The distribution of Recent benthic foraminifera in the Polar Front region, southwest Atlantic

Twenty-five core-top samples from the Maurice Ewing Bank (MEB) and Islas Orcadas Rise (IOR) were examined to determine the distribution of benthic foraminifera in the vicinity of the Polar Front in the southwest Atlantic Ocean. The Polar Front has a subsurface expression that effects the areal and depth distribution of benthic foraminifera in this region.Three faunal assemblages were identified by Q-mode factor analysis. The shallowest assemblage, dominated by Bulimina aculeata, is present from 1500 to 2600 m on the Maurice Ewing Bank and is associated with potential temperatures of 1.71-0.50°C, salinities of 34.74-34.70‰ and potential density values of 45.84–46.04 sigma-4. A second assemblage, dominated by Uvigerina peregrina, occurs in water depths from 2600 to 3100 m and is associated with potential temperatures of 0.40-0.26°C, salinities of 34.70-34.69‰ and potential density values of 46.05–46.07 sigma-4. The third assemblage is dominated by Nuttallides umbonifera, Ehrenbergina trigona and secondarily by Oridorsalis umbonatus and Pullenia bulloides (the N. umbonifera-E. trigona assemblage) is present form 2770 to 3120 m on the Islas Orcadas Rise. This assemblage is associated with potential temperatures of 0.36-0.14°C, salinities of 34.69-34.68‰, and potential density values of 46.06–46.09 sigma-4. Although the U. peregrina assemblage and the N. umbonifera-E. trigona assemblage overlap bathymetrically, they are present in waters of slightly different properties. The Bulimina aculeata assemblage is within the core of the Lower Circumpolar Deep Water (LCDW), while the other two assemblages occur within transition zones between the LCDW and Weddell Sea Deep Water (WSDW).The difference in the benthic foraminiferal assemblages at similar depths on the Islas Orcadas Rise and the Maurice Ewing Bank is the result of different water-mass regimes separated by the Polar Front.     

Bathymetric distribution of benthic foraminifera on the Australian-Irian Jaya continental margin,eastern Indonesia

We identified 164 taxa of benthic foraminifera in 35 selected box-core top samples collected on the Australian-Irian Jaya continental margin in waterdepths between 60 and 2119 m, along three systematically sampled transects across the Banda Arc. The bathymetric distribution pattern of the benthic foraminiferal faunas is related to the oceanographic situation of this area, where the watermasses of the Indian Ocean collide with the watermasses of the Pacific Ocean. With the results of cluster analyses and empirical depth-ranges of “isobathyal” taxa, four faunal depth-zones and four subzones can be distinguished:

• 1.(A) The Outer Shelf Biofacies (60–150 m), corresponding to the photic oxycline-zone of the Indonesian Surface Waters, and inhabited by a benthic foraminiferal association dominated by Amphistegina lessonii, Operculina ammonoides, Heterolepa dutemplei and various miliolids.
• 2.(B) The Upper Bathyal Biofacies (150–400 m), reflecting the aphotic, deeper Indonesian Surface Waters, dominated by Bolivina robusta, Heterolepa mediocris, Hanzawaia nipponica and Lenticulina spp. A major faunal break is situated at the lower boundary of this depth-zone.
• 3.(C) The Middle Bathyal Biofacies (400–1500 m), representing the Indonesian Intermediate Waters with minimum oxygen-contents, dominated by Bolivina robusta, Cassidulina carinata, Gavelinopsis lobatulus and Sphaeroidina bulloides. In this depth-zone many taxa occur with only limited depth-ranges, on which four subzones (C1–4) could be identified, allowing for a local (paleo)bathymetric resolution of a few hundred meters.
• 4.(D) The Lower Bathyal Biofacies (1500–2120 m) corresponds with the Indonesian Deep Waters. It is dominated by Pullenia bulloides and other cosmopolitan deep water indicators, such as Epistominella exigua, Laticarinina pauperata, Oridorsalis umbonatus and Planulina wuellerstorfi.

The sample fraction > 250 μm can be used to readily delineate the major faunal trends. Paleobathymetric resolution improves when the sample portion > 125 μm is used.     

Recent benthic foraminifera from the continental margin of northwest Africa: Community structure and distribution

Sediment grab samples were collected at 107 locations along the continental margin of northwest Africa. These samples form a series of depth transects between the Straits of Gibraltar and Dakar, Senegal. The greater than 250 μm size fraction was retained for a census of both the live and dead foraminifera. After deleting trace occurrences, Jaccard and correlation coefficient-based cluster analyses were performed to decipher the community structure for this margin.The geographic distribution of the four major faunal provinces recognized is conventional in that for both the live and dead assemblages three are related to depth: upper slope and shelf, middle slope, and lower slope and continental rise. The biotopes and thanatotopes within these provinces are strongly restricted along vertical or latitudinal boundaries when the data are analysed using Jaccard coefficients. Dendrograms constructed from correlation coefficients emphasize depth-related faunal communities. Dominant species such asCibicides lobatulus, Trifarina fornasinii, Planulina ariminensis, Uvigerina finisterrensis, andC. wuellerstorfi are generally distributed within a definite depth range along this margin and strongly influence the correlation coefficient-based dendrograms. Other distributions are clearly not depth-related but correspond to various environmental variables, for example:Cancris auriculus — coarse substrate;Bolivina subaenariensis — oxygen minimum;Cibicidoides kullenbergi andHoeglundina elegans — low organic carbon.Uvigerina peregrina is also a notable exception to depth-dependence in that this dominant species and the province it represents are generally confined to the continental slope south of Cape Blanc. Its preference for that region may arise from the high organic carbon and fine grain-size of the sediment there, from low salinity of the bottom water, or from other unknown variables. The most important vertical faunal boundary occurs between Cape Barbas and Cape Blanc (between 22° and 21°N latitude). In contrast, the Canary Island Ridge intersects the continental margin at a saddle depth of about 1500 m, but that physiographic barrier has little effect on provincialism of the deep-water benthic foraminifera.     

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.     

Foraminiferal distribution on the continental margin off Nova Scotia

Q-mode factor analysis of total foraminiferal abundance data (living plus dead) from 250 grab samples taken from the continental margin off Nova Scotia allows the determination of eight factor assemblages. On the northeastern shelf, an exclusively agglutinated assemblage dominated byAdercotryma glomerata occupies both banks and basins. Central shelf basins contain a predominantly calcareous assemblage dominated byGlobobulimina auriculata andNonionellina labradorica. Transitional between these two factor assemblages is an agglutinated assemblage dominated bySaccammina atlantica. Consistently present along the shelf edge is aTrifarina angulosa assemblage. In northeastern bays and a few samples near Sable Island, an agglutinatedEggerella advena assemblage is found. A relict and transport affected assemblage dominated byElphidium excavatum occurs in the southwestern approaches to Emerald Basin. Sandy/gravelly areas of the inner shelf and outer bank regions are characterized by aCibicides lobatulus assemblage.The statistical relationships of these defined assemblages to various aspects of the marine environment (depth, temperature, salinity, percent gravel, sand and mud) were investigated through multiple regression techniques. Results indicate that the present foraminiferal distribution patterns off Nova Scotia are influenced by the prevailing watermass characters and substrate. TheAdercotryma glomerata assemblage is influenced by the presence of cold, (0–4°C) low salinity waters (32–33‰) of arctic, Labrador Current origin. The central basin assemblage (G. auriculata) is related to warmer (8–12°C) more saline waters (35‰) of slope origin. The transition between these two bottom waters is marked by the opportunisticSaccammina atlantica assemblage. Preferred substrate character possibly determines the occurrence of theCibicides lobatulus, Islandiella islandica andEggerella advena assemblages.Trifarina angulosa shows a significant relationship to salinity and depth.Although the surficial sediments on the Nova Scotian Shelf are largely the product of reworking of glacial deposits during late glacial and early Holocene times, all but theE. excavatum factor assemblage appear to be in equilibrium with the modern oceanographic regime.     

Diversity and geographic distribution of benthic foraminifera: a molecular perspective

The diversity and distribution of modern benthic foraminifera has been extensively studied in order to aid the paleoecological interpretation of their fossil record. Traditionally, foraminiferal species are identified based on morphological characters of their organic, agglutinated or calcareous tests. Recently, however, new molecular techniques based on analysis of DNA sequences have been introduced to study the genetic variation in foraminifera. Although the number of species for which DNA sequence data exist is still very limited, it appears that morphology-based studies largely underestimated foraminiferal diversity. Here, we present two examples of the use of DNA sequences to examine the diversity of benthic foraminifera. The first case deals with molecular and morphological variations in the well-known and common calcareous genus Ammonia. The second case presents molecular diversity in the poorly documented group of monothalamous (single-chambered) foraminifera. Both examples perfectly illustrate high cryptic diversity revealed in almost all molecular studies. Molecular results also confirm that the majority of foraminiferal species have a restricted geographic distribution and that globally distributed species are rare. This is in opposition to the theory that biogeography has no impact on the diversity of small-sized eukaryotes. At least in the case of foraminifera, size does not seem to have a main impact on dispersal capacities. However, the factors responsible for the dispersal of foraminiferal species and the extension of their geographic ranges remain largely unknown. Special Issue: Protist diversity and geographic distribution. Guest editor: W. Foissner.     

Distribution and microhabitats of living (stained) benthic foraminifera from the Canadian Arctic Archipelago

The geographic distribution of live (Rose Bengal stained) foraminifera from ☐ cores taken in the Canadian Archipelago shows a dominance of agglutinated species in the western study area and an abundance of calcareous forms in the east. This distribution is attributed to the presence of differing water masses. The western channels are shallow and permit entrance only of the Arctic water mass whereas the eastern channels allow passage of the more saline and warmer Atlantic water mass. In habitat depth, both calcareous and agglutinated species exhibit highly variable vertical faunal distributions. Of the 6 cores studied, the depth above which 95% of the individuals occur ranges from 2.5 to 13 cm. Species microhabitat preference between localities also was found to be variable. Several factors are suggested to contribute to this variability. The cores were taken in shallow-water environments where physical and chemical conditions are less stable. Seasonal differences in ice cover affects productivity and thus the amount of food reaching the benthos. Sedimentation rates also are affected by differences in ice cover. Both of these factors control the rate of food burial which in turn must influence species vertical distribution patterns and microhabitat preferences. Although it has not been investigated, the role played by benthos in modifying sediment texture and in oxygenating subsurface layers may be an additional factor contributing to the variability observed in this study.     

The spatial and vertical distribution of living (stained) benthic foraminifera from a tropical,intertidal environment,north Queensland,Australia

We investigated the distribution of living (stained) benthic foraminifera across a tropical, intertidal shoreline adjacent to Cocoa Creek, Queensland, Australia for the purpose of better understanding the nature of test production and ultimately fossil assemblage development within such environments. Short cores (up to 1 m) were collected during the wet and dry season, along an elevational gradient comprising non-vegetated intertidal mudflat and higher-intertidal mangrove forest environments. The distribution of stained specimens can be broadly delineated into assemblages characterising ‘upper mangrove’ (2.64–2.91 m above Lowest Astronomical Tide (LAT)) and ‘low mangrove-mudflat’ (1.62–2.18 m above LAT) environments. Agglutinated species were generally limited to upper mangrove stations. Calcareous species occurred within all of the intertidal environments examined but differ in their composition between upper and lower intertidal settings. Upper mangrove faunas were characterised by the agglutinated species Arenoparrella mexicana, Haplophragmoides wilberti, Miliammina fusca, Miliammina obliqua and Trochammina inflata and the calcareous species Helenina anderseni. Live (stained) assemblages at lower intertidal elevations were dominated by the calcareous species Ammonia aoteana, as well as Rosalina spp., Elphidium oceanicum, Triloculina oblonga, Ammonia pustulosa and Shackoinella globosa.     

Kleptoplast distribution,photosynthetic efficiency and sequestration mechanisms in intertidal benthic foraminifera

Foraminifera are ubiquitously distributed in marine habitats, playing a major role in marine sediment carbon sequestration and the nitrogen cycle. They exhibit a wide diversity of feeding and behavioural strategies (heterotrophy, autotrophy and mixotrophy), including species with the ability of sequestering intact functional chloroplasts from their microalgal food source (kleptoplastidy), resulting in a mixotrophic lifestyle. The mechanisms by which kleptoplasts are integrated and kept functional inside foraminiferal cytosol are poorly known. In our study, we investigated relationships between feeding strategies, kleptoplast spatial distribution and photosynthetic functionality in two shallow-water benthic foraminifera (Haynesina germanica and Elphidium williamsoni), both species feeding on benthic diatoms. We used a combination of observations of foraminiferal feeding behaviour, test morphology, cytological TEM-based observations and HPLC pigment analysis, with non-destructive, single-cell level imaging of kleptoplast spatial distribution and PSII quantum efficiency. The two species showed different feeding strategies, with H. germanica removing diatom content at the foraminifer’s apertural region and E. williamsoni on the dorsal site. All E. williamsoni parameters showed that this species has higher autotrophic capacity albeit both feeding on benthic diatoms. This might represent two different stages in the evolutionary process of establishing a permanent symbiotic relationship, or may reflect different trophic strategies.Subject terms: Microbial ecology, Microbial ecology, Microbiology     

Modelling benthic habitats and biotopes off the coast of Norway to support spatial management

Habitat conservation, and hence conservation of biodiversity hinges on knowledge of the spatial distribution of habitats, not least those that are particularly valuable or vulnerable. In offshore Norway, benthic habitats are systematically surveyed and described by the national programme MAREANO (Marine AREAl database for NOrwegian waters). Benthic habitats and biotopes are defined in terms of the species composition of their epibenthic megafauna. Some habitats are of special conservation interest on account of their intrinsic value and/or vulnerability (e.g., long-lived species, rareness, to comply with international regulations such as OSPAR). In Norway, off Nordland and Troms, the following habitats of special interest can be found: Umbellula encrinus Stands, Radicipes sp. Meadows, Deep Sea Sponge Aggregations, Seapen and Burrowing Megafauna Communities, Hard Bottom Coral Gardens. In this paper, we used underwater video data collected within the MAREANO programme to define and describe benthic habitats and biotopes of special interest, and to map the geographic distribution thereof by means of habitat modelling.We first evaluated the community structure of each habitat in the list using a SIMPROF test. We determined that the class Deep Sea Sponge Aggregations, as defined by OSPAR, had to be split into at least three classes. We then re-defined seven new types of ecological features, including habitats and biotopes that were sufficiently homogeneous. Then we modelled the spatial distributions of these habitats and biotopes using Conditional Inference Forests. Since the purpose of the distribution maps is to support spatial planning we classified the heat maps using density thresholds.The accuracy of models ranged from fair to excellent. Hard Bottom Coral Gardens were the most rare habitat in terms of total area predicted (224 km², 0.3% of the area modelled), closely followed by Radicipes Meadows (391 km², 0.6%). Soft Bottom Demosponges (Geodid sponges and other taxa) represent the largest habitat, with a predicted area of 9288 km² (14%). Distribution maps of classes defined by habitat-forming species (Hard Bottom Coral Gardens) were more reliable than those defined by a host of species, or where no single species was a clear habitat provider (e.g. Seapen and Burrowing Megafauna Communities). We also put forward that a scale of patchiness larger than the scale of observation, and homogeneity of the community both play a role in model performance, and hence in map usefulness. These along with density threshold values based on observed data should all be taken into account in marine classifications and habitat definitions.     

Patterns of benthic fauna and benthic respiration on the celtic continental margin in relation to the distribution of phytodetritus

In-situ and shipboard measurements of sediment community oxygen consumption (SCOC), in conjunction with a study of the distributions of macro and megafauna and phytopigments, were used to determine and, where possible, explain the distribution of labile particulate organic matter (POM) on the NE Atlantic continental slope (Goban Spur, SW Ireland). A specific issue concerned the existence of depocentres of labile POM on the slope caused by lateral transport, a phenomenon that has been found previously in the NW Atlantic. The SCOC data from October 1993 and May 1994 showed a steady decrease with increasing water depth. SCOC values ranged from 5.4 mmol m⁻² d⁻¹ at the shelfbreak to 0.3 mmol m⁻² d⁻¹ at 4,500 m depth. No evidence was found for seasonal variation in SCOC. A clear seasonal signal was observed with regard to sediment phytopigments and phytopigment fluxes into sediment traps attached to the benthic lander. The upper- and mid-slope values of both parameters were much higher in May 1994 than in October 1993 and August 1995. This is consistent with the normal spring bloom pattern; but because of the degraded state of the May phytodetritus in the near-bottom water, reflected in the lack of a response in SCOC and the low chlorophyll-a concentrations, it was concluded that the material was not derived from the overlying photic zone, but instead transported from elsewhere in the benthic nepheloid layer (BNL). In August 1995, the lower slope (>3,000 m) had received a strong and fresh phytodetritus pulse (3 g C m⁻²) forming a mucous layer on top of the sediment. Using phytopigments and sterols as molecular markers, it was shown that the pulse was derived from an offshore bloom with an important contribution by dinoflagellates. By contrast, no mucous layer was found on the upper slope stations in August 1995. Macrofauna biomass showed a distinct decline from the upper slope down to the lower slope conforming to the diminishing supply of labile POM. The total wet biomass of megafauna reached relatively high values at the lower slope (>3,500 m) owing to large motile sea cucumbers. The presence of these “vacuum-cleaner” sea cucumbers is considered indicative of the occurrence of phytodetritus pulses. In spite of their assumed adaptation to periodic pulses, the estimated contribution by the sea cucumbers to the total benthic mineralization is minor. When combining data from different years/seasons we observed decoupling between the food supply to the lower slope and the upper and mid slopes. The major pulse to the former comes from an offshore summer bloom. The upper and mid slope appear to be fuelled by spring bloom material which is subsequently redistributed on the upper slope in a BNL. The quality of the seston in the BNL diminished in the offshore direction as indicated by the phytopigment concentrations.     

Sediment (grain size and clay mineralogy) and organic matter quality control on living benthic foraminifera

The paleoecological interpretation of fossil foraminiferal assemblages depends on an understanding of the ecological processes operating at the present. This study investigates both the quality of organic matter (OM) by elemental analysis as well as the sediment grain size and clay mineralogy to understand their relative influence on distribution and abundance of benthic foraminifera. This study is carried out on 15 samples regularly spaced from the mudflat to the tidal marsh. The results indicate that grain size is the most limiting parameter. Living (stained) benthic foraminiferal density and species richness are both very low within coarser sediments. OM is the second limiting factor. The density of foraminifera is the lowest and the species richness is the highest with the lowest organic carbon (C_org) contents and C/N < 12. Conversely, when the C_org is very high and C/N > 12, the density is high and the species richness medium. A high smectite proportion within the clay-size fraction seems to favor the development of Miliammina fusca. Trochammina inflata and Jadammina macrescens are both favored by an increase of organic carbon proportion but Trochammina inflata preferentially feeds on algal-derived OM when compared with Jadammina macrescens.     

The niche of benthic foraminifera,critical thresholds and proxies

Ecological studies of benthic foraminifera are carried out to explain patterns of distribution and the dynamics of communities. They are also used to provide data to establish proxy relationships with selected factors. According to niche theory, the patterns of distribution of benthic foraminifera are controlled by those environmental factors that have reached their critical thresholds. For each species, in variable environments, different factors may be limiting distributions both temporally and spatially. For a species or an assemblage to be useful as a proxy its abundance must show a strong correlation with the chosen factor. Since numerous factors influence each species, it is only in those environments where the majority of factors show little variation but one particular factor shows significant variation that the proxy relationship for that factor can be determined. On theoretical grounds, the reliability of using foraminiferal abundance as a proxy of a selected environmental factor should be restricted to the range close to the upper and lower thresholds. For oxygen, foraminifera are potential proxies for the lower limits but once oxygen levels rise to values of perhaps >1 or 2 ml l⁻¹, there is no longer a relationship between oxygen levels and abundance. By contrast, the flux of organic matter over a large range shows a sufficiently close relationship with foraminiferal assemblages so that transfer functions can be derived for the deep sea. However, the relationship at species level is far less clear cut. Much more accurate estimates of primary productivity and modern organic flux rates are required to improve the determination of past flux rates.     

Sewage effects on the food sources and diet of benthic foraminifera living in oxic sediment: A microcosm experiment

A microcosm experiment was conducted to investigate the effects of sewage-derived particulate organic matter (POM) on the food sources and diets of two species of intertidal benthic foraminifera, Ammonia beccarii and Haynesina germanica, using lipid biomarkers to determine trophic relationships. The lipid content of the sediment and associated micro-organisms was a guide to potential food sources while that of the foraminifera was a guide to what they had actually eaten. Six microcosm tanks were established, with constant salinity, temperature and oxygen content, and each with a thin layer of sediment containing living foraminifera. Three microcosms were used as controls and three were treatments to which the POM from secondary treated sewage was added. Each microcosm was treated as a single replicate (to avoid pseudoreplication). The experiment was run for 38 days. The results showed that the food sources (from the sediment) and the diet of the foraminifera did not significantly differ in the controls or the treatments, but quantities of fatty acids decreased in both the sediment system and the foraminifera over the duration of the experiment. It is concluded that sewage-POM (secondary treatment) does not have a direct effect on the food sources of the foraminifera or their diet. The foraminifera did not feed directly on the sewage-derived POM, nor did the addition of sewage stimulate growth of micro-organisms associated with the sediment system. However, recent field data collected by the authors provides evidence that season plays an important role in foraminiferal response to organic pollution (OP), and microcosm sediment might have been unknowingly collected at a time when foraminifera are now known not to respond to OP, i.e. in summer.     

Geomorphology and benthic cover of mesophotic coral ecosystems of the upper insular slope of southwest Puerto Rico

The upper insular slope of southwest Puerto Rico is defined as extending from the shelf break at ~20 m water depth down to a depth of ~160 m where there is a pronounced change in geomorphic character and the basal slope begins. The upper slope is divided into two geomorphic zones separated by a pronounced break in slope gradient at ~90 m water depth. Descending from the shelf break, these are Zone I (20–90 m) and Zone II (90–160 m). As orientation of the shelf margin changes, geomorphology of Zone I shows systematic variations consistent with changes in exposure to prevailing waves. Within Zone I, exposed southeast-facing slopes have a gentler gradient and lower relief than more sheltered southwest-facing slopes, which are steep and irregular. Mesophotic coral ecosystems (MCEs) are largely restricted to Zone I and concentrated on topographic highs removed from the influence of active downslope sediment transport. Accordingly, MCEs are more abundant, extensive and diverse on southwest-facing slopes where irregular topography funnels downslope sediment transport into steep narrow grooves. MCEs are more sporadic and widely spaced on southeast-facing slopes where topographic highs are more widely spaced and downslope sediment transport is spread over open, low-relief slopes inhibiting coral recruitment and growth. Relict features formed during preexisting sea levels lower than present include deep buttresses at ~45–65 m water depth and a prominent terrace at ~80 m. Based on correlations with existing reef accretion and sea-level records, it is proposed that the 80-m terrace formed during the last deglaciation ~14–15 ka and subsequently drowned during a period of rapid sea-level rise associated with meltwater pulse 1A at ~14 ka and deep buttresses at ~45–65 m formed between ~11.5 and 13.5 ka and then drowned during a period of rapid sea-level rise associated with meltwater pulse 1B at ~11.3 ka.     

Patterns in deep-sea macrobenthos at the continental margin: standing crop,diversity and faunal change on the continental slope off Scotland

Depth-related patterns of macrobenthic community structure and composition have been studied from box-core samples from the Scottish continental slope where deep-sea trawling and oil exploration are becoming increasingly important. There is a strong pattern of declining biomass and faunal abundance with increasing depth, but results also indicate reduced biomass and numbers of macrobenthos in the shallowest samples from just below the shelf edge where there are coarse sediments and a regime of strong bottom currents. There is also reduced species diversity at the shallowest stations, probably caused by hydrodynamic disturbance, but no clear mid-slope peak in species diversity as described from the northwest Atlantic. Taxonomic composition of the macrobenthic community shows most change between about 1000 and 1200 m, expressed as a major dichotomy in multivariate analysis by cluster analysis and ordination. It also shows up as a step-like increase in the rate of accumulation of new macrofaunal species. This corresponds to a change in hydrodynamic regime, from a seabed rich in suspension- and interface-feeding epifauna, to one where biogenic traces from large, burrowing deposit feeders are well developed, and visible epifauna rare in seabed photographs. It also corresponds to the depth zone where earlier study of megafaunal echinoderms in trawl and epibenthic sled samples also shows a clear peak in across-slope rate of change in faunal composition.     

The distribution and diversity of culturable aerobic heterotrophic benthic bacteria in the continental slope of the Bay of Bengal: Linked abiotic factors, including a tsunami

The culturable aerobic heterotrophic benthic bacterial population and community structure in relation to the physico-chemical parameters in the continental slope of the Bay of Bengal was studied. In addition, diversity indices were calculated and pretsunami (in 2004) and post-tsunami (in 2005) diversity values were compared. Sediment samples were collected from two cruises in the depth zone of 214–1000 m (10°36′ N–20°01′ N and 79°59′ E–87°30′ E). The vertical distribution of the total heterotrophic bacterial population during both cruises was higher in the top section (0–3 cm) of the sediment. The average total heterotrophic bacterial population was in the range of 0.42–37.38 × 10⁴ CFU/g to 1.66–19.73 × 10⁴ CFU/g dry sediment weight during the two cruises, respectively. The limiting physico-chemical factors were sediment pH, sediment temperature, TOC, porosity, and clay as revealed from multiple regression (r = 0.75) and BIOENV (Partial Correlation ρω = 0.447) analyses. The shannon-Wiener index (H′ log e), Simpson index (D), Margalef index (d) and Pielou’s evenness index (J′) were found to be higher in the 1000 m depth stations. Cluster analysis showed that the 500 m depth stations clustered either with the 200 m or with the 1000 m stations. The 200 m depth stations never formed a cluster with the 1000 m stations. Pre-tsunami diversity indices at two depth ranges (200 m and 1000 m) were higher than those of the post-tsunami indices, which was quite evident from the cluster analysis as well. This study confirms the effect of the tsunami surge in the sediments of the continental slope of the Bay of Bengal in the marine ecosystem, which is also attributed to the temporal variation of the heterotrophic bacterial population and diversity.