Continental Shelf Research Articles

Otolith chemistry reveals ontogenetic movement of the Antarctic toothfish (Dissostichus mawsoni) in the Amundsen Sea polynya, Antarctica

The Amundsen Sea polynya is one of the richest areas of primary productivity in Antarctica. The Antarctic toothfish (Dissostichus mawsoni, TOA) is a commercially valuable circum-Antarctic fish that undertakes ontogenetic movements on the continental shelf. However, the movement and distribution of this species remain unclear in the polynya, especially in the data-poor Amundsen Sea polynya. Along with hydrological and fishery survey data in the Amundsen Sea polynya, the otolith chemistry of TOA from three areas along the ice shelf of the Dotson–Getz Trough was analyzed to explore the movement pattern of this species. The elemental composition deposited during early life in the otolith core did not show significant differences, indicating that TOA in the Amundsen Sea polynya probably experienced a similar hydrological environment at the early life stage or might originate from the same spawning ground. However, the elemental composition deposited at the otolith edge showed that SrCa−1 and LiCa−1 differ in otolith chemistry between areas along the Amundsen Sea coast. In conjunction with additional data from the fish sampled in these areas, this study reveals that TOA show an ontogenetic movement from westward along the ice shelf in the Amundsen Sea polynya. A deeper area (with a depth of over 1000 m) in the west box provides a fish pool for TOA, which needs to be conserved in the fishery management of toothfish populations.

Dissolved trace elements and nutrients in the North Sea—a current baseline

Primary production is an important driver of marine carbon storage. Besides the major nutrient elements nitrogen, phosphorus, and silicon, primary production also depends on the availability of nutrient-type metals (e.g., Cu, Fe, Mo) and the absence of toxicologically relevant metals (e.g., Ni, Pb). Especially in coastal oceans, carbon storage and export to the open ocean is highly variable and influenced by anthropogenic eutrophication and pollution. To model future changes in coastal carbon storage processes, a solid baseline of nutrient and metal concentrations is crucial. The North Sea is an important shelf sea, influenced by riverine, atmospheric, Baltic Sea, and North Atlantic inputs. We measured the concentrations of dissolved nutrients (NH4+, NO3−, PO43−, and SiO44−) and 26 metals in 337 water samples from various depths within the entire North Sea and Skagerrak. A principal component analysis enabled us to categorize the analytes into three groups according to their predominant behavior: tracers for seawater (e.g., Mo, U, V), recycling (e.g., NO3−, PO43−, SiO44−), and riverine or anthropogenic input (e.g., Ni, Cu, Gd). The results further indicate an increasing P-limitation and increasing anthropogenic gadolinium input into the German Bight.

Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean

Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.

Settling velocity of atmospheric particles in seawater: Based on hydrostatic sedimentation method using video imaging techniques

When atmospheric particles deposit to the ocean, their settling velocities and residence times associated are critical for their effects on oceanic ecosystems. We developed a hydrostatic sedimentation method using video imaging techniques to track particles of 5–20 μm in diameter falling into seawater and determine the particle settling velocities in relation to their diameter, shape, organic matter contained, and seawater salinity. The measured settling velocities varied from 0.025 to 0.41 mm/s. Irregular particle shape and organic matter contained in particles also, however, reduced the values. The settling velocities were decelerated by the dissolution process of particle in seawater. Combined with the experimental results, a formula for calculating the settling velocity formulae for atmospheric particles was estimated. Using this equation, the residence time of particles is estimated to be less than one month in continental shelf sea and more than 100 days in the oceans.

Nitrogen uptake rates and phytoplankton composition across contrasting North Atlantic Ocean coastal regimes north and south of Cape Hatteras

Understanding nitrogen (N) uptake rates respect to nutrient availability and the biogeography of phytoplankton communities is crucial for untangling the complexities of marine ecosystems and the physical, biological, and chemical forces shaping them. In the summer of 2016, we conducted measurements of bulk microbial uptake rates for six 15N-labeled substrates: nitrate, nitrite, ammonium, urea, cyanate, and dissolve free amino acids across distinct marine provinces, including the continental shelf of the Mid-and South Atlantic Bights (MAB and SAB), the Slope Sea, and the Gulf Stream, marking the first instance of simultaneously measuring six different N uptake rates in this dynamic region. Total measured N uptake rates were lowest in the Gulf Stream followed by the SAB. Notably, the MAB exhibited significantly higher N uptake rates compared to the SAB, likely due to the excess levels of pre-existing phosphorus present in the MAB. Together, urea and nitrate uptake contributed approximately 50% of the total N uptake across the study region. Although cyanate uptake rates were consistently low, they accounted for up to 11% of the total measured N uptake at some Gulf Stream stations. Phytoplankton groups were identified based on specific pigment markers, revealing a dominance of diatoms in the shelf community, while Synechococcus, Prochlorococcus, and pico-eukaryotes dominated in oligotrophic Gulf Stream waters. The reported uptake rates in this study were mostly in agreement with previous studies conducted in coastal waters of the North Atlantic Ocean. This study suggests there are distinct regional patterns of N uptake in this physically dynamic region, correlating with nutrient availability and phytoplankton community composition. These findings contribute valuable insights into the intricate interplay of biological and chemical factors shaping N dynamics in disparate marine ecosystems.

A density surface model describing the habitat of the Critically Endangered Rice’s whale Balaenoptera ricei in the Gulf of Mexico

The newly recognized Rice’s whale Balaenoptera ricei is among the most endangered large whale species in the world and primarily occupies a region near the continental shelf break in the northeastern Gulf of Mexico (GoMex). We analyzed visual line-transect survey data collected throughout the northern GoMex from 2003-2019 and developed spatially explicit density maps using a density surface modeling approach to examine relationships between Rice’s whale density and bathymetric and oceanographic features. We identified water depth, surface chl a concentration, bottom temperature, and bottom salinity as key parameters that define the Rice’s whale habitat. This is consistent with upwelling of cold, high-salinity water along the continental shelf break and seasonal input of high-productivity surface water originating from coastal sources. The dominant circulation patterns in the GoMex, including the presence of Loop Current eddies, lead to increased productivity and likely play a role in maintaining high densities of forage species needed to support Rice’s whales. Extrapolation of the model suggests additional regions in Mexican waters of GoMex that may be suitable for Rice’s whales. This study informs the designation of critical habitat as defined by the US Endangered Species Act and will assist in marine spatial planning activities to avoid additional anthropogenic impacts to Rice’s whales associated with the development of wind energy and aquaculture.

Taxonomic distinctness and diversity patterns of a polychaete (Annelida) community on the continental shelf of the Southern Gulf of Mexico.

The spatial patterns of taxonomic diversity of annelid polychaete species from the continental shelf in the Southern Gulf of Mexico were examined in this study. We used taxonomic distinctness and its spatial variations to explore the diversity patterns and how they change between Southern Gulf of Mexico regions. In addition, using taxonomic distinctness as a dissimilarity measure and Ward's Clustering, we characterized three distinct faunal assemblages. We also investigated patterns of richness, taxonomic distinctness, and distance decay of similarity between sampling stations as a ß-diversity measure. Finally, we examined the spatial relationships between polychaete assemblages and environmental variables to test the relative importance of spatial and environmental components in annelid polychaete community structure from the Southern Gulf of Mexico. We used a combination of eigenvector-based multivariate analyses (dbMEMs) and distance-based redundancy analysis (dbRDA) to quantify the relative importance of these explanatory variables on the spatial variations of taxonomic distinctness. The significance level of spatial and environmental components to the distribution of polychaete species showed that the combined effect of spatial processes and sediment characteristics explained a higher percentage of the variance than those parameters could alone.

Decadal Variability of Ice‐Shelf Melting in the Amundsen Sea Driven by Sea‐Ice Freshwater Fluxes

AbstractThe ice streams flowing into the Amundsen Sea, West Antarctica, are losing mass due to changes in oceanic basal melting of their floating ice shelves. Rapid ice‐shelf melting is sustained by the delivery of warm Circumpolar Deep Water to the ice‐shelf cavities, which is first supplied to the continental shelf by an undercurrent that flows eastward along the shelf break. Temporal variability of this undercurrent controls ice‐shelf basal melt variability. Recent work shows that on decadal timescales the undercurrent variability opposes surface wind variability. Using a regional model, we show that undercurrent variability is induced by sea‐ice freshwater fluxes, particularly those north of the shelf break, which affect the cross‐shelf break density gradient. This sea‐ice variability is linked to tropical Pacific variability impacting atmospheric conditions over the Amundsen Sea. Ice‐shelf melting also feeds back onto the undercurrent by affecting the on‐shelf density, thereby influencing shelf‐break density gradient anomalies.

Asymmetry response of storm surges along the eastern coast of the Taiwan Strait

Spatiotemporal variation of storm surges in the Taiwan Strait (TWS) is studied using water level datasets from 14 tidal gauge stations located in the TWS from summer to fall of 2016. The effects of bathymetry on storm surges and tropical cyclone (TC)-induced continental shelf wave (CSW) are explored. By comparing water level response along the east coast of TWS, it is found that storm surges are asymmetric on the north and south sides of the Zhan-Yun Ridge (ZYR), regardless of the different categories and tracks of TCs passing by. Observations indicate that the ZYR could modulate the storm surges and the CSW propagation; ZYR can not only amplify the storm surges that generally peak around the ZYR, but also block the CSW propagation by dramatically dissipating its kinetic energy as revealed by the dispersion relation for the first mode CSW. Moreover, local wind work and the remote forcing, which are induced by TCs and cold fronts, respectively, can also modulate the synoptic variations of water level in the TWS.

Biogeochemical factors regulating photosynthetically dissolved organic carbon produced by phytoplankton in the Taiwan Strait

The distribution and mechanisms of photosynthetically dissolved organic carbon (PDOC) released by marine phytoplankton are frequently neglected and inadequately understood because most studies on carbon sequestration capacity have focused on photosynthetic particulate organic carbon. In this study, percentage extracellular release (PER) and its environmental influencing factors were investigated for 10 cruises in the Taiwan Strait during 2006–2023. The results indicated that the PER increased horizontally from the nearshore to the off-shelf and vertically from the surface to the bottom within the euphotic zone. PER tends to be low in eutrophic waters such as upwellings and estuaries and high in oligotrophic waters. The study revealed that the average contribution of PDOC to total primary productivity (TPP) in the Taiwan Strait could reach 18.2 ± 11.7%, which is similar to the previously estimated global oceanic values. PDOC production satisfied approximately 25% the carbon requirements of heterotropic bacteria (HB). A detailed analysis of the PER combined with model simulations proved that the distribution of the PER in the Taiwan Strait was caused by the joint contribution of irradiance, size-fractionated phytoplankton, and nutrient stoichiometry. Our results contradict the view that the PER is a constant factor that is unaffected by TPP. However, there was a significant negative correlation between the PER and TPP. The PDOC was always lower than the bacterial carbon demand for a broad range of bacterial growth efficiencies, suggesting a weak coupling between phytoplankton exudation and bacterial metabolism. This challenges the idea that there is a well-coupled relationship between bacteria and phytoplankton present on the continental shelf. These findings indicate significant discrepancies in PDOC mechanisms and the quantitative importance of nearshore eutrophic and off-shelf oligotrophic environments. Consequently, it is unwise to use uniform PERs without differentiation under trophic conditions when reevaluating and appraising marine carbon fixation.

Seafloor geomorphology of the Wrigley Gulf shelf, Amundsen Sea, West Antarctica, reveals two different phases of glaciation

AbstractKnowledge of the behaviour of marine‐based ice sheets during times of climatic warming, such as the last deglaciation, provides important information to understand how ice sheets respond to external forcing. We analysed swath bathymetric and acoustic sub‐bottom profiler data from Wrigley Gulf on the western Amundsen Sea shelf, West Antarctica, to identify glacial features and reconstruct past changes in the extent of the West Antarctic Ice Sheet (WAIS) and ice flow directions. Glacial bedforms mapped within a bathymetric cross‐shelf trough include features showing cross‐cutting and overprinting relationship and indicate changes in ice‐flow orientation. Here, we distinguish at least two phases of different ice‐flow patterns on the Wrigley Gulf shelf. During the earlier phase, seaward ice stream flow on the inner shelf was deflected towards the east due to the existence of an ice dome on the middle‐outer continental shelf. Retreat of grounded ice towards the centre of this dome is indicated by the asymmetric cross profile of recessional moraines mapped on the middle shelf. The later glaciation phase was characterized by fast, NNW‐directed ice flow across the shelf along a broad front and subsequent stepwise landward retreat, which is evident from the common occurrence and orientation of mega‐scale glaciation lineations and grounding zone wedges on the middle‐inner shelf. It is uncertain whether the two phases of glaciation recorded on the seafloor occurred during the last and penultimate glacial periods or at different times of the last glaciation. Reliable chronological constraints from sediment cores and additional geomorphological information are needed to understand the cause of the changes in WAIS dynamics reflected by the two ice‐flow phases.

Geohazard features of the Tyrrhenian Calabria

ABSTRACT This paper accompanies the Maps of Geohazard features of the Cilento and the Calabro-Tyrrhenian continental margin in the southern Tyrrhenian Sea (Italy). The main geohazard-related features were derived from extensive seafloor mapping through the collection of high-resolution multibeam data acquired during several oceanographic cruises. They encompass many fluids seepage features, fault scarps, landslides scars, gullies, channels, and canyons. Hazards related to coastal landslides and shelf-indenting canyons are very high in these sectors (especially in southern Calabria) due to active seismicity coupled with rapid uplift, high sedimentation rates and narrow or totally absent continental shelf, thus promoting a direct connection between steep slopes and coastal areas. In this setting, mass-wasting features can directly impact coastal or submarine infrastructures or indirectly create local tsunami waves, as observed in historical times. Moreover, this physiographic setting of the margin facilitates the transfer of marine litter toward deep-sea areas.

Field Observations Of The Influence Of Infragravity Waves On Wave Overtopping At A Dike On A Shallow Foreshore

Infragravity (IG) waves are low-frequency waves (0.0033 Hz < f < 0.05 Hz) that exist: (i) being bound to the wave groups of high-frequency sea-swell (SS) waves (0.05 Hz < f < 1 Hz), (ii) generated by the moving breakpoint due to the groupiness of the SS waves (Bertin et al., 2018), and (iii) as free IG waves released from the SS wave groups and reflected from neighbouring coasts (Rijnsdorp et al., 2021). Offshore in deeper water (h/Hm0,o > 4, with h the water depth and Hm0,o the offshore significant wave height), IG waves become most apparent during storm conditions, but are still rather limited in wave height (order of a couple of cm). However, at the toe of coastal structures with a shallow foreshore depth (1 < ht/Hm0,o < 4, with ht the water depth at the toe), the energy of IG waves becomes more significant relative to the SS wave frequency band of the wave spectrum (Hofland et al., 2017), becomes equally important for very shallow foreshores (0.3 < ht/Hm0,o < 1), and even dominates SS wave energy for extremely shallow foreshore depths (ht/Hm0,o < 0.3). IG wave growth on mildly sloped shallow foreshores is a result of IG wave shoaling and SS wave energy transfer to IG waves via nonlinear wave-wave interactions (Bertin et al., 2018), and the relative growth of IG to SS wave energy is further aided by SS wave breaking. Coastal urban areas along the Belgian coast (and in other low-lying countries worldwide) rely on a hybrid beach-dike coastal defense system for protection against flooding. During design storm conditions, the nourished beach erodes to a very to extremely shallow foreshore in front of a low-crested impermeable sea dike. Wave overtopping at this type of structure has been shown to be affected by IG waves, based on physical (Altomare et al., 2016) and numerical modelling (Lashley et al., 2021) for shallow to extremely shallow foreshores. However, field observations of the influence of IG waves on wave overtopping have been scarce (van Gent and Giarusso, 2003) and the influence of free IG waves has not been investigated yet. Indeed, the amount of IG wave energy that is expected to arrive at the coastline, not only depends on IG waves naturally bound to the SS wave groups, but also on free IG waves generated offshore or locally reflected from the coast (edge waves). The Belgian continental shelf is characterized by the prevalence of offshore sandbanks (see Figure 1a), which might be an additional source of free IG waves, due to ss wave breaking on the shallow crest of those banks. These free IG waves are location specific and their existence can only be determined by field observations. Knowledge of the offshore boundary conditions with detailed IG wave information is indispensable for the correct prediction and modelling of surf zone hydrodynamics and beach morphodynamics (Fiedler et al., 2019), and the consequent wave overtopping on the dike. Simultaneous measurements of IG waves, offshore and on the beach-dike system during storm conditions together with wave overtopping has not been done before, as it is complex and needs a specific setup with high precision measurement equipment. In-situ measurements provide an invaluable dataset (i.e., no model nor scale effects) and are essential for validating safety assessment and design methodologies of coastal defense systems.

Eurasian Arctic Rifted Margin Composite Tectono-Sedimentary Element

Abstract The Eurasian Arctic Rifted Margin (EARM) spans a poorly studied transition region between the Eurasian continental shelf and deep-water Nansen Basin. It is a conjugate margin to the Lomonosov Ridge as both domains were united before opening of the oceanic Eurasian Basin. The EARM is considered as a Composite Tectono-Sedimentary Element (CTSE) consisting of two individual tectono-sedimentary elements (TSEs): (1) an undefined group of poorly known pre-rift Mesozoic and Paleozoic sedimentary successions, and (2) the Late Cretaceous-Paleocene syn-rift strata. The latter are bounded by ∼56 Ma breakup unconformity and overlain by the Eurasia Basin prograded margin TSE, which is not included in EARM. In the absence of drilled wells, we utilize multichannel seismic reflection (MCS) data for seismic stratigraphic correlations with nearby, better-explored areas. The EARM is subdivided into three along-strike segments (from west to east): Svalbard-Barents Sea, the North Kara Sea, and the Taimyr-Laptev Sea. Across-strike, the EARM is composed of a proximal zone of stretched crust characterized by normal faulting, and a relatively narrow distal transition zone that may contain unroofed serpentinised mantle. The lack of drilled wells makes it challenging to evaluate petroleum potential of the CTSE, although some indications of a working petroleum system presence exist.

Downscaling the ocean response to the Madden–Julian Oscillation in the Northwest Atlantic and adjacent shelf seas

AbstractSubseasonal-to-seasonal (S2S) prediction is a global effort to forecast the state of the atmosphere and ocean with lead times between two weeks and a season. This study explores the feasibility of S2S prediction of the ocean using a variety of tools including statistical analysis, a statistical-dynamical mixed layer model, and a regional, high-resolution ocean circulation model based on physical principles. Ocean predictability on S2S timescales is analyzed by compositing winter sea surface temperature (SST) anomalies in the North Atlantic with respect to the state of the Madden–Julian Oscillation (MJO). It is found that statistically significant, large-scale SST changes, particularly along the eastern seaboard of North America, can be related to the MJO. This signal is shown to be driven by anomalous air–sea heat fluxes caused by atmospheric perturbations in response to the MJO. The high-resolution model of the Gulf of Maine and Scotian Shelf is used to downscale the mean ocean response to the MJO. The model is able to capture the observed relationship between the MJO and SST in the northwest Atlantic. It is also shown that the anomalous atmospheric circulation in response to the MJO leads to anomalous upwelling on the Scotian Shelf. Overall, this study demonstrates that it is feasible, and of value, to use regional ocean models for S2S prediction.

The Quaternary sedimentary history and dynamics of the deep-water margin off the Hudson Strait ice stream

Sites of ice-stream discharge to the continental slope build trough-mouth fans from glacigenic debris flows (GDFs), subglacial meltwater flows, and suspended sediment. Thus, past ice configurations may be interpreted from the sedimentological record. This study links the records in cores, high-resolution seismic profiles, and multibeam swath bathymetry of Hatton Slope and Fan to the changing glaciological history in Hudson Strait, one of the largest ice streams of the late Pleistocene Laurentide Ice Sheet. 14C-AMS dates, X-ray fluorescence geochemical profiles, and oxygen isotopes in sixteen cores document sediment supply during the last glacial cycle, and earlier history is interpreted from seismic data. The Frobisher Bay slope sector has a long record of GDF deposits from severe glacial periods between MIS 6 and 22, with little impact of meltwater discharge. Mass-transport events and aggrading gravel-floored valleys dominate the northern Hatton slope sector. The central slope sector shows the greatest shelf break progradation by till deposition, with kilometre-spaced gullies heading at 700–730 mbsl from the last major grounding of the ice-stream on the upper slope during Heinrich event 3 (H3). The gullies pass downslope into ∼5 km wide valleys, which have been sites of sediment bypass since widespread meltwater flow in MIS 6 and show little aggradation since MIS 16. Intervening >50 m high ridges show alternating levee accretion and erosion by shallow mass-transport events. Downslope on Hatton Fan, GDFs are common and fill wide erosional channels that lead to the sandy braid plain of the central Labrador Sea. The channels of the southern sector and Saglek Slope have channeled turbidity currents down the NAMOC, and stratified contourite deposits accumulated on the fan. Over the MIS 1–3 period widely represented by cores, ice was grounded in Hatton Basin and on the upper slope to 700 m water depth only immediately prior to H3. Older and younger H events had a floating ice-shelf in Hatton Basin, and sedimentation was dominated by the retreat of that ice shelf to Resolution Island. Distal cores show no systematic difference between the H1–top H3 interval and the base H3–H5 interval. Given the supply of carbonate detritus of consistent composition back to at least H5, any MIS 3 deglaciation of Hudson Bay would require improbably high growth rates of ice sheets in the centre of the continent.

Morphological insights into the three-dimensional complexity of rhodolith beds

The ecological importance of rhodolith beds stems from their role as structurally complex three-dimensional habitat formed by free-living red calcareous algae. Their structural singularity is due to the great variety of complex and branching morphologies exhibited by rhodoliths that create interstitial spaces and increase their surface area. This increases the ecological niches for cryptofauna and provide refuge for a high number of organisms, which is why rhodolith beds are considered biodiversity hotspots. In this work, we studied a rhodolith bed located in the Menorca Channel, formed by several species of red calcareous algae that exhibit a great variety of morphologies and form an extensive and heterogeneous habitat. This study explored the morphological diversity of the rhodolith bed, comparing the ‘Core Habitat’ (within the center of the bed with the highest densities of rhodoliths) with the boundaries or ‘Adjacent Habitat’ where rhodolith density was lower. Our results show that all rhodolith growth forms (branched, pralines and boxwork) in the Core Habitat had higher interstitial space and were larger than the ones from adjacent zones. Moreover, we explored the three-dimensional techniques to study the morphological characteristics that have historically been studied in two dimensions. This study contributes to the knowledge of morphological diversity in well-preserved rhodolith beds from continental shelves in the western Mediterranean Sea and reinforces the use of three-dimensional measurements, specifically the interstitial space of branched rhodoliths, to provide more accurate data on habitat complexity.

Seasonal abundance, lipid storage, and energy density of Calanus finmarchicus and other copepod preyfields along the Northwest Atlantic continental shelf

Abstract Calanus finmarchicus is an abundant calanoid copepod in the New York Bight (NYB) that has energy-dense lipid stores. This study measured C. finmarchicus abundance, lipid storage and energy density and compared it to two other abundant calanoid species in the region. We compared the NYB C. finmarchicus preyfield characteristics with data from two other NW Atlantic coastal ecosystems, the Gulf of Maine (GoM) and Cape Cod Bay (CCB). Copepods from net tows were identified, enumerated, photographed and measured. These data were used to calculate animal biovolume and lipid content as well as the volumetric energy density of the copepod preyfield. C. finmarchicus were most abundant in spring and summer and the mean lipid storage of C. finmarchicus remained largely constant regardless of season and location within the NYB. Centropages typicus were abundant enough in the winter to be more energy-rich than C. finmarchicus. Temora longicornis, while also abundant, never surpassed the energy density of C. finmarchicus. C. finmarchicus in the NYB were comparably lipid-rich to those from the GoM and CCB. C. finmarchicus in the NYB can be a reliable energy source for local predators due to their high spring and summer abundances and year-round storage of high-energy wax esters.

Social structure and site fidelity of an oceanic delphinid: the rough‐toothed dolphin (Steno bredanensis) off the southwest Pacific coast of Mexico

AbstractRough‐toothed dolphins (Steno bredanensis) typically inhabit deep oceanic waters from tropical to temperate waters worldwide. However, their ecology and behavior remain poorly understood. We investigated site fidelity and social structure of rough‐toothed dolphins using photo‐identification data collected between 2014 and 2022 in the continental shelf waters of the southwestern coast of Mexico. A total of 133 groups of rough‐toothed dolphins were recorded. Mean group size was 4.9 individuals (SD = 4.5), and dolphins were encountered in waters of a mean depth of 110 m (SD = 188). Of 228 individuals identified, 55% were observed once, 39% were seen between two and four times, and 6% were seen five times or more. Site fidelity analysis suggests three levels of site fidelity: transients, occasional visitors, and regular visitors, which are likely to affect the social structure. Through network analysis, nonrandom social patterns were identified (S = 1.25 ± 0.26) and dolphins exhibited both preferences and avoidances of social partners. Cluster analysis suggested significant divisions within the population (Q = 0.43 ± 0.05). Despite limitations related to sample size and design, this preliminary study suggests that rough‐toothed dolphins are best characterized by a fission‐fusion society, which is similar to most coastal delphinid populations.

Bathymetric LiDAR and Semi‐Automated Feature Extraction Assist Underwater Archaeological Surveys

ABSTRACTCritical data concerning key developments in global human history now lie submerged on continental shelves where investigations confront significant challenges. Whereas underwater excavations and surveys are expensive and weather dependent and require specialized training and equipment, remote sensing methods can improve chances for success offshore. A refinement in one method, a semi‐automated analysis protocol that can help to identify Pleistocene and Holocene era archaeological deposits in bathymetric LiDAR datasets, is presented here. This method employs contour mapping to identify potential archaeological features in shallow water environments in Apalachee Bay, Florida. This method successfully re‐identified multiple previously recorded archaeological sites in the study region and detected at least four previously undocumented archaeological sites. These results suggest that this procedure can expand on methods to identify and record submerged archaeological deposits in sediment‐starved, shallow‐water environments.