Shallow Soft Sediments Research Articles

Different approaches to estimate benthic metazoan diversity associated with free-living macroalgae (Fucus vesiculosus) on shallow soft sediments

Habitat complexity can boost biodiversity by providing a wide range of niches allowing species co-existence. Baltic Sea benthic communities are characterised by low species diversity. Thus the occurrence of the habitat forming macroalga Fucus vesiculosus may influence benthic communities and promote diversity. Here biodiversity estimates were obtained through conventional and eDNA approaches for the benthic assemblages associated with free-living Fucus and the adjacent bare-sediment habitats at six sites from the Northern Baltic Proper and the Gulf of Finland. Free-living F. vesiculosus habitats are heterogeneous with biodiversity estimates varying considerably among sites. The additional habitat complexity provided by F. vesiculosus tended to improve taxa richness as a result of additional epifauna assemblages, although macroinfaunal taxa richness and abundance was often reduced. Consequently the complex habitats provided by free-living F. vesiculosus often improve biodiversity, yet alters the composition of assemblages in soft sediment habitats and consequential ecosystem functioning. The study emphasised the disparity in biodiversity estimates achieved when employing different biodiversity approaches. Biodiversity estimates were more similar within approaches compared to between habitat types, with each approach detecting exclusive taxa. Consequently, biodiversity estimates can benefit from a multi-approach design where both conventional and eDNA approaches are employed in complement.

Near-surface characterization using distributed acoustic sensing in an urban area: Granada, Spain

SUMMARY The Granada Basin in southeast Spain is an area of moderate seismicity. Yet, it hosts some of the highest seismic hazards in the Iberian Peninsula due to the presence of shallow soft sediments amplifying local ground motion. In urban areas, seismic measurements often suffer from sparse instrumentation. An enticing alternative to conventional seismometers is the distributed acoustic sensing (DAS) technology that can convert fibre-optic telecommunication cables into dense arrays of seismic sensors. In this study, we perform a shallow structure analysis using the ambient seismic field interferometry method. We conduct a DAS array field test in the city of Granada on the 26 and 27 August 2020, using a telecommunication fibre. In addition to the existing limitations of using DAS with unknown fibre-ground coupling conditions, the complex geometry of the fibre and limited data recording duration further challenge the extraction of surface-wave information from the ambient seismic field in such an urban environment. Therefore, we develop a processing scheme that incorporates a frequency–wavenumber (f−k) filter to enhance the quality of the virtual shot gathers and related multimode dispersion images. We are able to use this data set to generate several shear-wave velocity (VS) profiles for different sections of the cable. The shallow VS structure shows a good agreement with different geological conditions of soil deposits. This study demonstrates that DAS could provide insights into soil characterization and seismic microzonation in urban areas. In addition, the results contribute to a better understanding of local site response to ground motion.

Habitat formed by the invasive macroalga Caulerpa filiformis (Suhr) Hering (Caulerpales, Chlorophyta) alters benthic macroinvertebrate assemblages in Peru

The green macroalga Caulerpa filiformis has been spreading on shallow soft sediment habitats along the Peruvian coast, colonizing previously unvegetated sediments to create monospecific meadows. We examined the nature of the impact of C. filiformis meadows on the density, taxonomic richness and assemblage structure of epifaunal and infaunal benthic macroinvertebrates. Specifically, we tested whether the spread of C. filiformis has resulted in different macroinvertebrate assemblages than those formed by the dominant native macroalgae (i.e., Rhodymenia spp.) and unvegetated sediments. Surveys were undertaken in two bays in each of two locations, in central and southern Peru, during winter 2017 and summer 2018. In general, our results show that macroinvertebrate assemblages were similar across all three habitats, although there were some differences, related to location and time, but with no clear patterns observed. Taxonomic richness and density was generally higher in the vegetated habitats than the unvegetated habitat, and where there were differences between the two vegetated habitats there was no consistent pattern of which habitat supported the highest richness or density. Given invading C. filiformis is primarily colonizing unvegetated habitats it would appear that this species is creating a new niche which supports similar assemblages, but higher taxonomic richness and density than unvegetated habitats. While our study suggests that C. filiformis is having a limited ecological impact we recommend that actions be put in place to limit the spread of this invasive species at the same time as increasing monitoring of the ecological impacts of this species as lags in the ecological impacts of invasive species are common.

Multi-drive level Vibroseis test to evaluate the non-linear response of soft soils

In this paper we present the results of a seismic experiment using data produced by an active Vibroseis source and varying the drive force level, during a standard seismic reflection survey. We observed a clear non-linear response that can be linked to the presence of shallow soft sediment layers. The variation of the applied vibration force induces a change in both propagation velocity and attenuation of the Rayleigh waves, and this implies that such parameters are non-linear functions of the wave energy. We analyze the soil non-linear behavior with respect to the estimated induced strain, in order to compare the collected data against the observed reduction of shear modulus and damping values as usually measured in laboratory experiments and reported in the literature. The use of multi-component receivers allowed us also to identify the Rayleigh wave ellipticity and hence the resonance frequency of the studied site. Using active source data, rather than passive micro-tremors of unknown origin, for these applications is a novel approach. Thus, we propose an alternative and efficient method to characterize the non-linear key parameters of the soil seismic response, based on the use of controlled sources of widespread use in exploration seismics. This approach may pave the way to the development of utterly new site characterization techniques.

An Invasive Mussel (Arcuatula senhousia, Benson 1842) Interacts with Resident Biota in Controlling Benthic Ecosystem Functioning

The invasive mussel Arcuatula senhousia has successfully colonized shallow soft sediments worldwide. This filter feeding mussel modifies sedimentary habitats while forming dense populations and efficiently contributes to nutrient cycling. In the present study, the density of A. senhousia was manipulated in intact sediment cores taken within an intertidal Zostera noltei seagrass meadow in Arcachon Bay (French Atlantic coast), where the species currently occurs at levels corresponding to an early invasion stage. It aimed at testing the effects of a future invasion on (1) bioturbation (bioirrigation and sediment mixing) as well as on (2) total benthic solute fluxes across the sediment–water interface. Results showed that increasing densities of A. senhousia clearly enhanced phosphate and ammonium effluxes, but conversely did not significantly affect community bioturbation rates, highlighting the ability of A. senhousia to control nutrient cycling through strong excretion rates with potential important consequences for nutrient cycling and benthic–pelagic coupling at a broader scale. However, it appears that the variability in the different measured solute fluxes were underpinned by different interactions between the manipulated density of A. senhousia and several faunal and/or environmental drivers, therefore underlining the complexity of anticipating the effects of an invasion process on ecosystem functioning within a realistic context.

Known unknowns: Conservation and research priorities for soft sediment fauna that supports a valuable SCUBA diving industry

Wildlife tourism can provide sustainable livelihoods, but can also significantly impact vulnerable species if improperly managed. To manage these impacts whilst continuing to support livelihoods, it is important to know the interests of tourists. Using the Best-Worst scaling method, we identified taxa that were most important to scuba dive tourism on shallow soft sediment habitats in Southeast Asia. We further identified differences in interest between demographic groups. We then investigated the current conservation status and research effort into the species driving this branch of tourism. The highest ranked taxa included fishes and invertebrates such as cephalopods and crustaceans. More than 200 respondents indicated that the species most important to muck dive tourism are mimic octopus/wunderpus, blue ringed octopus, rhinopias, flamboyant cuttlefish and frogfish. Diver interests were most influenced by sex, age and dive experience. The extinction risk of six of the top ten species has not yet been assessed by the International Union for Conservation of Nature. On average, the species driving this multi-million dollar tourism industry had less than one paper published every two years over the past two decades. The lack of research and conservation effort toward these species is at odds with their economic and social importance. Considering their high economic tourism value and unknown vulnerability, there is an urgent need for more research on fauna from shallow soft sediment and other habitats important to tourism.

Early Miocene brachiopods of the Waitemata and Waitakere groups, Auckland and Northland, New Zealand

ABSTRACTThe Early Miocene Waitemata and Waitakere group strata from Auckland and lower Northland, New Zealand encompass shallow rocky-shore, shallow soft-sediment and shallow-water through to outer-shelf facies. They contain an interesting brachiopod fauna comprising eight genera in six families and include the type localities for three species. Terebratella neozelandica is transferred to the genus Magasella and is confirmed as a separate species based on morphology and paleoecology. For the first time the type specimens of the species Liothyrella gravida and Magasella neozelandica are figured, Magadina browni and ‘Pachymagas’ are described and figured from North Island strata and Terebratulina suessi is described and figured from the Waitemata and Waitakere groups. The brachiopod fauna described here inhabited the margins of a subsiding marine basin in the Auckland and lower Northland area for c. 5 Ma in marginally tropical conditions in the Early Miocene. Six genera were also living in the warm–temperate Canterbury–Otago Basin at this time suggesting brachiopods may not be a reliable indicator of faunal provinces in New Zealand in the Early Miocene.

Methane emission through ebullition from an estuarine mudflat: 1. A conceptual model to explain tidal forcing based on effective stress changes

AbstractEbullition is an important pathway for transport of methane (CH4) to the atmosphere in wetlands. Water level changes have been suggested to trigger ebullition, especially in tidally flooded areas, although the controlling mechanisms remain uncertain. Bubble transport in submerged sediment represents a multiphase, dynamic interaction between gaseous and solid phases under the modulation of a liquid phase. An unvegetated sediment monolith was retrieved from an estuarine mudflat area at a tidal marsh site and maintained in a saturated state. Laboratory measurements on the mud monolith confirmed that not only ebbing tides, but also flooding tides could trigger ebullition releases of gas bubbles. We develop a Changing Stress for Simulating Ebullition (CSSE) model to describe mechanisms controlling bubble expansion in response to water level changes to unify these observations. Decreases in water level are assumed to lower the effective stress surrounding isolated trapped gas bubbles, driving upward transport via bubble expansion and deformation, with associated fracturing of overlying sediments. Increases in relative permittivity suggest that additional water invades macropores, with associated pore expansion, during the initial stage of increases in water level. We propose that subsequent matrix expansion under lowered effective stress on rising tides also leads to fracture propagation and bubble release. Our findings demonstrate the importance of effective stress changes in triggering ebullition from mudflat areas in tidal wetlands, modulated by the mechanical properties of shallow soft sediments.

Seismogeodesy of the 2014Mw6.1 Napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike‐slip fault

AbstractReal‐time high‐rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014Mw6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS‐only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real‐time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS‐only or seismic‐only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real‐time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Additional record of Rayllianassa amboinensis (de Man, 1888) from Japan, and description of a new species from Okinawa Island, Ryukyu Islands (Crustacea: Decapoda: Axiidea: Callianassidae).

Two species referred to the callianassid ghost shrimp genus Rayllianassa Komai & Tachikawa, 2008 are reported herewith. Additional locality records from Japan are provided for R. amboinensis (de Man, 1888), and the synonymy of Callianassa ngochoae Sakai, 1999 with R. amboinensis is discussed. It is shown that R. amboinensis is associated with sponges or alcyonacean soft corals, representing unusual habitats for callianassids. Rayllianassa rudisulcus n. sp. is described on the basis of a single ovigerous female from shallow soft sediment in Ohura Bay, Okinawa Island, Ryukyu Islands. The new species is distinguished from R. amboinensis by the absence of a dorsal oval on the carapace, the antennal peduncle being longer than the antennular peduncle, and the different shape of the third maxilliped. The status of Rayllianassa is also briefly discussed.

Confronting Feedbacks of Degraded Marine Ecosystems

In many coastal areas, marine ecosystems have shifted into contrasting states having reduced ecosystem services (hereafter called degraded). Such degraded ecosystems may be slow to revert to their original state due to new ecological feedbacks that reinforce the degraded state. A better understanding of the way human actions influence the strength and direction of feedbacks, how different feedbacks could interact, and at what scales they operate, may be necessary in some cases for successful management of marine ecosystems. Here we synthesize interactions of critical feedbacks of the degraded states from six globally distinct biomes: coral reefs, kelp forests, seagrass beds, shallow soft sediments, oyster reefs, and coastal pelagic food webs. We explore to what extent current management captures these feedbacks and propose strategies for how and when (that is, windows of opportunity) to influence feedbacks in ways to break the resilience of the degraded ecosystem states. We conclude by proposing some challenges for future research that could improve our understanding of these issues and emphasize that management of degraded marine states will require a broad social–ecological approach to succeed.

Influence of an altered salinity regime on the population structure of two infaunal bivalve species

Hydrological alterations in watersheds have changed the flows of freshwater to many nearshore marine environments. The ensuing alterations to the salinity environment of coastal waters may have implications for species distribution. This study describes the response of two common bivalves to a modified salinity environment imposed by freshwater inputs from a hydroelectric power station in Doubtful Sound, New Zealand. Populations of Austrovenus stutchburyi and Paphies australis inhabiting river deltas near the outflow of the power station in inner Doubtful Sound were more than an order of magnitude smaller in abundance than populations in neighbouring Bradshaw Sound where the salinity regime is unaltered. In addition, there was a lack of small size classes of both species in inner Doubtful Sound, suggesting that these populations are unsustainable over the long term (10–20 years). Laboratory experiments demonstrated that sustained exposure (>30 days) to low salinity (<10) significantly decreased bivalve survivorship; however, both species survived periods of exposure to freshwater up to at least 20 days in duration if followed by a period of return to normal seawater salinity. Examination of the extant salinity regime in light of these results indicates the current salinity environment in Doubtful Sound restricts bivalves to deeper waters (5–6 m depth). The observed discrepancy in the total biomass of these active suspension feeders between altered and control sites has potential implications for the flux of organic matter in the food webs of Fiordland's shallow soft sediment communities.

Naphthalene disrupts pheromone induced mate search in the amphipod Corophium volutator (Pallas)

Many crustaceans communicate with pheromones during mating. In the infaunal amphipod Corophium volutator (Pallas), females release gender-specific waterborne pheromones that guide males in their search for receptive burrowed females. Pollutants that affect the pheromone release or impair the ability of males to detect and respond to chemosensory cues, could profoundly affect mating. C. volutator is commonly found on shallow soft sediments in estuaries and coastal waters, and their reproductive season coincides with intense traffic of leisure boats, which discharge high concentrations of fuel into the water and sediment of these areas. This study shows that sublethal exposure to naphthalene, a polycyclic aromatic hydrocarbon abundant in motor fuel, disturbs chemical communication in C. volutator. This was demonstrated in Y-maze bioassays, where males were allowed to follow female pheromones after separately exposing the males or females to naphthalene spiked sediments (0, 0.5, 5 or 50 μg g −1 dw, nominal concentrations) for 3 days. The results show that exposure to naphthalene at the two lower concentrations significantly affects the males’ response to female pheromones ( p < 0.05 and p < 0.01, respectively). Male search activity was reduced by 27–45% and males could no longer find females by the use of the olfactory sense. Analysed sediment samples, however, indicate that the naphthalene concentrations causing this effect were 2–20 times lower than nominal concentrations. At the highest naphthalene treatment, no significant difference from the controls was found, probably explained by a different exposure regime for these amphipods since they seemingly avoided burrowing in sediments. Female C. volutator produce and release pheromones, despite naphthalene exposure. Disrupted chemical communication in C. volutator may affect the reproductive fitness of males and females with possible consequences for populations in contaminated areas.

Episodic disturbance events modify predator–prey interactions in soft sediments

Physical disturbance events are common in shallow soft-sediment habitats and can have significant effects on predator–prey interactions. While several studies have reported on predator aggregations following disturbance events, few studies have investigated the mechanisms and interactive effects of predation and physical disturbance on prey survival in shallow soft-sediment habitats. In this study the interactive effects of sediment resuspension and predation by two contrasting epibenthic predator species were tested on the survival of the amphipod Corophium volutator in a laboratory experiment. The shrimp Crangon crangon and juvenile plaice Pleuronectes platessa were used as predators, both numerical dominants in shallow soft sediments on the Swedish west coast. In addition we quantified epibenthic predator aggregation in the field following small-scale disturbances. In the laboratory, synergistic negative effects of predation and non-lethal disturbance on Corophium survival were found with both predator species, and rapid aggregation of several mobile epibenthic predator species following disturbance was demonstrated in the field. Abundances of C. crangon, the numerically dominant predator in the field, were doubled in disturbed patches within 2 min following disturbance. Our study emphasises the importance of considering episodic small-scale disturbances when interpreting predation effects and trophic interactions in shallow soft-sediment systems.

S-wave velocity structures of sediments estimated from array microtremor records and site responses in the near-fault region of the 1999 Chi-Chi, Taiwan earthquake

The 1999 Chi-Chi, Taiwan earthquake, MW = 7.6, caused severe damage in the near-fault region of the earthquake. In order to evaluate site effects in the near-field strong motions we estimate S-wave velocity structures of sediments at four sites using array records of microtremors. We also recalculated S-wave velocity structures at other four sites previously reported. To show the validity of the estimated S-wave velocity structures we separate empirical site responses from aftershock records using the generalized inversion method and show the agreement between empirical and theoretical site responses. We also show an observed fact that suggests soil nonlinearity during the Chi-Chi earthquake by comparing horizontal-to-vertical spectral ratios (HVRs) for main shock records with HVRs for aftershock records. Then we calculate one-dimensional equivalent-linear site responses using the estimated S-wave velocity structures and the main shock records observed on the surface. It is found that site amplification due to thick (about 6 km) sediments is one of the important factors for explaining the long-period velocity pulses of about 5 to 10 sec observed at sites in the footwall during the Chi-Chi earthquake. It is also found that the theoretical site responses of shallow soft sediments at sites that sustained severe damage in the hanging wall shows significant amplification around 1 sec. As the amplitude of velocity pulses with period around 1 sec is most critical in causing damage to ordinary buildings of moderate heights, our results suggest that the 1-sec period velocity pulses, amplified by the site response of shallow sediments should contribute to the severe damage during the Chi-Chi earthquake.

Small scale patterns in the structure of macrofaunal assemblages of shallow soft sediments

The spatial scale of local patterns in the fauna of two contrasting shallow water sediment assemblages has been investigated using hierarchical sampling. Replicate samples were taken at separations of 50 cm, 5 m, 50 m and 500 m. No significant differences between samples could be detected on any of these scales in a fine sand assemblage. However, in a heavily bioturbated sandy mud, samples separated by more than 50 m were significantly different from each other. This separation was largely a result of changes in the pattern of dominance among the most abundant species, particularly annelids. In a complimentary set of analyses, animals were regrouped either by higher taxa or by body size. The numerically important annelids and crustaceans showed a pattern of spatial similarity close to the full data set, but that shown by molluscs was clearly distinct. Large bodied animals were also independent of the main pattern while smaller species conformed to it. In the case of the latter, separation of sites at the 50 m scale could be related to patterns of species richness as well as dominance.