, savannas showed high spatial turnover in ground-layer composition: 89.3 ± 12.0 species/20 m2, compared with 41.5 ± 8.3 for prairies and 42.8 ± 8.0 for forests. At large spatial scales, a survey of 22 savanna remnants (42 ha) showed extensive floristic differentiation: 507 native plant species, ∼27% of Wisconsin's indigenous vascular flora (∼14 × 106 ha). Contrary to previous reports, Midwest oak savannas are forb dominated, except on the sandiest or sunniest microsites. Release of forbs from competition with C4 grasses under partly shaded conditions may help to account for the high diversity of savanna ground layers relative to prairies. Divergent distributions of plants with different adaptations for energy capture, together with large variation within sites in ground-layer light regime and among sites in soil texture, suggest that partitioning of light and soil gradients is important for maintaining the high plant diversity of oak savannas. Mass effects (involving dispersal subsidies from favorable microsites) may also play a role in maintaining diversity within these mosaic communities.

GRADIENTS IN THE COMPOSITION, STRUCTURE, AND DIVERSITY OF REMNANT OAK SAVANNAS IN SOUTHERN WISCONSIN

Savannas with scattered oak canopies, once the most widespread communities in southern Wisconsin, now are among the most endangered. Surprisingly little is known about the composition, structure, and horizontal patterning of their species-rich ground layers. This study relates the distribution and ecological characteristics of 417 ground-layer species to local and regional gradients in soil composition and light regime, based on an analysis of 722 1-m2 quadrats in 12 remnant savannas. Our findings have important implications for efforts to restore/conserve midwestern oak savannas. Ground-layer composition was strongly related to among-site differences in soil texture and within-site differences in light availability, with variation in sand vs. silt content accounting for twice as much turnover in species composition as that accounted for by direct photon flux density (PFD) estimated from hemispherical photographs. Most species reached peak coverage under sunny or partly shaded conditions. Flowering/fruiting was often skewed toward sunnier microsites. Absolute forb cover increased with silt content and declined with PFD. Graminoid cover showed a curvilinear relationship to soil texture and light, being highest in well-lit, moderately sandy microsites. Total ground-layer cover increased with silt content at a given irradiance; it increased with PFD on silty sites and decreased with PFD on sandy sites. Forb cover increased regularly with PFD and sand content. When quadrats were stratified by sand content and PFD, species richness of forbs and graminoids increased linearly with coverage by each group, with far more forb species present at a given coverage. Among graminoids, C4 grasses were common only in bright, sandy microsites; C3 grasses and sedges had broader ecological distributions. Among forbs, leaf width increased and leaf inclination became more horizontal toward shadier and siltier microsites; tall herbs were generally found in silty areas with dense ground-layer cover. Plants with N-fixing symbioses were found mostly on sandy, well-lit microsites, although climbing species occured on shadier and/or siltier microsites, where N-fixation might be expected to be less advantageous. Most ground-layer species were perennial (88%), with few annuals (6%) or biennials (6%). For the 85 most common species, breadth of distribution across savanna microhabitats (4 soil × 4 light) was significantly correlated with presence across 34 Wisconsin community types, suggesting that similar factors help to constrain distributions at local and regional scales. Species in the two largest genera (Aster and Solidago) differed significantly in distribution according to the Syrjala test, supporting habitat partitioning as a mechanism of coexistence. Oak savannas are unusually diverse. At small spatial scales, there were 16.1 ± 1.3 species/m2 (mean ± 1 sd), compared with 11.4 ± 2.7 for prairies and 8.2 ± 2.5 for forests. At slightly larger scales, savannas showed high spatial turnover in ground-layer composition: 89.3 ± 12.0 species/20 m2, compared with 41.5 ± 8.3 for prairies and 42.8 ± 8.0 for forests. At large spatial scales, a survey of 22 savanna remnants (42 ha) showed extensive floristic differentiation: 507 native plant species, ∼27% of Wisconsin's indigenous vascular flora (∼14 × 106 ha). Contrary to previous reports, Midwest oak savannas are forb dominated, except on the sandiest or sunniest microsites. Release of forbs from competition with C4 grasses under partly shaded conditions may help to account for the high diversity of savanna ground layers relative to prairies. Divergent distributions of plants with different adaptations for energy capture, together with large variation within sites in ground-layer light regime and among sites in soil texture, suggest that partitioning of light and soil gradients is important for maintaining the high plant diversity of oak savannas. Mass effects (involving dispersal subsidies from favorable microsites) may also play a role in maintaining diversity within these mosaic communities.

Nature and ecological implications of algal pigment diversity on the Molenplaat tidal flat (Westerschelde estuary, SW Netherlands)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 180:51-64 (1999) - doi:10.3354/meps180051 Nature and ecological implications of algal pigment diversity on the Molenplaat tidal flat (Westerschelde estuary, SW Netherlands) Cathy H. Lucas*, Patrick M. Holligan School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom *E-mail: catherine.h.lucas@soc.soton.ac.uk ABSTRACT: Depth profiles of chlorophyll a (chl a), its degradation products, and accessory pigment markers from 5 sites with differing sediment properties highlighted the role of physical and biological processes in re-distributing microalgal biomass both horizontally and vertically around the Molenplaat tidal flat. Intermediate-scale distribution of chl a was highly correlated with sedimentological parameters, with biomass greatest at siltier sites. Fucoxanthin:chl a ratios of 0.35 to 1.60 indicated that the microphytobenthos community was dominated by diatoms in both silty and sandy sediments. Cyanobacteria (zeaxanthin) were also present, mainly in September. Other sources of algae came in the form of deposited water column material. Peridinin, 19'hexanoyloxyfucoxanthin, and to a lesser extent chl b were the main pigments, primarily found at silty sites. Between June and September, the relative amounts of accessory pigments increased suggesting that inputs of material from the water column occurred during this period. Although benthic and water column pigments were found in the surface layers of silty and sandy sites, there were pronounced differences in biomass and vertical distribution. At sandy sites, homogenous vertical distributions of chl a in the upper 2 cm of sediment, together with low phaeopigment levels and the absence of water colum pigments below the surface layer, suggest that accumulation of in situ and allochthonous sources of microalgae is insignificant. Material is subjected to a continuous cycle of burial and resuspension during each tidal cycle and through the action of bioturbators such as Arenicola marina. There was a pronounced peak in chl a biomass at the silty sites in June. Rapidly decreasing chl a over the top 1 cm suggests that degradation was more rapid than sediment mixing. Large amounts of phaeopigments were produced, with the relative contribution switching from phaeophytins (March/April) to phaeophorbides (September) as the role of the herbivorous community in chl a cycling became more significant. An increase in relative contributions of accessory pigments both at the surface and in deeper layers between spring and autumn suggest that silty sites are a sink for sedimenting algae from the water column during this time. During the late autumn-winter, chl a biomass and pigment diversity decreased considerably, coincident with reduced biological activity and increased physical mixing. KEY WORDS: Microphytobenthos · Diatoms · HPLC · Algal pigments · Degradation processes · Sedimentation · Tidal flats Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 180. Publication date: May 03, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Taxon-specific tidal resuspension of protists into the subtidal benthic boundary layer of a coastal embayment

Sediment resuspension has widespread effects on microbial processes, primary and secondary production, and nutrient cycles, but its influence on protists other than microalgae is largely unknown. Distributions and abundances of protists in subtidal benthic boundary layers (BBL), in particular, are poorly known. We measured vertical profiles of protists in the BBL and underlying sediment at a subtidal silty site in Buzzards Bay, Massachusetts, USA, to determine cell-resuspension patterns. Tidal flow produced maximal bottom shear velocities of 1.4 to 2.2 cm s -1 . Near-bottom turbidity increased during each slack tide, when the suspended load settled, and it decreased during tidal exchange, presumably after a thin veneer of sediment resuspended from the sediment-water interface (SWI) and mixed into the upper water column. Tidal periodicities in protistan vertical profiles were taxon- and functional-group specific. Heterotrophic nanoflagellates (HNan) and ciliates, including scuticociliates, oligotrichs, and hypotrichs of the genera Euplotes and Urostrongylum, showed periodicities in distribution consistent with cycles of resuspension and deposition. BBL concentrations of HNan and scuticociliates were elevated during tidal exchange by factors of ≤2.1 and 4.6, respectively, within 5 cm of the SWI; oligotrichs were found consistently in the BBL but were in the sediment only during slack tide; Euplotes was present consistently in the sediment but was in the BBL only during tidal exchange. Total resuspended cells in the bottom 1 m were of the order 10 8 to 10 9 HNan m -2 and 10 5 to 10 6 ciliates m -2 , and in some cases the measured cell disappearance from surficial sediment during tidal exchange balanced the increase in the BBL. In contrast, pigmented nanoflagellates, pennate diatoms, and ciliates, including karyorelictids and other hypotrichs, maintained constant profiles throughout tidal cycles. Specificity of results among protistan groups might be due to behavioral adaptations such as depth zonation in the sediment, associations with particles, and vertical migration. We know of no other documentation in the field of cyclical emergence of heterotrophic protists and re-entry into sediment. Our data suggest complex taxon-specific linkages between sedimentary and water-column protistan communities that may be controlled by flow in the BBL, potentially influencing food-web dynamics.

High-frequency acoustic observations of benthic spatial and temporal variability

A bottom-mounted sonar operating at 40 kHz has been used to measure the variation of bottom acoustic scattering over extended time intervals at two shallow sites as part of the Coastal Benthic Boundary Layer Special Research Project. The acoustic data were analyzed using a correlation method that measures the spatial and temporal dependence of benthic change. The rate of decorrelation was two orders of magnitude more rapid at a sandy site near Panama City, Florida, USA, than at a silty site in Eckernforde Bay, Germany, and both sites were characterized by hot spots or localized regions of activity.

Herbage Dynamics on 2 Northern Great Plains Range Sites

Quantity and quality of forage produced are primary determinants of level of livestock production derived from grazing lands. Moreover, species composition of herbage is often considered a primary determinant of the ecological condition of rangelands. The broad objective of this study was to quantify the productivity, growth dynamics, and quality of herbage growing on 2 Northern Great Plains range sites and to concurrently relate magnitude and composition of production to the ecological condition of the sites. Using frequent harvest techniques, the 2-year study showed herbage production on the highly productive silty range site averaged 219 g m-2 as compared to 218 g m-2 on the supposedly less productive clay pan range site. The primary reason the clay pan site proved to be as productive as the silty site was attributed to the greater amounts of introduced annual grasses on the clay pan (148 g m-2) than the silty site (104 g m-2). The annual grass component on the clay pan was a near equal mix (71 vs 51 g m-2) of Japanese brome (Bromus japonicus Thunb.) and cheatgrass (B. tectorum L.) whereas the overwhelming dominant on the silty site was cheatgrass (73 g m-2). Western wheatgrass [Pascopyrum smithii Rydh. (Love)] was the dominant perennial grass on both sites averaging 49 g m-2 on the clay pan site and 57 g m-2 on the silty site. There were minimal differences between sites in terms of nutrient quality values (i.e., crude protein, acid detergent fiber, neutral detergent fiber) with results showing clearly that age of tissue was the major factor altering seasonal forage quality values. Range condition analyses revealed the clay pan site was in fair ecological condition and the silty site was in good condition. Study results demonstrate the need for land management agencies to continue to refine productivity estimates as well as adopt new techniques for assessing the ecological condition of rangelands.

High-frequency bottom backscattering: Roughness versus sediment volume scattering

High-frequency bottom acoustic and geoacoustic data from three well-characterized sites of different bottom composition are compared with scattering models in order to clarify the roles played by interface roughness and sediment volume inhomogeneities. Model fits to backscattering data from two silty sites lead to the conclusion that scattering from volume inhomogeneities was primarily responsible for the observed backscattering. In contrast, measured bottom roughness was sufficient to explain the backscattering seen at a sandy site. Although the sandy site had directional ripples, the model and data agree in their lack of anisotropy.

The STRESS sonar experiment

A high‐frequency scanning sonar experiment has been conducted as part of the STRESS Program (sediment transport events: slopes and shelves). An autonomous platform was used to scan a circular region 100 m in radius with 5° angular resolution. The acoustic frequency was 40 kHz and the transmitted signal was an FM pulse of 2‐kHz bandwidth. The objective was to quantify changes in bottom scattering due to storm‐induced sediment transport events. The acoustic experiment was conducted in water of depth 90 m at a silty site off the California coast. Other experiments measured environmental parameters such as sediment properties, bottom currents, and suspended sediment concentration. Scans were made at a rate of ten per day over a period of 13 weeks during the winter of 1988–1989. The acoustic data have been processed to determine scattering strength and the correlation between scans as a function of time separation. These results are presented in the form of images showing the evolution of the scattering process during quiet and stormy periods. Very little change is seen in the scattering strength, but the correlation magnitude exhibits a monotonic decrease that accelerated during a storm. The phase of the complex correlation provides a very sensitive indicator of small changes. Two applications of these phase data will be presented. [Work supported by ONR.]

Establishment of four Salicaceae species on river bars in interior Alaska

Patterns of seed and vegetative reproduction were investigated for three floodplain species, balsam poplar Populus balsamifera L., feltleaf willow Salix alaxensis (Anderss.) Cov. and sandbar willow Salix interior Rowlee, and one upland species, trembling aspen Populus tremuloides Michx., in interior Alaska. All four species have similar patterns of seed germination in response to moisture stress and high salt concentrations when tested under laboratory conditions. In field experiments, percent germination of all four species was also very similar, ranging from 0% on dry sandy sites, to greater than 60% on mesic silty sites. Germination on salt crusts ranged from 0–40% for all species, depending on the physical characteristics of the soil surface. Colonization of mesic silty sites was almost exclusively by seed, whereas colonization of dry sandy sites was limited to those species which were able to root sucker under floodplain conditions. Root suckering was also an important means of establishment on frequently inundated sites where establishment by seed was limited by flooding. Differences between the species in their distribution on the floodplain were related to differences in patterns of vegetative reproduction, but not seed germination.

A study of the dynamics of hummocky microrelief in the Canadian north

Microrelief consisting of mounds or hummocks was examined at selected sites in the discontinuous and continuous permafrost zones of the Canadian north. Observations of the macromorphology and micromorphology of the mounds were used to interpret the dynamics of their formation. There was evidence that pressures were created inside the mounds as the freezing interface migrated towards the permafrost table during early winter. These pressures moved material inwards and upwards within the mounds, causing the plasmic lamellae and flattened quartz grains in the microfabric to rotate into a preferred alignment. Except at the most silty sites, the mounds were fractured at their centres. Subsoil spread over the surface of the sandier mounds, or the surface was severely heaved. This kind of microrelief can easily be damaged by traffic.