Water Depth Gradient Research Articles

Effects of water depth and water level fluctuation on the total and bio-available element concentrations in riverine reed stands

• Chemical composition in riverine reed stands depends on water depth and water level fluctuation. • Both the total and the bio-available element concentrations increase towards the deep water. • Floods homogenize the surface layer of the sediment, but water depth gradient influences the bio-available fractions. • Chemistry in reed stands can indicate redox conditions in the rhizosphere and anthropogenic pollution sources. This paper evaluates the chemical composition of reed stands grown in different riverine habitats. In addition to the total concentrations, we first present data on the bio-available element fractions of the sediment of reed stands. The results demonstrate clear separation between the plant and sediment samples and reveal the same concentration order of organs in all of the investigated sites irrespective of the water depth and water level fluctuation of the habitats. In reed stands with static water depth gradient, both the total and the bio-available element concentrations of the sediment increase towards the deep water. In habitats regularly flooded by the river, the deposition of suspended matter homogenizes the surface layer of the sediment and, therefore, total element concentrations in that layer is not influenced by the deepening river bed. After the flood event, however, the water depth gradient does influence the bio-available fractions. Finally, it is also demonstrated that chemistry in reed stands can indicate redox conditions in the rhizosphere and anthropogenic pollution sources.

Spatiotemporal patterns of cladoceran community responses to water level variation in Haixi Lake, southwest China

ABSTRACT Hydrological regulation of lakes has been increasingly practiced around the world, with significant ecological consequences. However, little is known on how lake regulation affects community organization and succession. Crustacean zooplankton play a central role in energy transfer along lake food webs because of their intermediate trophic level. Haixi Lake, a mesotrophic deep-water reservoir in southwest China, was a shallow lake by nature but underwent dam construction and reinforcement in 1957 and 1987–1990, respectively. We applied the sediment records of cladoceran remains in Haixi to uncover the spatial and temporal changes of cladoceran assemblages along a gradient of water depth by combining a spatial survey of 27 surface sediment samples with a temporal survey of a well-dated sediment core covering the last 2 centuries. Lake depth was identified to be a paramount factor in driving the temporal and spatial variation of cladoceran assemblages, which was dominated by planktonic cladocerans such as Bosmina. With increasing water levels, a significant loss of benthic cladocerans and species diversity was noted, as well as a long-term decrease in antennule lengths of bosminids. Specifically, a threshold water depth of ∼8 m was found, possibly linked to lake water thermal stability and littoral habitat coverage. Our results highlight that water level serves as a significant factor in structuring zooplankton species composition and body sizes and should be considered for sustainable water management in regulated inland waters.

南四湖沉水植物物种多样性和功能多样性对水深梯度的响应

南四湖沉水植物物种多样性和功能多样性对水深梯度的响应

Responses of the submerged macrophyte Vallisneria natans to a water depth gradient

This study investigated responses of the submerged macrophyte Vallisneria natans to a water depth gradient of 0.3–1.5 m in shallow lakes, and examined changes of morphology, physiological parameters, leaf-epiphytic bacteria community, and water purifying ability. Results of the morphological and physiological parameters (shoot height, root length, total chlorophyll, contents of soluble protein (SP) and malondialdehyde, activities of superoxide dismutase, catalase, peroxidase, glutamine synthetase, and alkaline phosphatase) indicated that 0.9–1.2 m was the optimal water depth for planting. Vallisneria natans suffered photoinhibition at the shallow water depth of 0.3–0.6 m and lipid peroxidation damage in water 1.2–1.5 m deep. Microbial analyses indicated that at the water depth of 0.6 m, the accumulated cyanobacteria led to the suppression of microbial organics decomposition and nutrient metabolism in the leaf biofilms. The water quality indicators (chemical oxygen demand, total nitrogen, total phosphorus, and fluorescent dissolved organic matter) also confirmed that 0.9–1.2 m was the optimal planting depth of Vallisneria natans. The results of this study provided theoretical guidance and technical support for the restoration of submerged macrophytes in natural shallow lakes.

Experimental characterization of various scale hydraulic signatures in a flooded branched street network

ABSTRACTUnderstanding urban flood flow repartition mechanisms within street network remains challenging. This paper presents an experimental dataset of water depth and velocity profiles in the Icube experimental rig. The spatial variability of discharge along a street is highlighted. Comparing the water depth gradient orientation and the flow repartition promotes for complex repartition mechanisms at small scales. Comparable unit-discharge values are observed in both large and narrow streets. Interestingly, the intensity of the discharge deviation at the crossroad scale is related with the asymmetry of the velocity profile entering a crossroad. Moreover, discharges at the crossroad and the subdistrict scales show that downscaling of the flow repartition relationship is difficult. At real scale, results show that inflow velocity profile asymmetry may persist on a distance of several street widths downstream of a crossroad. Such distances are insufficient to consider disconnected crossroads as done in the literature to establish discharge repartition relationships.

Large accumulations of 34S-enriched pyrite in a low-sulfate marine basin: The Sturtian Nanhua Basin, South China

The Cryogenian of the Nanhua Basin (eastern Guizhou Province, South China) consists of a continuous succession of glacial and post-glacial deposits from the Sturtian Glaciation, including diamictite of the Tiesi’ao Formation and Mn-carbonate and black shale of the overlying lower Datangpo Formation. Here, we analyzed the sulfur chemistry of these units with the goal of understanding regional to global changes in the marine sulfur cycle accompanying a Snowball Earth event. The study units are characterized by elevated carbonate-associated sulfate (CAS) δ34S (mean +56.0‰, range +49.6 to +62.6‰) and pyrite δ34S compositions (mean +57.5‰, range +48.8 to +66.8‰). Both CAS and pyrite δ34S show water-depth gradients, with mean values increasing from the shallower Lijiawan area (CAS: +49.3‰; pyrite: +44.5‰) to the deeper Xixibao (CAS: +57.9‰; pyrite: +59.8‰) and Gaodi areas (CAS: +62.4‰; pyrite: +61.6‰), reflecting a density-stratified water column with limited vertical mixing. △34S values (i.e., δ34SCAS – δ34Spy) range from −6.5‰ to +8.0‰ with a mean of +0.7‰. These features, which are similar to those for coeval strata globally, are consistent with low seawater sulfate concentrations, but several additional features of the Nanhua Basin deposits do not conform to existing Cryogenian sulfur-cycle models: (1) high total sulfur contents (mean 2.2 ± 1.1%), which are difficult to reconcile with low seawater sulfate, and (2) frequent negative △34S values, which indicate that in situ microbial sulfate reduction (MSR) cannot have been the sole control on pyrite δ34S. These features point to quantitatively important hydrothermal sulfur inputs to the Nanhua Basin watermass. Based on these considerations, we propose a new sulfur-cycle model for the Sturtian Nanhua Basin in which hydrothermal emissions supplied large amounts of 34S-enriched H2S to the water column. The released H2S was partly precipitated as syngenetic framboidal pyrite and partly oxidized to sulfate that was removed to the sediment as CAS, thus accounting for the unusual combination of high total sulfur concentrations, similar strongly 34S-enriched sulfur-isotopic compositions for CAS and pyrite, and frequent negative △34S values. As a result of low seawater sulfate concentrations, both δ34SCAS and δ34Spy developed water-depth gradients through vertical mixing of strongly 34S-enriched hydrothermal sulfide from deep-graben vents with moderately 34S-enriched sulfate from the global ocean. Our model provides new insights into Sturtian-glacial sulfur cycling processes within a semi-restricted marine basin that are likely to have wider applicability to Neoproterozoic marine systems.

Waste water discharge from a large Ni-Zn open cast mine degrades benthic integrity of Lake Nuasjärvi (Finland)

The Talvivaara/Terrafame multi-metal mining company is Europe’s largest nickel open cast mine, it is also known for the largest wastewater leakage in the Finnish mining history and a series of other accidents. In this paleolimnological study, influences of a recently constructed treated waste water discharge pipeline into Lake Nuasjärvi were investigated by analyzing past (pre-disturbance) and present community compositions of key aquatic organism groups, including diatoms, Cladocera and Chironomidae, along spatial (distance, water depth) gradients. In addition to defining ecological changes and impacts of saline mine waters in the lake, chironomids were used to quantitatively reconstruct bottom water oxygen conditions before and after the pipe installation (in 2015). The diatom and cladoceran communities, which reflect more the open-water habitat, showed only relatively minor changes throughout the lake, but a general decrease in diversity was observed within both groups. Chironomids, which live on substrates, showed more significant changes, including complete faunal turnovers and deteriorated benthic quality, especially at the sites close to the pipe outlet, where also chironomid diversity was almost completely lost. Furthermore, the reconstructed hypolimnetic oxygen values indicated a major oxygen decline and even anoxia at the sites near the pipe outlet. The limnoecological influence of the pipe decreased at sites located counter-flow or behind underwater barriers suggesting that the waste waters currently have location-specific impacts. Our study clearly demonstrates that whereas the upper water layers appear to have generally maintained their previous state, the deep-water layers close to the pipe outlet have lost their ecological integrity. Furthermore, the current hypolimnetic anoxia close to the pipe indicates enhanced lake stratification caused by the salinated mine waters. This study clearly exhibits the need to investigate different water bodies at several trophic levels in a spatiotemporal context to be able to reliably assess limnoecological impacts of mining.

Sedimentary Organic Carbon Budget Across the Slope to the Basin in the Southwestern Ulleung (Tsushima) Basin of the East (Japan) Sea

AbstractWith the total sediment oxygen uptake rates measured using an in situ benthic chamber, vertical distributions of organic carbon, and sedimentation rates estimated by excess 210Pb across the slope to the basin sediment of the southwestern region of the Ulleung (Tsushima) Basin (UB), the partitioning of organic carbon fluxes in the sediment was estimated to understand the biogeochemical cycles of organic carbon in the high productivity marginal sea. The results of depth attenuation of total oxygen uptake (TOU) demonstrate that the organic carbon oxidation of the UB sediment was 2.5 times higher than that obtained from the empirical relationship of the global's depth attenuation of TOU. Similar to TOU, the high mass accumulation rates observed in the slope region were 9.5 times higher than the rate in the basin, indicating that the slope may act as the depocenter of organic carbon. The organic carbon budget with water depth gradient implies that a significant fraction of the organic carbon deposited into sediment is supplied by lateral transport down the slope. Definite increasing C/N ratio with water depth indicates that the refractory organic carbon seems to be successively transported later from shelf to slope. The total burial flux in the sediment of southwestern UB was estimated to be 0.46±0.04 Tg C/year, which is similar to the megadepocenter of the Congo River fan. Our results imply that the UB sediment may be an important biogeochemical reaction place not only for organic carbon but also materials linked to primary production.

Distribution of subfossil chironomids (Diptera, Chironomidae) along a water depth gradient in the shallow Lake Spore, northern Poland

Subfossil chironomid (Diptera, Chironomidae) remains are often used as indicators of lake level changes in palaeolimnological studies. However, their usefulness as a water depth proxy can vary between the sites, depending on the lake morphology, mode of taphonomic processes or amplitude of past water level fluctuations, among other factors. In this study, we have examined the distribution of subfossil chironomids in the shallow Lake Spore (northern Poland) to assess the influence of water depth on the fauna. Our aim was to evaluate the site-specific utility of subfossil chironomids for lake level reconstruction at Lake Spore. The subfossil chironomid assemblages in Lake Spore have heterogeneous distribution, suggesting they are predominately composed of remains deposited close to the sampling location. A strong relationship between the water depth and the chironomids is marked by the 25.12% variance explained by water depth in the taxonomic data. Moreover, according to generalized linear models (GLMs) out of 44 dominant taxa, 12 have significant relationships with water depth. However, the sensitivity of our chironomid fauna to water depth changes is not continuous along the entire depth gradient. The most abrupt assemblage change occurs at 2.6–3.7 m water depth, in proximity to the depth where macrophytes become less dense and finally disappear. We conclude that, despite these strong chironomid-water depth relationships, only major water level fluctuations can be satisfactorily reconstructed due to the limited turnover rates of the fauna along a depth gradient and relatively small amplitude of the lake level variations characteristic for East-Central Europe.

The Relative Abundance of Benthic Bacterial Phyla Along a Water-Depth Gradient in a Plateau Lake: Physical, Chemical, and Biotic Drivers.

Water-depth biodiversity gradient, one of the typical biogeographical patterns on Earth, is understudied for bacteria in freshwater ecosystems, and thus left the underlying mechanisms poorly understood especially for benthic bacteria. Here, we investigated the water-depth distribution of surface sediment bacterial phyla and their driving factors in Lake Lugu, a plateau lake in Southwest China. Our results revealed that the relative abundance of 11 dominant bacterial phyla showed various water-depth patterns, such as increasing, decreasing, hump-shaped, and U-shaped patterns. These patterns across phyla were consistent with their different niche positions of water depth, while the occupancy-abundance relationships were not dependent on phylum attributes. Consistently, phylum abundance was best explained by water depth; other physical and chemical factors, such as metal ion concentrations, SiO2, and pH, can also explain the variations in some bacterial phyla. Chemical variables were the main drivers of the dominant bacterial phyla. However, biotic variables also showed substantial importance for some phyla, such as Planctomycetes, Actinobacteria, and WS3. This work could provide new insights into the general water-depth patterns and underlying mechanisms of the relative abundance of bacterial phyla in freshwater ecosystems.

Relative Diazotroph Abundance in Symbiotic Red Sea Corals Decreases With Water Depth

Microbial dinitrogen (N2) fixation (diazotrophy) is a trait critical for coral holobiont functioning. The contribution of N2 fixation to holobiont nitrogen (N) supply likely depends on the ecological niche of the coral holobiont. Consequently, coral-associated diazotroph communities may exhibit distinct activity patterns across a water depth gradient. We thus compared relative abundances of diazotrophs in the tissues of two common hard coral species, Podabacia sp. and Pachyseris speciosa, along their water depth distribution (10 – 30 m and 30 – 50 m, respectively) in the Central Red Sea. The relative gene copy numbers of the nifH gene (i.e. referenced against the eubacterial 16S rRNA gene), as proxy for N2 fixation potential, were assessed via quantitative PCR. We hypothesized that relative nifH gene copy numbers would decrease with water depth, assuming a related shift from autotrophy to heterotrophy. Findings confirmed this hypothesis and revealed that nifH gene abundances for both corals decreased by ~97 % and ~90 % from the shallowest to the deepest collection site. However, this result was not significant for Pachyseris speciosa due to high biological variability. The observed decrease in nifH gene abundances may be explained by the relative increase in heterotrophy of the coral animal at increasing water depths. Our results underline the importance of interpreting microbial functions and associated nutrient cycling processes within the holobiont in relation to water depth range reflecting steep environmental gradients.

Transient scour and fill. The case of the Pilcomayo River

Transient scour and fill refers to the general scour and fill in a riverbed due to flood passage. It was given much attention in times of Leopold and Maddock (1953) and Colby (1964) from records of outstanding case studies. Field data from a station in a contracted, long reach of the large sand-bed Pilcomayo River –1192 newly typed in forms– are presented, with sediment concentration in the database up to 60 kg/m3, mostly wash load. Water surface rises and falls during a flood event, whereas streambed evolves in reverse (it renders the analogy to an accordion). Streambed scouring accounts for 36% of the change in flow area during floods and data reveal scouring up to 4 m from the streambed elevation when flood starts. Velocity data display large hysteresis loops inexplicable by unsteady flow only. Once streambed starts to fall, river bed scour is self-sustaining. Water depth gradient appears to be a crucial variable in a novel equation for transient scour and fill derived from Exner equation. It turns out that flow non-uniformity is relevant to explain transient scour and fill in contrast to flow unsteadiness, which is not. Quasi-steady gradually varied flow equation solved along the contracted reach is the base for a model that captures well the recorded bed evolution. The model explains most of the transient scour and fill by the downstream expansion, but the non-uniform flow and the bed material suspended load also play a role. The model fails on the days of highest sediment concentrations.

Biodiversity patterns across taxonomic groups along a lake water-depth gradient: Effects of abiotic and biotic drivers

Understanding biodiversity patterns and the role of biotic attributes in governing these patterns remains one of the most important challenges in ecology. Here, taking water depth in Lake Lugu as a typical geographical gradient, we studied how these different taxa, that is bacteria, diatoms and chironomids, respond to the water depth and environmental gradients using molecular and morphological methods. We further evaluated the relative importance of water depth, environmental variables and biotic attributes in explaining biological characteristics, such as biomass, species richness, and community composition. The biomass of chironomids and the richness of bacteria and chironomids showed a nonlinearly decreasing pattern associated with increased water depth, while biomass and species richness of diatoms showed U-shaped and hump-shaped patterns, respectively. The three taxonomic groups all showed increasing dissimilarity with water depth changes, and there was clear cross-taxon congruence among the variations in community composition. Abiotic variables were pivotal in structuring biological characteristics; however, the biotic attributes also explained a unique portion of their variations. This suggests that biotic interactions significantly influenced the patterns of biomass, species richness, and community compositions along the water depth gradient for the three taxonomic groups studied. Our results provide new evidence that biotic attributes could help in predicting the biodiversity of aquatic communities along geographical gradients, such as water depth.

Distribution of benthic testate amoeba assemblages along a water depth gradient in freshwater lakes of the Meshchera Lowlands, Russia, and utility of the microfossils for inferring past lake water level

Testate amoebae are important components of benthic communities in freshwater lakes, where they play an essential role in decomposer food webs. They are used widely in paleoecological investigations because of their high taxonomic diversity, well-defined ecological preferences and decay-resistant tests. Studies of testate amoeba assemblages in lake surface sediments are necessary to better understand lake ecosystem function and improve the use of these organisms as bio-indicators in paleoecology. This study explored the use of testate amoebae as proxies for inferring past water level in freshwater lakes, and expanded upon the limited body of research into lake testate amoebae in Russia. Our results indicate that species composition of testate amoeba assemblages in the lakes was typical for such biotopes, with most of the species belonging to the genera Difflugia, Centropyxis, Arcella and Euglypha. Analysis of variation of testate amoebae along a water-depth gradient showed that three assemblage types could be distinguished: shallow-water (0–4.5 m), intermediate-water-depth (4.5–20.5) and deep-water (20.5–33 m). Deep-water assemblages did not contain any unique taxa and were dominated by eurybiotic and planktonic species. Species diversity was highest in the intermediate-water-depth assemblages and lowest in deep-water ones. Although variations in testate amoeba assemblages across water depth in freshwater lakes are complex and context-dependent, there are clear patterns in species composition and diversity, which can be used to infer past lake water levels. Future studies on the effect of water depth on testate amoeba assemblages in diverse types of freshwater lakes should increase the utility of the method.

Location and Energy Saving of Underwater Sensor Network for Monitoring Seabed Landslide

Xu, Y.; Shan, H., and Jia, Y., 2018. Location and energy saving of underwater sensor network for monitoring seabed landslide. In: Liu, Z.L. and Mi, C. (eds.), Advances in Sustainable Port and Ocean Engineering. Journal of Coastal Research, Special Issue No. 83, pp. 29–34. Coconut Creek (Florida), ISSN 0749-0208.The traditional positioning system of underwater sensor network for monitoring submarine landslide could complete the location of submarine landslide, but the positioning offset was large and it was unable to work for a long time. Therefore, the research on positioning and energy saving of underwater sensor network for monitoring submarine landslide was proposed. The gradient of water depth was calculated based on multibeam bathymetric data, and the gradient change was recognized. Combined with topography features, the scarp of submarine landslide was recognized, and the identification of position of submarine landslides was determined. Based on SLMP positioning algorithm, the mechanism of UPS framework was used to realize the accurate coordinate positioning of submarine landslide. DCT transform, LDPC coder-decoder and Key frame coder-decoder of encoding algorithm of underwater sensor were used to transform Wyner-Ziv domain code and thus to realize energy-saving operation of underwater sensor network monitoring. Experiment proves that the positioning accuracy of method in this paper is improved by 58.33%, and the energy can be saved more than 55.75%. Meanwhile, it has high efficiency.

Upper Miocene molluscs of Monti Livornesi (Tuscany, Italy): Biotic changes across environmental gradients

The upper Miocene mollusc assemblages of Monti Livornesi, used as a means to study the nature of Mediterranean benthic communities at the edge of the Messinian salinity crisis, are framed in a high-resolution stratigraphic scheme and quantitatively approached by the study of historical museum collections and modern samples. Instead of a single assemblage from either Tortonian or Messinian age, as previously thought, this fauna comes from three consecutive shallowing-upward depositional sequences bounded by regional unconformities, the Luppiano (upper Tortonian-lowermost Messinian), Rosignano and Raquese units (early Messinian). Facies analysis, taphonomy and quantitative paleoecology show that the Luppiano assemblage is characterised by aragonitic species from a eutrophic brackish-water shallow marine muddy bottom, the Rosignano assemblage by calcitic species from an oligotrophic coarse-grained seafloor close to a coral reef and the Raquese assemblage from an open-marine muddy bottom. Published comparisons between Miocene and Pliocene faunal lists should be considered only crude estimates of faunal change until more is known about the distribution of species along paleoenvironmental gradients. The analysis of Miocene and Pliocene abundance data allows us to frame the Monti Livornesi molluscs along carbonate-siliciclastic and water depth gradients and to reach a better understanding of the effects of salinity crisis on the Mediterranean biota.

Water depth-dependent notonectid predatory impacts across larval mosquito ontogeny.

Context-dependencies can modulate the strength of predatory interactions and often remain unquantified. In particular, differences in water depth within aquatic systems could influence predator efficiencies towards prey which utilise three-dimensional space through the water column. Differences in prey size could drive prey size-refuge effects, influencing the efficacy of natural enemies towards vector species. We thus quantify the predatory impact of two notonectid predators, Anisops breddini and Anisops sardeus, towards four different larval instars of Culex quinquefasciatus prey across a water depth gradient, using functional responses (FRs). Consumption rates differed significantly between the predators, and interspecific differences in responses to variations in water depth were emergent. Both notonectids were able to handle C. quinquefasciatus prey across all instar stages, yet predation rates were generally higher towards early as opposed to late instar prey. Anisops sardeus was most voracious, and differential predation rates of this species were most pronounced in shallow waters. Type II FRs were displayed by notonectids in the majority of treatments; however, Type III FRs were emergent in specific treatment groups, with potential implications for prey population stability. Both capture rates and handling times were often greater at greater depths, and thus maximum feeding rates reduced as depth increased. Our results further demonstrate substantial predatory impacts of notonectid predators towards mosquito, and quantify biotic and abiotic context-dependencies which modulate their impact. Given notonectids are capable of aerial dispersal between ephemeral aquatic habitats of varied volumes, their promotion in aquatic systems could help reduce proliferations of medically important mosquitoes. © 2019 Society of Chemical Industry.

Effects of water depth gradient on physiological characteristics of Vallisneria natans

For the purpose of studying the response of the Vallisneria natans antioxidant enzyme defense system to different water depths, we selected seedlings with well-preserved rhizomes, consistent germination and positive growth as experimental plants and put them into 10 plexiglass barrels at depth gradients. After 60 days of simulation experiments, we tested the malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), chlorophyll and soluble protein contents and root activity. The results showed that (1) the contents of Chla, Chlb, Chl (a + b) and Car first increased with water depth and then decreased, reaching maximum values of 0.99, 0.38, 1.37 and 0.20 mg/g, at 110 cm respectively; (2) SOD and POD decreased with increased water depth, with a significant decreasing trend particularly between 30 and 60 days, and MDA showed the same trend as POD. CAT decreased with increased water depth but then showed a two-fold increase trend, probably due to the gradual effects of long-term water stress the antioxidant enzyme system in V. natans leaves. (3) Soluble protein had the same trend as MDA. Root activity implied that the more active the root system is, the more ramets and biomass are produced. In this experiment, the plant grew well at depths of less than 120 cm and showed clear signs of aging or death beyond this depth; the results showed that the antioxidant enzyme defense system engaged a series of antioxidant protection mechanisms to increase plant stress resistance ability and reduce or eliminate the damage of plants, but when the stress increased beyond the tolerance of the plant, the antioxidant defense system was destroyed, which eventually led to the death of the plant.

High diversity of benthic bacterial and archaeal assemblages in deep-Mediterranean canyons and adjacent slopes

Submarine canyons and slopes increase the topographic heterogeneity of continental margins and enhance mass and energy transfer from the shelf to the deeper basins, profoundly influencing the biodiversity and functioning of deep-sea ecosystems. However, information on the distribution and diversity of microbial assemblages in such habitats is extremely scant. In the present study, for the first time, we investigated abundances of benthic prokaryotes and their diversity (i.e., bacterial and archaeal) in two of the largest canyons of the Mediterranean Sea (Bisagno and Polcevera Canyons, in the Ligurian Margin) and their adjacent open slope along a water-depth gradient from 200 to 2000 m. We found that prokaryotic abundance and the richness of Operational Taxonomic Units (OTUs) varied within a narrow range despite the high variability of thermohaline and trophic conditions in the habitats investigated. Conversely, the taxonomic composition (in terms of OTUs) was highly variable among the different habitats and depths investigated. The largest fraction of OTUs (representing >80% of the total OTUs identified) were unique of each sampling site, indicating a high prokaryotic β-diversity among the investigated systems. Thermo-haline and trophic conditions and water depth only partially influenced the composition of the prokaryotic assemblages suggesting that other factors, potentially related to physical forcing typically occurring in margin ecosystems, could promote a high diversification of benthic bacterial and archaeal assemblages. This study provides new insights into the benthic prokaryotic diversity of Ligurian canyons and suggests that microbial assemblages thriving in continental margins can largely contribute to the whole diversity of Mediterranean deep-sea ecosystems.

Depositional environments beneath the shelf-edge slopes of the Great Barrier Reef, inferred from foraminiferal assemblages: IODP Expedition 325

To understand sea-level changes since the Last Glacial Maximum (LGM) and their effects on coral reef systems, the shelf-edge slopes of the Great Barrier Reef (GBR) were cored during the Integrated Ocean Drilling Program (IODP) Expedition 325. Recovered unconsolidated sediments beneath the submerged shelf edge reefs contain abundant foraminiferal tests, which record changes in depositional environments and paleo-water depths. A total of 177 sediment samples were collected from 17 holes along three transects located within two geographical areas (Noggins Pass and Hydrographers Passage), and were analyzed to determine stratigraphic changes in foraminiferal assemblages (2–0.5 mm size fraction). Results show that four foraminiferal assemblages (A, B, C and D) are delineated by multivariate analyses (Q-mode cluster analysis and non-metric multidimensional scaling: NMDS), and these assemblages correspond to a back-reef to reef margin zone (0–10 m deep; Assemblage A), an upper photic zone (10–30 m deep) associated with hard substrates (Assemblage B), an intermediate to lower photic zone (30–90 m deep) characterized by soft substrates (Assemblage C), and a lower photic zone (90–130 m deep) only found in modern shelf slopes (Assemblage D). Gradual shifts in these four foraminiferal assemblages superimposed on a two-dimensional NMDS ordination mainly reflect water-depth gradients and the relative dominance of two substrate types (hard and soft substrates). Pre-LGM (older than Marine Isotope Stage 3: ≥MIS3) sediments along transects at Hydrographers Passage were deposited at intermediate to lower photic zones. In contrast, relatively shallow-water sequences found in ≥MIS3 deposits at Noggins Pass likely owe their origin to either turbidite and/or land slide processes. The lack of Assemblage D in ≥MIS3 deposits from all transects could be related to lowering temperature and/or increasing terrestrial influences (i.e. more light attenuation). Shallowing upward sequences found in LGM (MIS 2) deposits at Hydrographers Passage are likely related to stepwise sea-level falls to the full extent of the LGM. These foraminifer-based paleoenvironmental interpretations suggest that the GBR shelf edge slopes have changed their depositional environments continuously in response to sea-level fluctuations during the last glacial cycle.