Influence of the mode of macrofauna-mediated bioturbation on the vertical distribution of living benthic foraminifera: First insight from axial tomodensitometry

We investigated the influence of bioturbation by macrofauna on the vertical distribution of living (stained) benthic foraminifera in marine intertidal sediments. We investigated the links between macrofaunal bioturbation and foraminiferal distribution, by sampling from stations situated on a gradient of perturbation by oyster-farming, which has a major effect on benthic faunal assemblages. Sediment cores were collected on the French Atlantic coast, from three intertidal stations: an oyster farm, an area without oysters but affected by oyster biodeposits, and a control station. Axial tomodensitometry (CT-scan) was used for three-dimensional visualization and two-dimensional analysis of the cores. Biogenic structure volumes were quantified and compared between cores. We collected the macrofauna, living foraminifera, shells and gravel from the cores after scanning, to validate image analysis. We did not investigate differences in the biogenic structure volume between cores. However, biogenic structure volume is not necessarily proportional to the extent of bioturbation in a core, given that many biodiffusive activities cannot be detected on CT-scans. Biodiffusors and larger gallery-diffusors were abundant in macrofaunal assemblage at the control station. By contrast, macrofaunal assemblages consisted principally of downward-conveyors at the two stations affected by oyster farming. At the control station, the vertical distribution of biogenic structures mainly built by the biodiffusor Scorbicularia plana and the large gallery-diffusor Hediste diversicolor was significantly correlated with the vertical profiles of living foraminifera in the sediment, whereas vertical distributions of foraminifera and downward-conveyors were not correlated at the station affected by oyster farming. This relationship was probably responsible for the collection of foraminifera in deep sediment layers (> 6 cm below the sediment surface) at the control station. As previously suggested for other species, oxygen diffusion may occur via the burrows built by S. plana and H. diversicolor, potentially increasing oxygen penetration and providing a favorable microhabitat for foraminifera in terms of oxygen levels. By contrast, the absence of living foraminifera below 6 cm at the stations affected by oyster farming was probably associated with a lack of biodiffusor and large gallery-diffusor bioturbation. Our findings suggest that the effect of macrofaunal bioturbation on the vertical distribution of foraminiferal assemblages in sediments depends on the effects of the macrofauna on bioirrigation and sediment oxidation, as deduced by Eh values, rather than on the biogenic structure volume produced by macrofauna. The loss of bioturbator functional diversity due to oyster farming may thus indirectly affect infaunal communities by suppressing favorable microhabitats produced by bioturbation.     

Quantification of biogenic 3-D structures in marine sediments

Burrow and tube structures of marine infauna were quantified in sediments from cores obtained at 3 depths (75, 95 and 118 m) at 3 occasions over a 1-year period. The benthic communities at the two deeper stations were re-establishing subsequent to re-oxygenation after hypoxic conditions; the shallow station was a reference station unaffected by hypoxia. The benthic macrofauna was simultaneously quantified from sieved grab samples. The sediment cores were frozen and later sliced with a plane in horizontal layers. Biogenic structures in these sections were digitally analysed for numbers, area and volume. Number of tubes and burrows showed a general decline with depth in the sediment. The biogenic structures in the sediment at the reference station increased the sediment-water interface about 1.5 times compared to the surface area. The surface areas of burrows and tubes were 2 to 3 times larger at the reference station than at the deeper stations. The benthic fauna at the 95-m and 118-m stations showed a temporal increase in density and number of species. In contrast, development of biogenic surface areas in the sediment at these stations showed no significant temporal trend during the benthic faunal succession. Relic tubes of Melinna cristata were found throughout the sampling period at the 118-m station. The ecological importance of bioirrigation for biogeochemical processes is discussed.     

Small-scale heterogeneity in deep-sea nematode communities around biogenic structures

The unexpected high species richness of deep-sea sediments gives rise to the questions, which processes produce and maintain diversity in the deep sea, and at what spatial scales do these processes operate? The idea of a small-scale habitat structure at the deep-sea floor provides the background for this study. At small scales biogenic structures create a heterogeneous environment that influences the structure of the surrounding communities and the dynamics of the meiobenthic populations. As an example for biogenic structures, small deep-sea sponges (Tentorium semisuberites Schmidt 1870) and their sedimentary environment were investigated for small-scale distribution patterns of benthic deep-sea nematodes. Sampling was carried out with the remotely operated vehicle Victor 6000 at the Arctic deep-sea observatory HAUSGARTEN. In order to investigate nematode community patterns sediment cores around three small sponges and corresponding control cores were analysed. A total of approx. 5800 nematodes were identified. The comparison of the nematode communities from sponge and control samples indicated an influence of the biogenic structure “sponge” on diversity patterns and habitat heterogeneity. The increased number of nematode species and functional groups found in the sediments around the sponges suggest that on a small scale the sponge acts as a gradient and creates a more divers habitat structure. The nematode community from the sponge sediments shows a greater taxonomic variance and species richness together with lower relative abundances of the species compared to those from control sediments. Obviously, the more homogeneous habitat conditions of the control sediments offer less micro-habitats than the sediments around the sponges. This seems to reduce the number of functional groups and species coexisting in the control sediments.     

Biodiversity of benthic invertebrates and organic matter processing in shallow marine sediments: an experimental study

The main objective of this study was to measure the impact of benthic invertebrate diversity on processes occurring at the water-sediment interface. We analyzed the effects of interactions between three shallow water species (Cerastoderma edule, Corophium volutator, and Nereis diversicolor). The impacts of different species richness treatments were measured on sediment reworking, bacterial characteristics, and biogeochemical processes (bromide fluxes, O₂ uptake, nutrient fluxes, and porewater chemistry) in sediment cores. The results showed that the three species exhibited different bioturbation activities in the experimental system: C. edule acted as a biodiffusor, mixing particles in the top 2 cm of the sediments; C. volutator produced and irrigated U-shaped tubes in the top 2 cm of the sediments; and N. diversicolor produced and irrigated burrow galleries in the whole sediment cores. C. edule had minor effects on biogeochemical processes, whereas the other species, through their irrigation of the burrows, increased the solute exchange between the water column and the sediment two-fold. These impacts on sediment structure and solute transport increased the O₂ consumption and the release of nutrients from sediments. As N. diversicolor burrowed deeper in the sediment than C. volutator, it irrigated a greater volume of sediments, with great impact on the sediment cores.Most treatments with a mixture of species indicated that observed values were often lower than predicted values from the addition of the individual effects of each species, demonstrating a negative interaction among species. This type of negative interaction measured between species on ecosystem processes certainly resulted from an overlap of bioturbation activities among the three species which lived and foraged in the same habitat (water-sediment interface). All treatments with N. diversicolor (in isolation and in mixture) produced similar effect on sediment reworking, water fluxes, nutrient releases, porewater chemistry, and bacterial characteristics. Whichever species associated with N. diversicolor, the bioturbation activities of the worm hid the effect of the other species. The results suggest that, in the presence of several species that use and modify the same sediment space, impact of invertebrates on ecosystem processes was essentially due to the most efficient bioturbator of the community (N. diversicolor). In consequence, the functional traits (mode of bioturbation, depth of burrowing, feeding behaviour) of an individual species in a community could be more important than species richness for some ecosystem processes.     

Benthic and hyporheic macroinvertebrate distribution within the heads and tails of riffles during baseflow conditions

The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. Riffle heads were found to contain a greater proportion of interstitial fine sediment than riffle tails. Significant differences in the composition of benthic communities were associated with the amount of fine sediment. Riffle tail habitats supported a greater abundance and diversity of invertebrates sensitive to fine sediment such as EPT taxa. Shredder feeding taxa were more abundant in riffle heads suggesting greater availability of organic matter. In contrast, no significant differences in the hyporheic community were recorded between riffle heads and tails. We hypothesise that clogging of hyporheic interstices with fine sediments may have resulted in the homogenisation of the invertebrate community by limiting faunal movement into the hyporheic zone at both the riffle heads and tails. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale.     

Persistence of egg pocket architecture in redds of chum salmon, Oncorhynchus keta

Synopsis Analysis of frozen cores of gravel surrounding the egg pockets of chum salmon, Oncorhynchus keta, collected in the fall revealed that spawning activity by females purged about 75 % of the fine sediments from the stream bed. The egg pocket was one of four distinct vertical strata detected in the cores. There was an undisturbed layer below the egg pocket, and separate bridge and cover strata above the egg pocket, all defined by different particle size distributions. However, by spring most of the egg pockets had been infiltrated with fine sediment and the particle size distribution approached background levels. The most likely physical factors responsible for these results were: (1) intrusion of fine sediments through the cleaned surface gravel, (2) lateral subsurface migration of fine sediment into interstitial voids, (3) scour of the surface gravel and subsequent deposition of a sand rich bedload, and (4) superimposed spawning activity of other fish, causing disturbance of the cleaned surface gravel and exposing the egg pocket to intrusion of fine particles. We conclude that, while female salmon substantially affect the physical environment of their embryos, subsequent sediment transport processes and fine bedload flux tend to return this environment to pre-spawning conditions.     

Influence of Bed‐Sediment Features on the Interstitial Habitat Available for Macroinvertebrates in 15 French Streams

The interstitial space of bed‐sediments is a major component of the stream system as it influences both physical and biological processes and patterns, such as exchanges between surface water and groundwater or the assemblage of invertebrates. To investigate the influence of grain‐size distribution and vertical bed‐packing on the interstitial habitat, we used 99 frozen cores collected from 15 French streams. On our data, overall invertebrate density, density of Chironomidae and taxon richness were more strongly related to effective porosity than to other physical variables describing bed‐sediments. We built two models to quantify the influence of grain‐size distribution and vertical bed‐packing on effective porosity. The effective porosity decreased with increasing grain‐size heterogeneity and was strongly related to grain‐size distribution. These relationships differed with vertical bed‐packing, but were consistent across three geologic types of sediments that varied in particle shape and particle texture. These general relationships suggest that effective porosity could be helpful to better understand the distribution patterns of invertebrates in bed‐sediments.     

Hyporheic community composition in a gravel-bed stream: influence of vertical hydrological exchange, sediment structure and physicochemistry

Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini‐piezometers, each installed in a different upwelling or downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates. 3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L^?1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than downwelling areas. 4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter. 5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations. 6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.     

Historical Assessment of the Impacts of Chemical Contaminants in Sediments on Benthic Invertebrates in the Tidal Passaic River,New Jersey

A historical impact assessment was conducted for chemical contaminants in sediments of the tidal Passaic River in northern New Jersey. The assessment was based on sediment cores collected from 1990 to 1995. Each sediment core was segregated into fractions and the fractions dated using radioisotope analysis. Chemical concentrations, including a variety of metals and organic compounds, were estimated by decade for most of the 20th century in five reaches of the River. The chemical data for each decade were compared to available benchmark sediment quality values that represent levels of chemicals that may be toxic to benthic invertebrates in estuarine systems. Benchmark exceedances in the River were then calculated and characterized spatially and temporally using quantitative Geographic Information System (GIS) analyses, and the area of “impacted” sediments was calculated for each chemical and decade. Results of this assessment suggest that the ability of Passaic River sediments to support “normal” benthic invertebrate populations was limited by sediment contaminants throughout the 20^th century. Conditions have improved somewhat since the 1950s, although impacts to benthic populations remain from several metals and organic compounds despite some overall improvements in sediment quality in recent years.     

Video-logging for examining biogenic structures in deep heterogeneous subsurface sediments

Video-logging through a transparent tube is described as an alternative method for examining in situ the distribution of biogenic structures within the deep layers of heterogeneous sediments in which undisturbed cores are hard to be obtained. The method comprises four successive phases: the installation of transparent tubes into the sediment, the lowering of a video-camera for scanning sediments through the tube walls, the numerical treatment of images for correcting deformations and obtaining panoramic views of discrete sediment layers, and the mapping of biogenic structures. The method was used successfully to determine the density and depth distribution of galleries made by the oligochaetae Tubifex tubifex (Müller, 1874) within the infiltration bed of a stormwater basin.     

Spatial distribution of zoobenthos in the watercourses of the Biya River basin (Altai)

The composition and spatial distribution of macrozoobenthos were studied in 26 watercourses of the Biya River basin from 1997 to 2004. The bottom fauna of the basin’s rivers includes 382 species, 217 genera, 81 families, 21 orders, 11 classes, and 7 types of invertebrates. Species common in the northern and eastern Palearctic prevail, presumably evidencing the oriental origin of the rheophylic benthic fauna. Four basic and two intermediate types of benthic communities were revealed. These communities differ in the structures of the complexes dominant by biomass. The successive substitution of the communities from the upper reaches of Lake Teletskoye tributaries to the Biya mouth depends on factors related to the basin’s relief and corresponds to changes in the landscape and hydrological conditions. This substitution generally conforms to the concept of river continuum.     

Experimental study on the influence of bioturbation performed by Nephtys caeca (Fabricius) and Nereis virens (Sars) annelidae on the distribution of dinoflagellate cysts in the sediment

Dinoflagellates include noxious microalgae responsible for the formation of toxic red tides and the poisoning of molluscs and crustaceans, resulting in important economic losses. As a consequence, the life cycle of these algae has been extensively studied, but the dormancy phase (cyst) in the sediment record is little known. In the intertidal zone, bioturbation, an important biological process resulting from the activities of benthic fauna, significantly influences the movement of particles in the sediments. Laboratory experiments have allowed comparing and quantifying the movements of fluorescent microspheres resulting from the activity of two polychaetes annelidae, Nereis virens and Nephtys caeca. The particles, which simulate 45 µm diameter dinoflagellate cysts, are deposited in flat aquaria at the surface or deep in the sediment. Photographs of the aquaria were taken at regular intervals, to observe, in a non-destructive manner, the movement of the particles and to calculate, using adapted software, the optical reworking coefficient (ORC) over time. A difference appears between the movements of the particles generated by both species of polychaetes. Nereis virens create “permanent” galleries that carry the microspheres deeply in the sediment during the digging, bioirrigation and feeding, and Nephtys caeca homogenize the particles in the first centimetres of sediment during its erratic movements. The study shows that the bioturbation generated by these organisms can modify the distribution of the 45 µm diameter dinoflagellate cysts in the sedimentary column, burying them or raising them back to the water-sediment interface.     

Macrozoobenthos of the Small Aral Sea

Benthic assemblages of the Small Aral Sea were studied in 1999. The species composition, density, and biomass of benthic invertebrates were determined. The vertical distribution pattern and its dynamics in the course of desiccation of the sea were analyzed. It is shown that the vertical distribution of the benthos has changed substantially over the last 30 years. The biomass of the benthos in the inshore zone increased, while in the central part of the sea, benthic invertebrate populations declined and gradually disappeared. These changes are likely to be due to the accumulation of organic matter in the bottom sediments and the weakening of hydrodynamic processes, which could lead to oxygen depletion in deep water. In the 1990s, the benthic assemblages of the Small Aral Sea were relatively stable and highly productive. It is concluded that the Aral Sea is a promising area for fishery restoration.     

Flood effects on invertebrates, sediments and particulate organic matter in the hyporheic zone of a gravel-bed stream

1. We investigated the effects of a flood on the fauna and physical habitat of the hyporheic zone of the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. Freeze core hyporheic samples (to 50 cm depth) and benthic samples (to 10 cm) were taken, along with measurements of vertical hydrological gradient, before, 2 days after and 1 month after the flood (estimated return period: 1.5 years, estimated Q_max = 10.4 m³ s^?1). 3. The composition of the hyporheos differed over the three sampling occasions with fewer taxa collected immediately postflood than preflood. The equitability of the community was higher on both postflood occasions, consistent with the reduced densities of two abundant taxa (Leptophlebiidae and Copepoda). 4. Total invertebrate abundance was lower on the postflood occasions than preflood in both benthic (0–10 cm) and hyporheic (10–50 cm) sediments. Several taxa, including asellotan isopods and amphipods, recovered within 1 month of the event. Hyporheic densities of larval Hydora and nematodes did not differ among the three sampling occasions, but the water mite Pseudotryssaturus was more abundant 1 month after the flood than preflood. There was no evidence of vertical movements (to 50 cm) by any taxa in response to the flood. 5. The proportion of fine sediments (<1 mm) in the subsurface sediments (10–50 cm) increased over the three sampling occasions and median particle size declined, but sediment porosity did not change. More particulate organic matter was found in the sediments after the flood. 6. Our study provides little evidence that the hyporheic zone (to 50 cm) acted as a significant refuge during the flood event, although movements to or recolonisation from sediments deeper than 50 cm could explain the recovery of many crustacean and mite taxa within 1 month.     

Glacial Holocene environment of the southeastern Okhotsk Sea: evidence from geochemical and palaeontological data

Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 ¹⁴C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 ¹⁴C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.     

Pelagic-benthic transition of the harmful alga, Heterosigma akashiwo: Changes in swimming and implications for benthic cell distributions

Many harmful algal blooming (HAB) species transition between a vegetative, motile phase in the water column and a dormant, non-motile resting phase in the sediments. These life history transitions potentially regulate the timing, location and persistence of bloom events. Motility promotes aggregation and influences vertical distributions in the water column. However, the contribution of this behavior to benthic distributions of resting cells is currently unknown. We used video-tracking techniques to test the hypothesis that algal cells use active down-swimming during pelagic-benthic transition to favorably influence benthic distributions. In an experimental water column, we monitored cell swimming trajectories of Heterosigma akashiwo for 14 days after cells were signaled to enter the benthic resting stage. Using the statistical characteristics of individual cell trajectories, we developed a video-based motion assay to assign each tracked Heterosigma cell to one of three cell states known to occur during pelagic-benthic transition: induced motile, transitional and resting. The primary swimming characteristic influencing benthic distribution, net vertical velocity, was essentially the same for all three cell states. Hence, we found no evidence that active down-swimming influences benthic distributions. Our data suggest that benthic distributions of Heterosigma resting cells are similar to distributions of slowly sedimenting passive particles. These observations suggest that Heterosigma benthic resting cell distributions can be predicted by modeling the effects of cell sedimentation rates combined with geophysical flow patterns.     

Longitudinal variation in the composition of the benthic macroinvertebrate fauna of a typical North coast Jamaican river

Benthic macroinvertebrate fauna plays a major role in river ecosystems, especially those of tropical islands. Since there is no information on the distribution of benthic invertebrates along a Jamaican river, we report here on the composition of the benthic fauna of the Buff Bay river, on the Northern coast of Jamaica. A total of 14 samples were collected from five sites, using kick nets and a Surber sampler, between May 1997 and October 1998. We also examined the applicability of the rhithron/potamon model, and some of the premises of the River Continuum Concept (RCC) in relation to the distribution of invertebrate taxa. The results showed a total of 38 taxa of identified invertebrates. A group of dominant taxa, composed mainly of immature stages of insects, occurred at all sites. Two notable characteristics of the river were the absence of a true potamonic fauna and the low representation of the shredder functional feeding group in the community We conclude that, while there was minor variation in the composition of the benthic macroinvertebrate fauna among the sites, this was a response to local conditions within the river system. The characteristics of the community did not conform to either of the models.     

Comparison of the Vertical Distribution of Nematodes from Two Contrasting Abyssal Sites in the Northeast Atlantic Subject to Different Seasonal Inputs of Phytodetritus

The vertical distribution of nematodes in the sediments of three cores from the phytodetritus-influenced Porcupine Abyssal Plain station is compared with three cores from the Madeira Abyssal Plain station in the DEEPSEAS programme. Nematode vertical distributions are compared with sediment organic chemistry data sampled at the same time from the same sites. The results support the two hypotheses erected by Thiel (1983) that the penetration of meiofauna into deep-sea sediments might be correlated with the input of organic matter and that a greater biological activity of larger organisms would increase the penetration of nematodes into the sediment through increase downward transport of food material.     

Latest Pleistocene—Holocene paleoceanographic trends on the continental margin of eastern Canada: Foraminiferal,dinoflagellate and pollen evidence

Micropaleontological studies were made of cores from four shelf basins on the eastern Canadian Margin: Emerald and Canso basins on the Scotian Shelf (44°–46° N), Notre Dame Channel, Newfoundland Shelf (50° N) and Cartwright Saddle, Labrador Shelf (55°). Events were correlated using a combination of¹⁴C dates and pollen stratigraphies. Surface- and bottom-water changes were compared on the basis of dinoflagellates and benthic foraminifera, respectively. The results indicate significant paleoceanographic shifts along a north—south gradient both prior to and during the Holocene.Distinct Late Pleistocene—Holocene paleoceanographic events were distinguished in the Emerald, Canso and Notre Dame basins; these events are less obvious in Cartwright Saddle which is in deeper water and further off-shore. Pleistocene glaciomarine sediments in all basins contain a fauna dominated byElphidium excavatum f.clavata; dinoflagellates and pollen are rare or absent. The widespreadElphidium fauna probably reflects turbid glacial meltwater and/or a permanent ice shelf cover from 20,000-10,000 yrs BP. The Notre Dame core also penetrates older sediment with an outer Labrador Current fauna which may represent a late Wisconsinian interstade at about 23,000 yrs BP. From 7,000–10,000 yrs BP a cold water fauna occurred which is similar to modern outer Labrador Current faunas. From about 5000–7000 yrs BP, a warm interval is indicated by a relatively warm-water calcareous benthonic foraminiferal fauna and increased representation of typical Gulf Stream dinoflagellates. The most recent change occurred in the last 2000 years with an abrupt cooling associated with stronger flow of the arctic inner Labrador Current. This cooling event is marked by an increase in arctic dinoflagellates and by an exclusively agglutinated benthonic foraminiferal fauna at two sites (Canso and Notre Dame). These Holocene paleoceanographic changes are not clearly seen in the benthic fauna of the deep northern basin (Cartwright Saddle) although dinoflagellate data at this site indicate that surface-water changes have occurred that are similar to those found in shallower basins.Shifts in the zonal position of the Gulf Stream and changes in the relative mass transports of the West Greenland and Labrador currents are mechanisms which may account for the paleoceanographic events. The glacial—interstadial—glacial sequence recorded in the Notre Dame Channel, in conjunction with other theories on glacial triggering mechanisms, provides biostratigraphic evidence which suggests the onset of a glacial stage in the near future.     

Benthic invertebrate community structure is influenced by forest succession after clearcut logging in southeastern Alaska

To assess the effects of timber harvesting on headwater streams in upland forests, benthic community structure was contrasted among four dominant forest management types (old growth, red alder-dominated young growth, conifer-dominated young growth, clearcut) and instream habitats (woody debris, cobble, gravel) in southeastern Alaska. Benthos in streams of previously harvested areas resulted in increased richness, densities and biomass relative to old growth types, particularly in young growth stands with a red alder-dominated riparian canopy. Woody debris and gravel habitats supported a combination of higher densities and biomass of invertebrates than cobble habitats. In addition, woody debris also supported a richer and more diverse invertebrate fauna than either cobble or gravel substrates. Maintaining both a woody debris source and a red alder component in regenerating riparian forests following timber harvesting should support greater invertebrate densities and diversity following clearcutting.