Using tsunami deposits to determine the maximum depth of benthic burrowing

Deep-sea infauna with calcified exoskeletons imaged in situ using a new 3D acoustic coring system (A-core-2000)

The deep ocean is Earth’s largest habitable space inhabited by diverse benthic organisms. Infauna play crucial roles in shaping sedimentary structures, relocating organic matter, porewater chemistry, and hence biogeochemical cycles. However, the visualization and quantification of infauna in situ inside deep-sea sediment has been challenging, due to their sparse distribution and that deep-sea cameras do not visualize animals living below the sediment surface. Here, we newly developed a 3D acoustic “coring” system and applied it to visualize and detect burrowing bivalves in deep-sea sediments. The in situ acoustic observation was conducted at a dense colony of vesicomyid clams in a hydrocarbon seep in Sagami Bay, Japan, focusing on a patch of juvenile clams with a completely infaunal life style. We clearly observed strong backscatters from the top and lower edges of animals in our 3D acoustic data. At least 17 reflectors were identified in the survey area (625 cm²), interpreted to correspond to living clams. The estimated depths of the lower edge of clams ranged between 41 and 98 mm. The acoustic system presented here is effective for detecting and monitoring infauna with calcified exoskeletons. This novel tool will help us better assess and understand the distribution of deep-sea infauna, particularly those groups with hard exoskeletons, as well as biogeochemical cycles.

Biological traits of marine benthic invertebrates in Northwest Europe

Biological traits analysis (BTA) provides insight into causes and consequences of biodiversity change that cannot be achieved using traditional taxonomic approaches. However, acquiring information on biological traits (i.e., the behavioural, morphological, and reproductive characteristics of taxa) can be extremely time-consuming, especially for large community datasets, thus hindering the successful application of BTA. Here, we present information on ten key biological traits for over a thousand marine benthic invertebrate taxa surveyed in Northwest Europe (mainly the UK shelf). Scores of 0 to 3 are provided to indicate our confidence that taxa exhibit each possible mode of trait expression. The information was acquired over a decade through an extensive appraisal of relevant sources, including peer-reviewed papers, books, online material and, where necessary, professional judgement. These data may be inspected, used, and augmented by fellow researchers, thus assisting in the wider application of BTA in marine benthic ecology.

Analysis of environmental and microbiological changes in Onagawa Bay immediately after the tsunami of the Great East Japan Earthquake based on sediment cores

We investigated two sediment cores to understand whether a tsunami in Onagawa Bay, Japan caused environmental changes. The value of δ¹³C ranged from -21.9‰ to -24.3‰ and of δ¹⁵N ranged from 5.1 to 5.9‰. We conclude that the source of the sediment in the present study area was mainly oceanic and not terrestrial. The chlorophyll concentration ranged from 1.8 to 4.0 μg/g ww, and did not vary greatly between surface and bottom layers. We inferred that all layers were deposited after the tsunami. The major phytoplankton taxa in sediments were diatoms from DNA sequencing. The presence of harmful dinoflagellates was minor. The concentrations of several heavy metals decreased slightly after the tsunami. We inferred that heavy metals in sediments were diluted by the tsunami disturbance. The land in Onagawa suffered serious damage, but disturbance of the seabed was much less evident.

The effects of sediment transport on temporal variation in radiocesium concentrations in very shallow water off the southern coast of Fukushima, Japan

We studied the very shallow coastal water off Iwaki City, Fukushima Prefecture, and investigated: (1) temporal variation in ¹³⁷Cs concentrations; (2) particle-size distribution of sediments; and (3) the effect on variation by waves, component-fractionated concentration of radiocesium and mineral composition at three sampling stations (Yotsukura, Ena rocky reef and Ena sandy station). There was a decline in ¹³⁷Cs concentrations in sediment samples at all sampling stations between 425 and 1173 days after the accident. All stations had fluctuations in ¹³⁷Cs concentrations between 425 and 800 days. At Ena sandy station and Ena rocky reef stations the declines in ¹³⁷Cs concentrations slowed from about 800 days after the accident. Fluctuations in particle median diameters were seen, as well as in ¹³⁷Cs concentrations. At Yotsukura, where the fluctuation in median diameter was small, a constant decrease in ¹³⁷Cs concentrations was observed. We considered that bioturbation may contribute the constant decrease. At Ena sandy station, where the fluctuation of the median diameter was large, the fluctuation in ¹³⁷Cs concentrations was also large. The movement of sediments was evaluated by the Shields parameter, and results indicated that at any station where the sediment was moved more frequently, the fluctuation in ¹³⁷Cs concentrations was also large. The highly contaminated small particles moved from our stations due to wave action between 425 and 800 days after the accident. The remaining relatively large particles might contribute to the slowing down in reduction of ¹³⁷Cs concentrations from 800 days after the accident. However, the ¹³⁷Cs concentrations in sediments in very shallow water off the southern coast of Fukushima may continue to decline over time.