Large Depth Gradient Research Articles

Vertical distribution of epibenthic megafauna of a large seamount west of Cape Verde islands (tropical North Atlantic)

Seamounts are thought to function as hotspots of megafauna diversity due to their topology and environmental characteristics. However, assessments of megafauna communities inhabiting seamounts, including diversity and density, are scarce. In this study, we provide megafauna diversity and density estimates for a recently discovered, not yet characterized seamount region (Boetius seamounts) west of Cape Verde (N17° 16′, W29° 26′). We investigated the distribution of epibenthic megafauna over a large depth gradient from the seamount’s summit at 1400 m down to 3200 m water depth and provided qualitative and quantitative analyses based on quantified video data. In utilizing an ocean floor observation system (OFOS), calibrated videos were taken as a horizontal transect from the north-eastern flank of the seamount, differentiating between an upper, coral-rich region (−1354/−2358 m) and a deeper, sponge-rich region (−2358/−3218 m). Taxa were morphologically distinguished, and their diversity and densities were estimated and related to substrate types. Both the upper and deeper seamount region hosted unique communities with significantly higher megafauna richness at the seamount’s summit. Megafauna densities differed significantly between the upper (0.297 ± 0.167 Ind./m2) and deeper community (0.112 ± 0.114 Ind./m). The seamount showed a vertical zonation with dense aggregations of deep-sea corals dominating the seamount’s upper region and colonies of the glass sponges Poliopogon amadou dominating the deeper region. The results are discussed in light of detected substrate preferences and co-occurrence of species and are compared with findings from other Atlantic seamounts.

Exploring Coral Reproduction in Great Depth

Photo 1. Great depths at the Gulf of Aqaba and Eilat, Red Sea, host flourishing reefs with thriving communities of corals, fish, and many other animals. The photos here represent scientific explorations of deep coral reefs (termed “mesophotic reefs”) at depth of 80 m (upper photos) using a remotely operated vehicle (ROV) and at depths of 45–60 m using (lower photos), closed-circuit rebreather dives with different gas mixtures as part of a 5-year study, investigating the reproduction of soft corals along with the depth. Photo credits: TASCMAR project (upper photos), Tom Shlesinger (lower photos). Photo 2. The encrusting soft coral Rhytisma fulvum is a common dweller on many Indo-Pacific reefs, and can be found along a large depth gradient of 0–50 m. In the Red Sea, this species usually displays one of two distinct color morphs: yellow colonies dominate the shallow areas (0–25 m) and purple-white colonies dominate the deeper areas (25–50 m). Photo credit: Tom Shlesinger. Photo 3. Between 2016 and 2020, from June–August, we monitored breeding events of the soft coral Rhytisma fulvum to document the exact timing and intensity of reproductive events across the species' entire depth range (i.e., 0–45 m). In the upper panel, Ronen Liberman (who led this study) observes a yellow morph Rhytisma fulvum colony brimming with 1-day old bright yellow embryos, which are entangled on the coral's surface. In the lower panel, Ronen is collecting samples at a mesophotic reef at Eilat, Red Sea, using decompression technical dives. Photo credits: Jessica Bellworthy (upper photo); Tom Shlesinger (lower photo). Photo 4. Within 6 days following fertilization, the embryos develop into elongated larvae-planulae and are ready to detach from the colony's surface to explore the surrounding reef area for a suitable settlement ground. In the upper panel, elongated yellow-colored planulae that are still caught on the surface of its maternal colony. In the lower panel, the white elongated planulae that were released by the purple Rhytisma fulvum colony begin to crawl on the substrate in search of a suitable location for settlement. Photo credit: Tom Shlesinger. These photographs illustrate the article “Soft coral reproductive phenology along a depth gradient: Can “going deeper” provide a viable refuge?” by Ronen Liberman, Tom Shlesinger, Yossi Loya, and Yehuda Benayahu published in Ecology. https://doi.org/10.1002/ecy.3760

Simulation of non-stationary two-phase mass transfer in a large reservoir

The Kuibyshev Reservoir was formed on the Volga River and is the largest in Eurasia. The length of the reservoir along the Volga is 510 km with its maximum depth of 41 m and average depth of 8 m; the area of water mirror is 6,450 km2. Conditions of runoff formation in catchment area, as well as seasonal and daily regulation are the reasons for unsteady regime of the reservoir. Specificity of flow control in the Kuibyshev Reservoir predetermines water masses motion, flow speed pattern, as well as degree of water saturation with suspended sediments and, therefore, reservoir bed siltation intensity. The lowest flow velocity in the reservoir is observed in summer-autumn period of year; at this time, the reservoir bottom is reshaped least actively. During spring filling, flow velocity in the reservoir is at maximum, hence, favourable conditions to transfer small and large fractions of suspended sediments are created. Most of them enter the reservoir during its spring time filling, although sometimes, sediments maximum was observed earlier, before reaching the peak of flood wave in the reservoir. Flow pattern unsteadiness leads to increase or decrease in flow transfer capacity and, hence, changes in bottom levels. Complex configuration of banks, large depth gradients and basin morphology’s particulars pre-determine instability of sedimentation processes in water area. 3D hydrodynamic model of the reservoir (A. Rakhuba) forms the basis for calculating bottom level motions of the Kuibyshev reservoir. This model is supplemented with analytical formula for sediment consumption (M. Shmakova) and hydraulic ratios to calculate erosion and sediments accumulation (M. Shmakova). Four ice-free months runoff simulation of the Kuibyshev reservoir includes the following main water content phases: low water period and periods of snow flood and rain floods. The study of spatial and temporal patterns of bottom reshaping has shown that greatest intensity of such reshaping occurs during the period of large level gradients, i.e. high water. Flood swelling is featured by intensive bottom erosion. When flood recesses, transfer capacity decreases that leads to sedimentation. The greatest motions in bottom reshaping occurs in the places of channel contraction, which is explained by sharp changes in channel capacity.

Deep-Reef Fish Communities of the Great Barrier Reef Shelf-Break: Trophic Structure and Habitat Associations

The ecology of habitats along the Great Barrier Reef (GBR) shelf-break has rarely been investigated. Thus, there is little understanding of how associated fishes interact with deeper environments. We examined relationships between deep-reef fish communities and benthic habitat structure. We sampled 48 sites over a large depth gradient (54–260 m) in the central GBR using Baited Remote Underwater Video Stations and multibeam sonar. Fish community composition differed both among multiple shelf-break reefs and habitats within reefs. Epibenthic cover decreased with depth. Deep epibenthic cover included sponges, corals, and macro-algae, with macro-algae present to 194 m. Structural complexity decreased with depth, with more calcified reef, boulders, and bedrock in shallower depths. Deeper sites were flatter and more homogeneous with softer substratum. Habitats were variable within depth strata and were reflected in different fish assemblages among sites and among locations. Overall, fish trophic groups changed with depth and included generalist and benthic carnivores, piscivores, and planktivores while herbivores were rare below 50 m. While depth influenced where trophic groups occurred, site orientation and habitat morphology determined the composition of trophic groups within depths. Future conservation strategies will need to consider the vulnerability of taxa with narrow distributions and habitat requirements in unique shelf-break environments.

Hard substrate in the deep ocean: How sediment features influence epibenthic megafauna on the eastern Canadian margin

Benthic habitats on deep continental margins (> 1000m) are now considered heterogeneous – in particular because of the occasional presence of hard substrate in a matrix of sand and mud – influencing the distribution of megafauna which can thrive on both sedimented and rocky substrates. At these depths, optical imagery captured with high-definition cameras to describe megafauna can also describe effectively the fine-scale sediment properties in the immediate vicinity of the fauna. In this study, we determined the relationship between local heterogeneity (10–100sm) in fine-scale sediment properties and the abundance, composition, and diversity of megafauna along a large depth gradient (1000–3000m) in a previously-unexplored habitat: the Northeast Fan, which lies downslope of submarine canyons off the Gulf of Maine (northwest Atlantic). Substrate heterogeneity was quantified using a novel approach based on principles of computer vision. This approach proved powerful in detecting gradients in sediment, and sporadic complex features (i.e. large boulders) in an otherwise homogeneous environment because it characterizes sediment properties on a continuous scale. Sediment heterogeneity influenced megafaunal diversity (morphospecies richness and Shannon-Wiener Index) and community composition, with areas of higher substrate complexity generally supported higher diversity. However, patterns in abundance were not influenced by sediment properties, and may be best explained by gradients in food supply. Our study provides a new approach to quantify fine-scale sediment properties and assess their role in shaping megafaunal communities in the deep sea, which should be included into habitat studies given their potential ecological importance.

Spatial patterns in the distribution of benthic assemblages across a large depth gradient in the Coral Sea, Australia

The Queensland Plateau in the Coral Sea off north-eastern Australia supports numerous submerged and emergent reefs. Osprey Reef is an emergent reef at the northern tip of the plateau ~1500 m in elevation. Over such a large depth gradient, a wide range of abiotic factors (e.g., light, temperature, substratum etc.) are likely to influence benthic zonation. Despite the importance of understanding the biodiversity of Australia’s Coral Sea, there is a lack of biological information on deep-water habitats below diving depths. Here, we used a deep-water ROV transect to capture video, still photos and live samples over a depth range spanning 92 to 787 m at North Horn on Osprey Reef. Video analysis, combined with bathymetry data, was used to identify the zones of geomorphology and the benthic assemblages along the depth gradient. There were marked changes in the geomorphology and the substrate along this depth gradient which likely influence the associated benthos. Cluster analysis indicated five benthic assemblage groups, which showed clear zonation patterns and were generally predictable based on the depth and sedimentary environment. These results are the first quantitative observations to such depths and confirm that the waters of the Coral Sea support diverse benthic assemblages, ranging from shallow-water coral reefs to mesophotic coral ecosystems, to deep-water azooxanthellate corals and sponge gardens. The knowledge provided by our study can inform management plans for the Coral Sea Commonwealth Marine Reserve that incorporate the deeper reef habitats and help to minimise future damage to these marine ecosystems.

Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions

AbstractWe used bulk tissue δ13C and δ15N values and δ15N values of individual amino acids (AA) to characterize the trophic structure of a pelagic fish assemblage from the North Pacific Subtropical Gyre (NPSG) ecosystem. We focus on energy flow between fishes inhabiting distinct epipelagic, mesopelagic, and upper bathypelagic habitats and on predatory fish foraging across and within these depth habitats. Trophic positions (TPs) estimated from a combination of trophic and source AA δ15N values (TPTr‐Src) spanned a narrow range of 0.7 TP for 10 species of large fishes, including tunas, billfishes, and gempylids (TPTr‐Src 4.3‐5.0). Similarly, 13 species of small micronekton fishes encompassed a range of 1.2 TP (TPTr‐Src 2.6‐3.8). The δ15N values of three source AAs were found to increase with increasing depth of capture across the 13 micronekton fish species (δ15NPhe range = 6.6‰; δ15NGly range = 13.4‰; δ15NSer range = 13.6‰), indicating that some species from epipelagic, mesopelagic, and upper bathypelagic communities access distinct food resources, such as suspended particles. These isotopic depth trends are consistent with previous observations in particulate organic matter and zooplankton from the NPSG, providing new evidence that large pelagic and micronekton fishes access a food web fueled by particles formed in surface waters but that are highly modified by microbes as they slowly settle to remote depths. On the contrary, no significant relationships between the δ15N values of source AAs and habitat depth were observed in the large predator fish group, of which many species move and forage across large depth gradients.

Sharing the slope: depth partitioning of agariciid corals and associated Symbiodinium across shallow and mesophotic habitats (2-60 m) on a Caribbean reef

BackgroundScleractinian corals and their algal endosymbionts (genus Symbiodinium) exhibit distinct bathymetric distributions on coral reefs. Yet, few studies have assessed the evolutionary context of these ecological distributions by exploring the genetic diversity of closely related coral species and their associated Symbiodinium over large depth ranges. Here we assess the distribution and genetic diversity of five agariciid coral species (Agaricia humilis, A. agaricites, A. lamarcki, A. grahamae, and Helioseris cucullata) and their algal endosymbionts (Symbiodinium) across a large depth gradient (2-60 m) covering shallow to mesophotic depths on a Caribbean reef.ResultsThe five agariciid species exhibited distinct depth distributions, and dominant Symbiodinium associations were found to be species-specific, with each of the agariciid species harbouring a distinct ITS2-DGGE profile (except for a shared profile between A. lamarcki and A. grahamae). Only A. lamarcki harboured different Symbiodinium types across its depth distribution (i.e. exhibited symbiont zonation). Phylogenetic analysis (atp6) of the coral hosts demonstrated a division of the Agaricia genus into two major lineages that correspond to their bathymetric distribution (“shallow”: A. humilis / A. agaricites and “deep”: A. lamarcki / A. grahamae), highlighting the role of depth-related factors in the diversification of these congeneric agariciid species. The divergence between “shallow” and “deep” host species was reflected in the relatedness of the associated Symbiodinium (with A. lamarcki and A. grahamae sharing an identical Symbiodinium profile, and A. humilis and A. agaricites harbouring a related ITS2 sequence in their Symbiodinium profiles), corroborating the notion that brooding corals and their Symbiodinium are engaged in coevolutionary processes.ConclusionsOur findings support the hypothesis that the depth-related environmental gradient on reefs has played an important role in the diversification of the genus Agaricia and their associated Symbiodinium, resulting in a genetic segregation between coral host-symbiont communities at shallow and mesophotic depths.

Testing depth-related multivariate patterns of macrofauna on the Indian continental shelf using reduced taxonomic resolution and data transformation

Although taxonomic sufficiency (TS) was developed for rapid environmental assessments, it has recently been adopted to study the spatial patterns of macrobenthos in relation to natural environmental gradients. To accept TS as a routine approach in wider benthic studies, it needs to be valid for various taxa and geographically different water masses. This study examined the effects of taxonomic resolution on depth-related multivariate patterns of macrobenthic polychaetes on the western Indian continental shelf. An extensive data set based on samples collected from a wide geographical area (7°–22°N latitudes) covering a large depth gradient (30–200 m) has been analysed. Species level polychaete community data had shown a significant variation between shallow and deeper waters. Our results indicated that generic and family level data also can detect community shifts along a depth gradient in a similar way to species level data. The entire western continental margin (200 m depth) is lying in the oxygen minimum zone (dissolved oxygen <0.5 ml l−1). This study indicated that family level results were sufficient to document the correlations of low oxygen on macrobenthic polychaetes. This study also tested the effects of transformations on depth-related patterns of polychaetes. In multivariate analyses, transformations play a role in defining the balance between contributions from common and rare species in the measure of similarity of two samples. Our results showed that the type of transformations did not make any prominent differences in the multivariate analyses.

Stable isotope variations in benthic filter feeders across a large depth gradient on the continental shelf

Spatial variations in carbon and nitrogen stable isotopes (δ 13C and δ 15N) of benthic filter feeders were investigated on an inshore-offshore gradient (0–250 km) along the continental shelf of the northern Bay of Biscay (NE Atlantic Ocean). δ 13C and δ 15N values were measured in muscle tissues of four filter-feeding mollusc species (epifaunal: Pecten maximus, Aequipecten opercularis; infaunal: Glycymeris glycymeris, Venus casina) and in benthic particulate organic matter along a corresponding water-depth gradient from 6 to 220 m. All four species showed a decreasing pattern of muscle δ 13C and δ 15N values with increasing depth. At the Ushant front (∼130 m water depth), where there is a decrease in both bottom water chlorophyll a and suspended particulate matter, muscle δ 13C and δ 15N values decreased in all species. Although δ 13C values of infaunal clams initially decreased at 30 m depth, δ 13C values of epifaunal scallops decreased around 120 m depth, far below the expected depth reduction in microphytobenthic production suggesting that carbon isotopes might not simply track microphytobenthic utilization. The difference between infaunal and epifaunal bivalve stable isotope values may reflect differences in feeding strategies. Muscle δ 15N values at the deepest stations (∼2‰) were lower than expected considering the typical trophic enrichment value of 3–4‰ between prey and consumers. These low δ 15N values may result from low metabolic rates and suggest the classic trophic enrichment may not hold true in species inhabiting deep waters. Stable isotopes in benthic filter feeders can reveal much information regarding their ecology and environment, but are not straightforward recorders of stable isotope baseline variations as is often assumed.

A CT-based software tool for evaluating compensator quality in passively scattered proton therapy

We have developed a quantitative computed tomography (CT)-based quality assurance (QA) tool for evaluating the accuracy of manufactured compensators used in passively scattered proton therapy. The thickness of a manufactured compensator was measured from its CT images and compared with the planned thickness defined by the treatment planning system. The difference between the measured and planned thicknesses was calculated with use of the Euclidean distance transformation and the kd-tree search method. Compensator accuracy was evaluated by examining several parameters including mean distance, maximum distance, global thickness error and central axis shifts. Two rectangular phantoms were used to validate the performance of the QA tool. Nine patients and 20 compensators were included in this study. We found that mean distances, global thickness errors and central axis shifts were all within 1 mm for all compensators studied, with maximum distances ranging from 1.1 to 3.8 mm. Although all compensators passed manual verification at selected points, about 5% of the pixels still had maximum distances of >2 mm, most of which correlated with large depth gradients. The correlation between the mean depth gradient of the compensator and the percentage of pixels with mean distance <1 mm is −0.93 with p < 0.001, which suggests that the mean depth gradient is a good indicator of compensator complexity. These results demonstrate that the CT-based compensator QA tool can be used to quantitatively evaluate manufactured compensators.

Depth-related patterns of meiofauna on the Indian continental shelf are conserved at reduced taxonomic resolution

Taxonomic sufficiency (TS) has been used in impact assessment studies of various pollution effects on marine benthic communities and found appropriate to identify the effects of pollution on marine communities. Cost, in terms of the expertise and time needed to identify organisms, increases with the level of taxonomic accuracy. Recently, TS has been adopted to study spatial patterns of macrobenthic community structure. In order to accept TS as a routine approach in wider benthic studies, it needs to be proved valid for various taxa and in geographically different areas. The present study investigates the value of TS in meiofaunal nematodes by analyzing an extensive data set based on samples collected from a wide geographical area covering a large depth gradient. For this study, samples were collected from every degree square of the western Indian continental shelf (7°–22°N latitudes). Our high resolution data showed that with increase in depth, nematode species richness and diversity decreased and communities showed significant variation between shallow and deeper waters. The present study tests whether lower taxonomic resolution nematode data can explain community shifts along a depth gradient in a similar way to species level data from the same data set. Meiofauna have often been neglected from benthic studies, and most attention has been given to macrofauna. This is mainly due to the difficulty in the taxonomic identification of meiofauna. The results of this study based on univariate and multivariate analyses support the use of family level data of nematodes to explain some aspects of depth variation in a similar way to species level data.

In situ photobiology of corals over large depth ranges: A multivariate analysis on the roles of environment, host, and algal symbiont

We applied a multivariate analysis to investigate the roles of host and symbiont on the in situ physiological response of genus Madracis holobionts towards light. Across a large depth gradient (5‐40 m) and for four Madracis species and three symbiont genotypes, we assessed several variables by measuring chlorophyll a fluorescence, photosynthetic pigment composition, or symbiont population descriptors. Most of the variation is explained by two major photobiological components: light‐use efficiency and symbiont cell densities. Two other minor components emphasize photoprotective pathways and light‐harvesting properties such as secondary pigments. Statistics highlight the role of irradiance on coral physiology and reveal mechanisms that are either genetically constrained, such as symbiont cell sizes, or environmentally dependent, such as photochemical efficiencies. Other parameters, such as cellular light‐harvesting and photoprotective pigment concentrations, are regulated by host, symbiont, and environment. The interaction between host and environment stresses the role of host properties in adjusting the internal environment available for the endosymbionts. Different holobiont strategies, relating to symbiont cell density, vary in their physiological optimization of light‐harvesting or photoprotective mechanisms and link to host‐species distribution and dominance over the reef slope. Symbiont functional diversity appears to have a significant role but does not explain host vertical distribution patterns per se, highlighting the importance of species‐specific morphological and physiological properties of the coral host.

3D Analytic Model for Testing Numerical Tidal Models

A 3D analytic solution is presented for tides in channels with arbitrary lateral depth variation. The solution is valid for narrow channels in which the lateral variation of the amplitude of tidal elevation is small. The error introduced by the solution is on the order of a few percent in a tidal channel of a few kilometers in width. The solution allows an arbitrary lateral depth variation and thus provides a wide choice of depth functions, especially those with large bottom slopes. The largest amplitude of the along-estuary velocity appears on the surface in the deepest water. The depth-averaged velocity is the largest in the deepest water. The time of flood (ebb) in deep water lags that in shallow water. The time of flood (ebb) on the surface lags that at the bottom. Since this solution is simple and allows arbitrary lateral depth variations, it can be used to demonstrate the first-order tidal flow in narrow tidal channels of variable depth, and to test high-resolution numerical models with large depth gradients.

Determination of the object surface function by structured light: application to the study of spinal deformities

The projection of structured light is a technique frequently used to determine the surface shape of an object. In this paper, a new procedure is described that efficiently resolves the correspondence between the knots of the projected grid and those obtained on the object when the projection is made. The method is based on the use of three images of the projected grid. In two of them the grid is projected over a flat surface placed, respectively, before and behind the object; both images are used for calibration. In the third image the grid is projected over the object. It is not reliant on accurate determination of the camera and projector pair relative to the grid and object. Once the method is calibrated, we can obtain the surface function by just analysing the projected grid on the object. The procedure is especially suitable for the study of objects without discontinuities or large depth gradients. It can be employed for determining, in a non-invasive way, the patient's back surface function. Symmetry differences permit a quantitative diagnosis of spinal deformities such as scoliosis.