Dead Benthic Foraminifera Research Articles

Response of inner shelf benthic foraminiferal community to different concentrations of dissolved oxygen under laboratory culture experiment

Dissolved oxygen, a vital parameter for a majority of marine organisms, may decline in the near future due to anthropogenic eutrophication in coastal waters. The effect of depleting dissolved oxygen on the marine benthic community is difficult to assess from field studies, as dissolved oxygen often co-varies with several other parameters, especially organic matter. The controlled laboratory culture experiments can help to assess the effect of a specific parameter on marine organisms. Foraminifera constitute a substantial fraction of the marine benthic organisms, and a few species are vulnerable to depleted oxygen. A majority of the previous culture experiments have used isolated specimens of individual benthic foraminiferal species. Although such studies helped to understand the response of individual species to a particular parameter, they have to be scaled up to understand the community-level response of benthic foraminifera. Here, we assess the community-level response of the shallow subtidal benthic foraminiferal community to five different oxygen concentrations (1.67 mL/L to 5.01 mL/L) in a laboratory culture experiment. The living benthic foraminiferal abundance was considerably high at the intermediate oxygen concentration, whereas it decreased at both the lower and higher concentrations. The decreasing dissolved oxygen caused poor preservation of dead benthic foraminifera due to a drop in pH. The relative abundance trend of the species belonging to the same genus was different, suggesting a species-specific response to the dissolved oxygen. The maximum abundance at 2.91 mL/L and the varying response of individual species clearly suggest a non-linear response of the benthic foraminiferal community to the dissolved oxygen.

Benthic foraminifera as environmental indicators in extreme environments: The marine cave of Bue Marino (Sardinia, Italy)

The coast of the Gulf of Orosei (Sardinia, Italy) consists of impressive cliffs set up on dolostones and limestones characterized by wide karst systems connected to the sea. Marine caves, which are part of these system flooded by seawater through marine entrances, may be considered as extreme environments because of wide spatial and temporal environmental variability due to changing marine and terrestrial contributions. This study presents the results of the third survey carried out in summer 2016 in the Bue Marino cave, as part of a research project started in 2014 aimed at the application of Benthic Foraminifera (BF) as ecological indicators in Mediterranean marine caves for the identification of different habitats and their environmental interpretation. Sediment and water samples were collected from a total of 25 stations from two distinct sectors of the cave (North Branch and Middle Branch); sediments were analysed for living and dead BF and grain size, while Temperature, Salinity, pH and Dissolved Oxygen were measured in water samples collected close to sediment water interface. Two main foraminiferal assemblages, with distinct characteristics with respect to the typical Mediterranean shallow-water ones, were recognized by means of Hierarchical Cluster Analysis and Non-metric Multidimensional Scaling, and a Canonical Correspondence Analysis deduced their environmental significance. A well oxygenated, less saline environment with coarse bottom sediment, correlated with a mixed calcareous-agglutinated assemblage (Gavelinopsis praegeri, Rosalina spp., Eggerelloides advenus and Reophax dentaliniformis) with high species diversity (H-index 2.32–3.57) and low foraminiferal density, was exclusive of the North Branch. A scarcely oxygenated, more saline environment with fine bottom sediment enriched in vegetal debris was related to a prevalently agglutinated assemblage characterized by low species diversity (H-index 1.60–2.68), with high dominance of E. advenus (up to 83.6%) associated to Ammonia tepida, and high foraminiferal density, recognized in the Middle Branch. These different environments were interpreted considering the different modes of feeding the karst systems of the two branches. They also corresponded to two distinct ecozones, Entrance and Confluence, already recognized in earlier studies. The environmental significance of the foraminiferal ecozones recognized in this study and their comparison with the ones identified in the previous years, helped to consider the ecological zonation as a tool for detecting seasonal and, possibly, long term annual environmental variability in the marine system.

Living and dead benthic foraminiferal distribution in two areas of the Ross Sea (Antarctica)

We investigated the living (stained) and dead benthic foraminiferal assemblages collected in surface sediment samples (0–1 cm) from two different areas (JOIDES Basin and Mawson Bank) of the Ross Sea (Antarctica). Samples were collected during the BEDROSE oceanographic cruise from January to February 2017. Four living and dead benthic foraminiferal fauna assemblages have been distinguished based on cluster analysis. The differences observed in the living and dead foraminiferal content from the two investigated areas are the result of taphonomic processes induced by the different oceanographic settings and environmental conditions. In the JOIDES Basin, agglutinated taxa Rhabdamminella cylindrica, Lagenammina difflugiformis, Adercotryma glomeratum, Recurvoides contortus, Reophax subfusiformis with high percentages of Trochammina group and Reophax spiculifer associated with the calcareous species Nonionella bradii and Astrononion echolsi characterize the living assemblages. The comparison between living and dead benthic foraminifera reveals considerable similarities in terms of the presence/absence of agglutinated species and differences in relative abundance of calcareous taxa. However, the major influencing factor in foraminiferal preservation appears to be the carbonate dissolution. Results from Mawson Bank show an almost exclusive presence of calcareous taxa with high percentages of Globocassidulina group in both living and dead assemblages. The dead fauna assemblage differs from the corresponding living assemblage by being more diverse documenting high-energy current influence on marine sedimentation. Graphic abstract

Relationship between substrate, physico-chemical parameters and foraminiferal tests in the Doñana National Park, a Biosphere Reserve in SW Spain

A multidisciplinar analysis of sediments collected in different environments of the Donana National Park (Guadalquivir estuary, SW Spain) provides an overview of the textural, mineralogical and physico-chemical parameters that control the distribution of benthic foraminiferal tests in this Biosphere Reserve. These microorganisms are absent in the fine quartzitic sands that constitute the substrate of temporary ponds with brief hydroperiods located in the dune systems and spits, as well as in other ponds with low conductivities or hypersaline conditions located in the inner marshlands or near the Guadalquivir river banks. Dead benthic foraminifera are mainly found on phyllosilicate-rich, silty-clayey substrates. The taphonomic analysis of the main species (Ammonia tepida, Haynesina germanica, Trochammina inflata, Entzia macrescens) points to its deposit in situ. Cluster analysis permits to delimitate six foraminiferal assemblages. Cluster II (A. tepida + H. germanica) is the dominant assemblage in the central ponds and the margins of the main channels, while cluster IV (T. inflata + E. macrescens) is restricted to some ponds located on the high marsh and cluster VI (Ammonia beccarii + Quinqueloculina spp.) is abundant on external beaches. Tidal fluxes cause the transport of these last marine benthic species and some plaktonic forms both to the inner areas of the estuary and to these beaches.

Live and dead deep-sea benthic foraminiferal macrofauna of the Levantine basin (SE Mediterranean) and their ecological characteristics

The present study sought to quantitatively characterize the live and dead benthic foraminifera communities of the deep southeastern Levantine basin of the Mediterranean Sea (33.4º–31.7 ºN, 31.3º–34.9 ºE; 100–1900 m water depth) and their relationships to environmental conditions. Box corer samples were collected at 50 sites between June and July 2013. The foraminiferal macrofauna (> 250 µm) were enumerated and identified (76% to the species level). Six live foraminiferal assemblages were identified, inhabiting six biotopes, the shelf margin (SM), two upper continental slopes (UCS1 and UCS2), the lower continental slope (LCS), the eastern bathyal plain (EBP) and the western bathyal plain (WBP). The dead communities were divided into four biotopes, generally compatible with the live ones, excluding the UCS2 and the EBP. The foraminiferal density in the various live biotopes was relatively stable across the studied area, excluding the UCS2 and EBP, unlike the density of the dead shells, which increased with depth. The number of taxa per biotope was estimated by rarefaction curves and compared to the observed numbers, with a decreasing number of live taxa with water depth. The alpha-diversity, which was evaluated in relation to the number of sampled individuals, reached an asymptote in all biotopes, with very low values in the WBP. The within-biotope heterogeneity was evaluated by the average Chao-Sørensen similarity index and by a beta-diversity index (exp(gamma diversity) - exp(alpha diversity)), revealing variable heterogeneities in both assemblages. Water depth, sediment grain size mode, CaCO3 (wt%), and clay fraction (wt%) were jointly but feebly correlated with live faunal composition.

Comparison of living and dead benthic foraminifera on the Portuguese margin: Understanding the taphonomical processes

Abstract Dead benthic foraminifera (> 150 μm) were studied in 23 sediment cores from the Portuguese Margin at water depths between 20 and 2000 m and located on 4 transects off the Douro, Mondego, Tagus and Sado river mouths and 1 transect in the Estremadura. For 10 stations, the dead faunal vertical distribution (0–8 cm) was first investigated in 4 different sediment horizons per core to evaluate the sampling effort necessary to have a representation of the dead fauna deposited under different environmental areas. As a result, it appears that the faunal vertical distribution is constant, except for the deepest environments where fragile taxa were identified in the top layers only. Dead foraminiferal assemblages in the 4–5 cm layer for all stations were then compared to previously published living foraminiferal assemblages (of March 2011) from the same cores to evaluate the taphonomical processes affecting major species. This improves the knowledge of the faunal distribution for a better benthic foraminiferal proxy for paleostudies. There was a considerable loss of some species in the dead fauna. Firstly, this concerns the fragile organic-cemented agglutinated taxa such as Reophax spp., Glomospira charoides, or Bathysiphon spp. Secondly, some calcitic species such as Nonion scaphum, Cancris auriculus, Ammonia beccarii or Bulimina aculeata that were particularly abundant in the living fauna on the inner shelf under the late winter high river discharge conditions, were also far less dominant in the dead fauna. Lastly, other species like Cassidulina carinata, Valvulineria bradyana, and Bulimina marginata systematically showed higher abundance in the dead fauna at the mid shelf. These species, related to eutrophic conditions occurring in summer during the upwelling activity, were therefore not well represented in the living fauna, collected in March. Transport of allochthonous specimens may also account for higher contribution in the dead community of some species like Cibicides lobatulus, Asterigerinata mamilla or Haynesina depressula, especially in coastal environments where hydrodynamic processes (river flood, winter storm, coastal drift) are more vigorous. Several species (U. mediterranea, U. bifurcata, T. agglutinans, H. balthica or B. costata), however, show little or no difference in both abundance and spatial occurrence between the living and dead faunas and provide a stable signal for paleoclimatic investigations.

Significance of replicates: Environmental and paleoenvironmental studies on benthic foraminifera and testate amoebae

Foraminifera (Rhizaria) and testate amoebae (Rhizaria and Amoebozoa) are single-celled organisms with marine and continental affinities. They are regarded as valuable bioindicators in transitional areas such as tidal and salt marshes and have been widely used for (paleo)-environmental characterization. Along-lasting debate regularly occurs on the use of living vs. dead fauna to accurately represent modern environments. Moreover, environmentally based benthic foraminiferal and testate amoebaean studies need a reliable sampling strategy to capture the spatial variability, particularly in transitional environments where patchiness complicates the data interpretations. The objective of the present study is therefore to define the minimum required number of replicates for capturing the variability of either living (environment) or dead (paleoenvironment) benthic foraminifera and testate amoebae. To address this question, 49 samples (i.e., replicates) were selected from a square meter zone in the tidal flat-salt marsh transitional zone along the Canche Estuary (Northern France). The range of faunal spatial variability was measured using geostatistical tools. The minimal number of samples for capturing the patchiness was determined using bootstrap resampling procedure. We provide evidence that for both living and dead fauna more than 26 samples (and even higher for some species) are needed to correctly evaluate the patchiness. Indeed, the living and dead fauna do not follow spatial homogenous trends, and this might bias paleoenvironmental interpretations. The commonly-suggested number of three replicates might not be enough to characterize the fauna in such heterogeneous environments.

Benthic foraminiferal response to changes in mining pattern: a case study from the Zuari estuary, Goa, India

Being sensitive to environmental changes, foraminifera have been extensively used to monitor pollution level in the marine environment, including the effect of mining in coastal areas. In the Goa state of India, the rejects from opencast mining on land largely find their way to the estuaries, as washout during monsoon. Additionally, the Mormugao Port at the mouth of the Zuari estuary is the hub of activities due to the transport of ore from hinterland areas by barges and its subsequent loading for export. On the directive of the Supreme Court of India, all the mining-related activities abruptly stopped throughout India, including that in Goa in 2012, and got reinstated in 2015. Therefore, it provided a fit case to test the effectiveness of benthic foraminifera as an indicator of environmental impact due to mining activities. A total of ten surface sediment samples from five locations in Zuari estuary were collected from a depth range of 4.5–8.5 m in the years of 2013 and 2016 and were analyzed for both the living (stained) and dead benthic foraminifera. The year 2013 represents a time interval immediately after the closure of extensive mining activity, and the sampling during 2016 represents minimal mining. The living benthic foraminiferal abundance was higher (19–54/g sediment) during 2013 and decreased substantially during 2016 (3–22/g sediment), suggesting an adverse effect of activities associated with mine closure on benthic foraminifera. Additionally, the relative abundance of Ammonia was also significantly low during the year 2016. The temporal variation in dead foraminifera was, however, different than that of the living foraminifera. The differential response was attributed to the terrigenous dilution as a result of change in sedimentation rate. Therefore, we conclude that living foraminifera correctly incorporate the changes in mining pattern and may be used as an effective tool to monitor the impact of mining. We further suggest that the potential counter effect of terrigenous dilution on total and living benthic foraminiferal population should be considered while interpreting temporal variations in foraminiferal abundance in marginal marine settings.

Benthic Foraminiferal Assemblages in the Pahang River Delta (Pahang, Malaysia)

Sixty-six surface sediments were collected to investigate the species composition and distribution patterns of living and dead benthic foraminifera from the Pahang River Delta (east coast Peninsular Malaysia). The foraminiferal assemblages comprised 50,411 individuals from 30 families (88 species within 50 genera) in the study area. Agglutinated foraminifera were dominant at the river mouth area while calcareous species dominated the offshore sampling sites. By using Q-mode cluster analysis, four faunal assemblages were associated with distant water depths and sediment substrates. The first assemblage, dominated by Asterorotalia pulchella, was present from the station with the highest abundance amongst the other stations in the study area from the water depths <15 m and associated with silty sand substrate. The second assemblage was mostly dominated by larger foraminifera that occurred at the deeper parts of the study area with mostly sand substrate. The third assemblage consisting of the agglutinated species Ammobaculites exiguus, occured in the river mouth at water depths less than 10 m and was associated with high content of silt and clay substrate. The fourth assemblage was dominated by the agglutinated species Ammonia and Elphidium and occurred at the shallow parts of the study area with fine sand and sandy mud substrate. Considering the above, the assemblage clustering suggests that the distribution patterns of benthic foraminifera in the study area were strongly influenced by water depth and type of sediment substrate.

Relationship between ‘live’ and dead benthic foraminiferal assemblages in the abyssal NE Atlantic

Dead foraminiferal assemblages within the sediment mixed layer provide an integrated, time-averaged view of the foraminiferal fauna, while the relationship between dead and live assemblages reflects the population dynamics of different species together with taphonomic processes operating over the last few hundred years. Here, we analysed four samples for ‘live’ (Rose-Bengal-stained) and dead benthic foraminifera (0–1cm sediment layer, >150µm) from four sites in the area of the Porcupine Abyssal Plain Sustained Observatory (PAP-SO; NE Atlantic, 4850m water depth). Two sites were located on abyssal hills and two on the adjacent abyssal plain. Our results indicate that the transition from live to dead benthic foraminiferal assemblages involved a dramatic loss of delicate agglutinated and organic-walled tests (e.g. Lagenammina, Nodellum, Reophax) with poor preservation potential, and to a lesser extent that of some relatively fragile calcareous tests (mostly miliolids), possibly a result of dissolution. Other processes, such as the transport of tests by bottom currents and predation, are unlikely to have substantially altered the composition of dead faunas. Positive live to dead ratios suggest that some species (notably Epistominella exigua and Bolivina spathulata) may have responded to recent phytodetritus input. Although the composition of live assemblages seemed to be influenced by seafloor topography (abyssal hills vs. plain), no such relation was found for dead assemblages. We suggest that PAP-SO fossil assemblages are likely to be comparable across topographically contrasting sites, and dominated by calcareous and some robust agglutinated forms with calcitic cement (e.g. Eggerella).

Benthic foraminiferal response to sedimentary disturbance in the Capbreton canyon (Bay of Biscay, NE Atlantic)

Living (Rose Bengal stained) and dead benthic foraminifera were investigated at 6 deep-sea sites sampled in the Capbreton canyon area (Bay of Biscay, France). Three sites were located along the canyon axis at 301m, 983m and 1478m and 3 stations were positioned on adjacent terraces at 251m, 894m and 1454m. Sedimentary features indicate that frequent sedimentary disturbances of different magnitudes occur along the Capbreton canyon axis and adjacent terraces. Such environmental conditions cause the presence of very particular benthic environments. Along the 6 studied sites, different foraminiferal responses to various sedimentary patterns are observed revealing the complexity of this canyon environment. Some sites (Gitan 3 (canyon axis), Gitan 5 (canyon axis) and Gitan 6 (terrace)) are characterized by moderate to low standing stocks and low diversity and are mainly dominated by pioneer taxa such as Fursenkoina brady, Reophax dentaliniformis and Technitella melo suggesting a recent response to turbidite deposits recorded at these sites. Others sites (Gitan 1 and Gitan 2) show extremely high standing stocks and are mainly dominated by the opportunistic Bolivina subaenariensis and Bulimina marginata. Such faunal characteristics belonging to a more advanced stage of ecosystem colonization indicates strongly food-enriched sediment but extremely unstable conditions. Moderate standing stocks and diverse assemblage composed of species such as Uvigerina mediterranea and U. peregrina has only been observed at the terrace site Gitan 4. More stable sedimentary conditions recorded at this terrace seem to be suitable to the development of a dense and diverse foraminiferal community. Numerous neritic allochtonous species were observed in the dead foraminiferal fauna. These allochthonous species mainly originate from shelf areas (<60m).

Recent benthic foraminifera in the Arctic Ocean and Kara Sea continental margin

Living (stained) and dead (unstained) benthic foraminifera from the Arctic Ocean and Kara Sea continental margin have been investigated to extend the knowledge on modern living and dead benthic foraminifera in the Arctic Ocean. Three living and three dead benthic foraminiferal fauna assemblages have been distinguished based on Q-mode cluster analysis. In the seasonally ice-free Nansen Basin, a living assemblage consisting of Ioanella tumidula–Stetsonia horvarthi assemblage has been established. On the permanently ice covered Lomonosov Ridge and Alpha Ridge in addition to the seasonally ice-free Nansen Basin, the living assemblage is characterized by I. tumidula–Oridorsalis tener–S. horvarthi assemblage. The seasonally ice-free and relatively shallow Kara Sea continental margin displays a living fauna assemblage with agglutinated Reophax species in addition to the calcareous species Cassidulina neoteretis, Epistominella nipponica, and Islandiella helenae. The dead assemblages from Alpha Ridge, Lomonosov Ridge and Nansen Basin with assemblages characterized by I. tumidula, O. tener and S. horvathi reflect the living assemblages well. The dead fauna assemblage from the Kara Sea continental margin differs from the corresponding living assemblage by showing less agglutinated specimens and dominance by C. neoteretis. However, there is also a relatively good potential reconstructing paleoenvironmental change from the Kara Sea continental margin, when it is taken into account that the agglutinated species are not well preserved in the dead assemblages.

Living and dead benthic foraminifera assemblages in the Bohai and northern Yellow Seas: Seasonal distributions and paleoenvironmental implications

Benthic foraminifera fossil shells are particularly useful in reconstructing water depth, temperature, the exported flux of organic carbon to the sea floor, and the level of bottom-water oxygenation in paleoenvironments. In this study, we investigated the living (stained) and dead (thanatocoenoses) benthic foraminifera assemblages collected in surface sediment samples from the Bohai Sea (BS) and the northern Yellow Sea (NYS). The samples were collected from different seasons, i.e. 50 samples in May (spring) and 46 samples in November (autumn). Our benthic foraminifera fauna analysis of these samples shows four living assemblages in May and three in November, indicating a sensitive response to seasonal environmental changes where the fauna live in the modern setting of the BS and NYS. Our redundancy analysis (RDA) of the living assemblages and their corresponding environmental parameters indicates that in spring, the abundances of Astrononion tasmanensis, Nonionella stella and Bulimina sp. have positive correlations with increased water depth, density and salinity, but are negatively correlated with increased FLOUR (chlorophyll). The abundances of Buccella frigid and Verneuilinulla advena are positively correlated with higher levels of dissolved oxygen (DO) and negatively correlated with increases in temperature. The abundances of Cribrononion subincertum are positively correlated with increased turbidity (TURB) and temperature, but negatively correlated with increased DO. In autumn, there is a positive correlation between the abundances of V. advena and increased salinity. Protelphidium tuberculatum, Ammonia beccarii (vars.) abundances, however, have negative correlations with salinity. During the autumn, the benthic foraminifera species P. tuberculatum was noticeably expanded and the dominance areas occupied by Elphidium magellanicum and C. subincertum were significantly reduced in the BS; along the southern coast of the Liaodong Peninsula, the dominant Buccella frigida in the spring was replaced by V. advena in autumn. The seasonal shifting of benthic foraminifera species are local responses to the combined changes in bottom water temperature, salinity, TURB and DO in the BS and NYS, and also to the more intensified autumn/winter intrusion of the Yellow Sea Warm Current (YSWC). The dead assemblages of benthic foraminifera, which are used to reconstruct paleoenvironments, exhibited considerable destruction of agglutinated assemblages. We found that such species as V. advena, Trochammina sp., Ammoscalaria sp. and Polskiammina asiatica are missing from the dead assemblages, possibly due to postmortem taphonomic processes in the BS and NYS. Considerable destruction of agglutinated species in dead assemblages may slightly bias our estimates of paleoenvironmental parameters when they are based on benthic foraminifera fossil shells.

Benthic foraminifera provide a promising tool for ecological quality assessment of marine waters

This study defines criteria for the use of benthic foraminifera (protists) as a quick and efficient bio-monitoring tool to implement marine legislation. Various sampling and preparation procedures are investigated in an attempt to find the optimal methodology for environmental monitoring using soft-sediment foraminifera with the objective of assessing ecological quality status (EcoQS). Twenty-seven sampling stations in silled basins along the Norwegian Skagerrak coast, NE North Sea, are investigated for environmental parameters and living (stained, including soft-shelled forms) and dead benthic foraminifera. Diversity, expressed as the effective number of species (expH′bc) and community composition are used to evaluate EcoQS using living (stained) benthic foraminifera. Correlation studies show that bottom-water dissolved oxygen concentration at the time of sampling ([O2]tos) is the main environmental factor controlling variation in diversity. Variables such as grain size, C/N, TOC and TN are less important. Correlation between foraminiferal diversity and [O2]tos, as well as correlation between community data and [O2]tos, suggest that benthic foraminifera represent an efficient bio-monitoring tool to evaluate EcoQS. A clear pattern from “bad” to “high” EcoQS is established using the strong link between the benthic foraminiferal diversity and the bottom-water oxygen gradient. The study shows that EcoQS can be evaluated quickly and accurately using the following method: sample the top 1cm of sediment, dry-pick about 250 living (stained) individuals of >125μm-sized fossilisable (i.e. most of those remaining subsequent to drying) foraminifera from each of three replicates. exp(H′bc) based on living benthic foraminifera is a promising tool to assess EcoQS. For fossil assemblages, exp(H′bc) has potential for evaluating temporal changes in in situ PaleoEcoQS and for defining reference conditions from pre-impacted times.

Two abyssal sites in the Southern Ocean influenced by different organic matter inputs: Environmental characterization and preliminary observations on the benthic foraminifera

Abstract The abundance and diversity of the deep-sea benthos are intimately linked to inputs of organic matter from the euphotic zone. However, it is often difficult to isolate the influence of surface productivity on benthic ecosystems from other environmental factors. To this end, two abyssal sites (∼4200 m water depth) located under contrasting productivity regimes around the Crozet Plateau, in the Indian Ocean sector of the Southern Ocean, were sampled during the austral summer of 2004/2005. One site (M5), east of the Crozet Isles, was located beneath an area where there was an enduring seasonal phytoplankton bloom. The second site (M6) was located in an oligotrophic high-nutrient low-chlorophyll (HNLC) region to the south of the islands. Organic fluxes to the seafloor at these sites are thought to reflect the overlying productivities, so that the benthic environment to the east of the islands was more eutrophic than at the southerly station. All other environmental variables were similar at the two sites, which are located just 460 km apart. The concentrations of chlorophyll- a and total organic carbon in the surficial sediments were significantly greater at the relatively eutrophic site, east of the islands (M5), than at the southerly site (M6). Total nitrogen, however, was similar at both sites. Significantly higher phytopigment concentrations were observed in the surficial sediments at the eutrophic site; in particular, the concentration of chlorophyll- a was 3 times greater than at the southern site, although the freshness of the labile component, as measured by chlorophyll- a to pheophorbide ratio, was not different between sites. These results confirm that fluxes of organic matter to the seafloor were higher at the site located beneath the bloom region. This was reflected in the abundance and diversity of live (stained) and dead benthic foraminifera (>125 μm), which were greater at the eutrophic site. The species composition of the dead foraminiferal assemblages were similar at both sites, however, and were dominated by Nuttallides umbonifera , Pullenia bulloides , and Melonis pompiloides . An exception was the “phytodetritus species” Epistominella exigua , which was more abundant at the eutrophic site, indicating a larger seasonal component to the export under the bloom region. Differences in the organic matter input regimes at the two sites appear to influence the abundance and diversity, but not the overall species composition, of the foraminiferal assemblages.

Live (Rose Bengal stained) and dead benthic foraminifera from the oxygen minimum zone of the Pakistan continental margin (Arabian Sea)

Live (Rose Bengal stained) and dead benthic foraminiferal communities (hard-shelled species only) from the Pakistan continental margin oxygen minimum zone (OMZ) have been studied in order to determine the relation between faunal composition and the oxygenation of bottom waters. Samples were taken from 136 m to 1870 m water depth during the intermonsoon season of 2003 (March–April). Live foraminiferal densities show a clear maximum in the first half centimetre of the sediment only few specimens are found down to 4 cm depth. The faunas exhibit a clear zonation across the Pakistan margin OMZ. Down to 500 m water depth, Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata dominate the assemblages. These taxa are largely restricted to the upper cm of the sediment. They are adapted to the very low bottom-water oxygen values (≈ 0.1 ml/l in the OMZ core) and the extremely high input of organic carbon on the upper continental slope. The lower part of the OMZ is characterised by cosmopolitan faunas, containing also some taxa that in other areas have been described in deep infaunal microhabitats. The contrast between faunas typical for the upper part of the OMZ, and cosmopolitan faunas in the lower part of the OMZ, may be explained by a difference in the stability of dysoxic conditions over geological time periods. The core of the OMZ has been characterised by prolonged periods of stable, strongly dysoxic conditions. The lower part of the OMZ, on the contrary, has been much more variable over time-scales of 1000s and 10,000 years because of changes in surface productivity and a fluctuating intensity of NADW circulation. We suggest that, as a consequence, well-adapted, shallow infaunal taxa occupy the upper part of the OMZ, whereas in the lower part of the OMZ, cosmopolitan deep infaunal taxa have repeatedly colonised these more intermittent low oxygen environments.

Foraminiferal response to an active methane seep environment: A case study from the Adriatic Sea

It has been analysed the taxonomic structure and isotopic composition of the cytoplasm and tests of living (Rose Bengal stained, protoplasm-full) and dead (empty tests) benthic foraminifera (> 63 μm) in surface sediments from sites with and without methane seepage at water depths of about 25 m water in the northern Adriatic Sea, and compared these data to isotope analyses of carbonate rocks at the same locations. Foraminiferal density is higher in seep samples than in control ones, despite the low oxygen and high sulphide contents in sediments. The δ 13C values in foraminiferal cytoplasm at the seep sites are lower than those in cytoplasm of the same species in control samples, suggesting that Beggiatoa may be a food source for the foraminifera, and explaining the higher foraminiferal density at the seep site. In contrast, the δ 13C values of tests of living and dead benthic foraminifera at seep sites were not significantly different from those in tests of the same species collected at the control sites. Therefore, the carbon isotope values of foraminiferal tests in this area do not reflect the ones of the methane seepage, although the authigenic carbonate in calcareous sandstones of the methane seepage area is significantly negative and indicates the contribution of methane seepage. The similar carbon isotope values measured in the foraminiferal tests from shallow settings of the north-eastern Adriatic Sea seem therefore to be independent of methane seepages.

Comparison of benthic foraminifera inside and outside a sulphur-oxidizing bacterial mat from the present oxygen-minimum zone off Pakistan (NE Arabian Sea)

Assemblages of live (Rose-Bengal-stained) and dead benthic foraminifera and stable oxygen and carbon isotopic composition of live benthic foraminifera were studied in and outside a bacterial mat composed of the large sulphur-oxidizing bacteria Thioploca and Beggiatoa from the oxygen-minimum zone off Pakistan (NE Arabian Sea). Two cores from the same Multicorer retrieved a bacterial mat and ambient sediment. The dominant species ( Globobulimina affinis, G. turgida, Bolivina pacifica, B. pseudopunctata, Uvigerina peregrina and Buliminella tenuata) in both cores are characteristic for dysoxic oxygen minimum zone conditions. The most significant difference between the two cores is the reduced number of stained benthic foraminifera (SBF) in the top 0.5 cm of the bacterial mat. Faunal densities of stained species are more than four times higher in the sediment surface sample (0–0.5 cm) outside the bacterial mat, at a distance of only 1.5 m. All stained species, however, observed outside the Beggiatoa/Thioploca mat were also observed in the core with the mat. Two species, Virgulinella fragilis and Bolivina dilatata, occur exclusively in the core with the bacterial mat. The diversity within the bacterial mat core is thus slightly higher than outside. Furthermore, the abundances of the species Buliminella morgani, B. tenuata and Alliatina primitiva are substantially higher in the bacterial mat than outside. Globobuliminids, on the other hand, seem to prefer the conditions outside the bacterial mat and are five times more frequent in the core taken outside the bacterial mat. Benthic foraminifers inhabit a broader microhabitat range outside the bacterial mat (∼5 cm) than within (3.5 cm). A marked decrease in SBF abundances was observed at the level of a black sulphur-rich layer which is interpreted to mark the shallow redox front below the bacterial mat. Stable carbon isotope analyses on live benthic foraminifera do not support a relation of the investigated Beggiatoa/Thioploca mat to a constant or seasonal seepage of methane at the continental slope off Pakistan. Surprisingly, however, stable oxygen isotope values of many species and especially of U. peregrina decrease with depth, which calls into question the suitability of U. peregrina as a recorder of bottom-water δ 18O.

Isotopic evidence for the incorporation of methane-derived carbon into foraminifera from modern methane seeps, Hydrate Ridge, Northeast Pacific

The presence of modern methane seeps at Hydrate Ridge, offshore Oregon, provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera. A series of cores were collected from the southern summit of Hydrate Ridge in 2002. Samples were preserved and stained to determine the δ 13C composition of three species of live (stained) and dead benthic foraminifera: Uvigerina peregrina, Cibicidoides mckannai, and Globobulimina auriculata. Specimens were examined under light and Scanning Electron Microscopy (SEM) and exhibit no evidence of diagenesis or authigenic carbonate precipitation. Individual living foraminifera from seep sites recorded δ 13C values from −0.4‰ to −21.2‰, indicating the isotopic influence of high methane concentrations. Average δ 13C values (calculated from single specimens) range from −1.28 to −5.64‰ at seep sites, and −0.81 to −0.85‰ at a control (off seep) site. Two distinct seep environments, distinguished by the presence of microbial mats or clam fields, were studied to determine environmental influences on δ 13C values. Individual foraminifera from microbial mat sites exhibited more depleted δ 13C values than those from clam field sites. We interpret these differences as an effect of food source and/or symbiotic microbes on foraminiferal carbon isotopic values, acting to magnify the negative δ 13C values recorded via the DIC pool. No statistical difference was found between δ 13C values of live vs. dead specimens. This suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions. However, a few dead specimens with extremely negative δ 13C composition (<-12‰) do indicate potential evidence for an authigenic influence on the recorded δ 13C composition.

Distribution of recent calcareous benthic foraminifera in the northern North Sea and relation to the environment

This paper compiles new and previously published data on recent calcareous benthic foraminifera (dead and living assemblages) in surface sediment samples from the northern North Sea area, focussing on the dead benthic foraminifera and their relation to the environment. Five dead benthic foraminiferal assemblages have been identified. In Scottish coastal areas Cibicides lobatulus and Rosalina sp. dominate in areas with strong current activity and coarse-grained sediments, whereas C. lobatulus and Trifarina angulosa dominate at similar conditions in the Norwegian coastal areas. Cassidulina laevigata assemblages occur in areas influenced by inflow of Atlantic water into the northern North Sea. In the central part of the Norwegian Channel Uvigerina mediterranea prevails in fine-grained sediments with high organic content and possibly low oxygen content. This species’ restricted distribution to the Norwegian Channel could possibly be related to the availability of food. Bulimina marginata and Hyalinea balthica dominate on the Fladen Ground where seasonal stratification is pronounced. This presumably leads to a decrease in the oxygen content in the bottom-waters during part of the year.