Pacific Coast Of Northeastern Japan Research Articles

Effects of the 2011 Tohoku‐oki tsunami and human activities on long‐term coastal geomorphic development in northeastern Tohoku, Japan

AbstractThe 2011 Tohoku‐oki tsunami caused large‐scale topographic changes along the Pacific coast of northeastern Japan. More than 10 years have passed since the tsunami waves struck the area. Today, because of reconstruction work, very few places exist where natural post‐tsunami topographic changes can be monitored continuously. For this study, the authors investigated topographic changes caused not only by the 2011 tsunami but also by natural and artificial activities during the 50 years before and after the tsunami based on aerial photographs, excavations and subsurface explorations using ground‐penetrating radar at the Osuka coast in Aomori prefecture, Japan. The site is rare because it is a protected area with few and superficial engineering activities, making it suitable for continuous observation of pre‐tsunami, syn‐tsunami and post‐tsunami topographic changes. The findings indicate that the 2011 tsunami waves generated large topographic changes: depositional and erosional features produced by the tsunami can be recognized, respectively, as tsunami deposits and erosional channels across the sand dunes. During the post‐tsunami phase, the sand volume at the coast quickly recovered naturally. Tsunami deposits and the erosional channels were well preserved underground even at 10 years after the event. However, dynamic movement of the dunes started after the tsunami. The shifting was attributable to the artificial clearing of coastal forests rather than the tsunami effects on the coast. Our results first indicate not only that the sedimentary features of paleo‐tsunamis but also the erosional features have some probability of being preserved in the subsurface of the beach and sand dunes at tsunami‐affected areas. Also, artificial activities such as deforestation are much more crucially undermining of the stability of the coastal geomorphology than the tsunami effects: the coast is now reaching a different status from its pre‐tsunami situation.

Morphological characteristics, molecular species validation and spatio-temporal distribution of myctophid eggs off the Pacific coast of northeastern Japan

Myctophids in the world's oceans have a huge biomass and act as an important link between secondary producers and upper trophic levels through their extensive diel vertical migrations from the epipelagic to mesopelagic layers. Dominance of myctophid larvae in oceanic ichthyoplankton has been reported from various parts of the major oceans; however, there are few records concerning their eggs. We established species-specific morphological characteristics for identifying myctophid eggs which were then validated by sequence analysis of mitochondrial 12S rRNA and cytochrome b genes using representative subsamples of the morphotypes, and then used the criteria to determine the species composition and spatio-temporal distribution of myctophid eggs. Samples were collected monthly in the epipelagic layer off the Pacific coast of northeastern Japan, resulting in 664 net tows that yielded a total of 10,172 myctophid eggs, defined by a smooth and delicate chorion, segmented yolk, and single oil globule. Subsamples of 160 small, 34 intermediate, and 20 large-sized eggs were sequenced, resulting in their classification as Diaphus theta, Diaphus garmani, and Myctophum asperum, respectively. Mean egg diameter was smallest in D. theta (0.75 mm), intermediate in D. garmani (1.06 mm), and largest in M. asperum (1.31 mm). Eggs of D. theta had a markedly narrower perivitelline space (0.17 mm) than D. garmani and M. asperum (0.49 and 0.71 mm, respectively). Based on morphology, the myctophid eggs were dominated by D. theta (86.0%), followed by D. garmani (12.6%) and M. asperum (1.4%). Abundances of early-stage eggs of D. theta and D. garmani were high during the daytime to dusk but low at night and dawn, indicating that spawning would not be active in the epipelagic layer at night. Spawning can be considered to occur in the mesopelagic layer during the early night, and then the eggs rise to reach the epipelagic layer in the daytime. High abundances of D. theta and D. garmani eggs were observed in the slope region from spring to summer, corresponding with the larval distribution reported for the study area.

Grazing pressure of sea urchin Mesocentrotus nudus estimated from seawater temperature and wave height, and its effects on the early growth of annual kelp Saccharina japonica in northeastern Japan

Grazing by the sea urchin Mesocentrotus nudus plays a vital role in the development of marine forests of annual kelp, Saccharina japonica, along the Pacific coast of northeastern Japan. The grazing rate of sea urchins, which are rigorously temperature dependent, affects the chance of germination/survival of the kelp during winter/spring when seawater temperatures in this region are highly variable among years. It has previously been found that if the mean seawater temperature between February and March remained below 5.5 °C, kelp forests in this region would establish later in the year. However, the mechanism leading to this success or failure of kelp forests to develop has yet to be investigated in detail.Laboratory experiments confirmed that the grazing rate and movement of the sea urchin decreased as seawater temperature declined from 12 °C to 3 °C. Field observations, using interval cameras from autumn to spring, revealed that the density of sea urchins on artificial reefs decreased at lower seawater temperatures and/or higher significant wave heights (SWH). Using these relationships, the grazing pressure by sea urchins (= grazing rate × density) in the site was estimated based on seawater temperature and SWH. Comparing the grazing pressures among past years with different states of kelp forests, conditions needed to establish kelp forests were revealed. Extremely low grazing pressure (< 0.5 g m−2 day−1) lasted for a total of 31 days in 2015 when kelp grew densely, while this condition occurred on 11 and 14 days respectively in 2019 and 2022 when kelp was sparse. Kelp did not grow in 2016 when the grazing pressure continuously remained above 0.5 g m−2 day−1. At this study site, such combinations of seawater temperature and SWH as 4 °C and 1.0 m, 6 °C and 2.3 m, and 8 °C and 3.7 m would equate to a grazing pressure of 0.5 g m−2 day−1. To develop a dense kelp forest, grazing pressure was required to be lower than 0.5 g m−2 day−1 for about a month as in 2015. Though these figures should be adjusted, a model and scheme of this study will be applied to the other areas where kelp forest restoration/management plans are required.

Deterioration of an annual kelp Saccharina japonica forest and its effects on dominant herbivores, sea urchin Mesocentrotus nudus and abalone Haliotis discus hannai, in northeast Japan

A marine forest of the annual kelp Saccharina japonica was monitored at three depth sites (6, 9 and 12 m) along a 200-meter-long transect in the Pacific coast of northeastern Japan from 2014 to 2021, during which the abundance of the kelp plummeted, and the forest shifted to a crustose coralline bed. Colder seawater temperature in the winter/spring (February and March), which is regulated by the extension of the cold Oyashio Current, plays an important role in establishing the kelp forest through reducing the grazing of herbivores during the critical period of kelp germination and early growth. The Oyashio Current reached the region in 2014 and 2015 but not from 2016 to 2021, making a fundamental change in the kelp forest. Macroalgal abundance was strikingly different between these two periods. Biomass of S. japonica reached up to 38.6–45.4 kg-ww m−2 in the first two years, whereas macroalgal biomass was less than 3.2 kg-ww m−2 in the latter 6 years except for in 2016 when the biomass reached 9.0 kg-ww m−2 at the 6 m site. Gonad and somatic growth of sea urchin Mesocentrotus nudus declined during the kelp deforestation, while the density of the sea urchin moderately increased in 2017. Similarly, the density of abalone Haliotis discus hannai plummeted by about 90% and the mean shell length decreased from 83 mm in 2014 to 31 mm in 2020. Under the ocean warming scheme, ecosystems depending on the primary production of S. japonica are expected to be altered in the future, and thus urging the need of environmental monitoring.

Seismogenesis of the 2021 Mw 7.1 earthquake sequence near the northeastern Japan revealed by double-difference seismic tomography

Recently, a Mw 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the Mw 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the Mw 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence.

Influence of normal tide and the Great Tsunami as recorded through hourly-resolution micro-analysis of a mussel shell

We report here hourly variations of Mg/Ca, Sr/Ca, and Ba/Ca ratios in a Mediterranean mussel shell (Mytilus galloprovincialis) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25–90 μm intervals, while no interpretable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established by (1) cessation of the external supply of Ca and organic layer forming when the shell is closed at low tide, and (2) the strong binding of Mg to the organic layer, but not of Sr and Ba. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, therefore serving a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

Significance of Fukushima-derived radiocaesium flux via river-estuary-ocean system

The environmental dynamics of Fukushima-derived radiocaesium from land to ocean and the impact of its flux on the marine environment are matters of concern because radiocaesium will be continually transported to the open ocean for the next several decades, or possibly more than one hundred years. In order to assess the distribution and flux of radiocaesium in a river-estuary-ocean system, we investigated the activity concentration of radiocaesium in Matsukawa-ura Lagoon, the largest lagoon in Fukushima, where it is very easy to carry out observations with a wide salinity gradient. Activity concentrations of dissolved 137Cs are elevated in seawater of low to intermediate salinity. It can thus be inferred that radiocaesium desorbs from suspended particles in an estuarine area. The porewater activity concentration of 137Cs in lagoon sediment was about 10 times higher than that in the overlying lagoon water. This direct measurement indicates that a significant amount of radiocaesium in sediment desorbs into porewater. From the results of a mass balance model, dissolved 137Cs flux from the lagoon's bottom is 15.3 ± 3.7 times greater than the riverine input, including desorption from particles. In the case of the whole Pacific coast of northeastern Japan (Miyagi, Fukushima, and Ibaraki Prefectures), dissolved 137Cs flux into the open ocean, including diffusion of porewater, is estimated to be up to 1.5 times greater than the sum of riverine input and the ongoing release from the Fukushima Dai-ichi Nuclear Power Station's harbor. Consequently, our results suggest that radiocaesium is transported to the open ocean under the control of various processes, not only by desorption from particles but also, for example, by the diffusion of porewater.

A case study on the swimming depth of Pacific bluefin tuna &lt;i&gt;Thunnus orientalis&lt;/i&gt; during the hauling of a bag net based on underwater observation using ROV in setnets off Sanriku, Pacific coast of northeastern Japan

A case study on the swimming depth of Pacific bluefin tuna &lt;i&gt;Thunnus orientalis&lt;/i&gt; during the hauling of a bag net based on underwater observation using ROV in setnets off Sanriku, Pacific coast of northeastern Japan

Evaluation of impacts of bottom fishing on demersal habitats: A case study off the Pacific coast of north-eastern Japan

Physical disturbance of the seafloor induced by bottom trawling is of increasing concern from the viewpoints of ecosystem conservation and sustainable fishery. We developed a method for evaluating the impacts of several kinds of trawl gears on the different seafloor habitats using bathymetry and sediment data and scientific survey data off the Pacific coast of north-eastern Japan. Ten types of habitats were classified by the cluster analysis applied to seafloor bathymetry, topography, and sediment hardness over the study area. Our mapping of these habitats demonstrated that the intensity of bottom fishing activities on the seafloor was different among habitat types and related to the abundance of commercial species living in each habitat. The ecological influences of fishing on these habitats were examined by focusing on the changes in demersal species diversity and biomass detected through scientific surveys. A decline in species diversity was more evident in the habitat types with complex topography and hard sediment in the intensely trawled areas. Comparison of fishing gear suggested that the otter trawl may be the most influential owing to the significant change in the demersal ecosystem.

Seasonal changes in depth and temperature of habitat for Japanese flounder Paralichthys olivaceus on the Pacific coast of northeastern Japan

Seasonal changes in habitat use in terms of depth and temperature were estimated for Japanese flounder Paralichthys olivaceus from the Pacific coast of northeastern Japan from 2006 to 2008, using data from trawl and gillnet fisheries logbooks, to which we assigned monthly estimates of bottom temperature. Estimated distributions of flounder for depth and temperature were validated by actual depth and temperature readings from data storage tags (DSTs) attached to flounder, and from collection records by trawl survey. Year-round, flounder > 300 mm in total length were present mainly in areas < 100 m in depth, although their depth range extended to ~ 100 m in summer and ~ 200 m in winter. Flounder experienced cold temperatures (< 10 °C) for several months during winter and spring, and cooler temperatures (< 18 °C monthly mean) in summer, with a range from 4.6 °C to 21.0 °C. This temperature range of the habitat is lower than recorded for this species elsewhere in Japan, and seemed to affect regional life-history traits such as growth and spawning season. DST readings indicated that flounder experienced higher temperatures during vertical swimming behavior than bottom temperatures when the water was stratified (May–October), which may also influence life-history traits.

Effects of the 2011 Tohoku earthquake and tsunami on the abundance and composition of anthropogenic marine debris on the continental slope off the Pacific coast of northeastern Japan

The abundance and composition of anthropogenic marine debris from 2012 to 2014 was assessed according to three bottom trawl surveys conducted on the upper continental slope between 198 m and 501 m off the Pacific coast of northeastern Japan. The surveys quantitated marine debris as follows: 33.52–164.62 items km−2 (January to July 2012), 91.68–215.11 items km−2 (November 2012 to May 2013), and 160.13–178.19 items km−2 (November 2013 to May 2014). Plastic bags or household materials mainly dominated terrestrial sources of debris. Principal component analysis latitudinally divided the study area according to debris abundance caused by geographical and hydrodynamic features. The long-term effect of tsunami-associated debris on the seafloor environment was recognized, because terrestrial sources such as heavy household materials were most abundant throughout the study period, with the additional accumulation of fishing gear and plastic bags.

Bottom temperature warming and its impact on demersal fish off the Pacific coast of northeastern Japan

Global climate change occurs not only at the ocean surface but also at the ocean bottom, which is the main habitat of demersal fish. To clarify the current status of bottom temperature warming off the Pacific coast of northeastern Japan, we examined gridded bottom temperature fields from 2003 to 2019. These fields were created by a newly developed gridding method using flexible Gaussian filter weighting with time, distance, and depth. Spatially averaged bottom temperature had a strong, significant warming trend of 0.083 to 0.115°C yr-1 in depth zones of 150-300 m, indicating bottom temperature warming. Corresponding to the warming, increases in landing amounts were found for warm-water species such as searobin in the middle region of our study area (37°50’-39°N). Seasonal catch amounts suggest that ribbon fish and swimming crab recently began to overwinter and reproduce in the area. The distribution shifts of non-target species in fisheries were also analyzed using bottom otter trawl survey data from the area from 2003 to 2019. Northward distribution shifts and increases in density were observed in blackbelly lantern shark and bighand grenadier, indicating that bottom temperature warming led to habitat expansion. Conversely, darkfin sculpin and jelly eelpout shifted northward with decreasing density, suggesting that bottom temperature warming had a negative effect on them. Deepsea bonefish shifted deeper into colder waters with increasing density and mean body weight. Thus, changes and responses of demersal fish to bottom temperature warming in the area were revealed.

Suitability of Japanese codling as a raw material for surimi-based products revealed by primary sequence analysis of myosin heavy chain and thermal gel properties

Complementary DNA encoding a part of myosin heavy chain was cloned from fast skeletal muscle of Japanese codling Physiculus japonicus. Japanese codling, a frequent catch off the Pacific coast of northeastern Japan, is a typical underutilized fish species, especially in summer. Phylogenetic analysis of the deduced amino acid sequence of Japanese codling myosin heavy chain loop 2 region revealed that this fish belongs to a group comprising white croaker Pennahia argentata and walleye pollack Gadus chalcogrammus, which are commonly used as raw materials for surimi-based products. Thus Japanese codling meat was considered a promising candidate ingredient for surimi-based products. Salt-ground meat was prepared from Japanese codling in the presence of 0.5 M NaCl and subjected to a two-step heating procedure: pre-heating at 15–70 °C for 30 min, and secondary heating at 85 °C for 20 min. Thermal gels with a maximum breaking strength of > 10 N were produced by this process. The breaking strength was comparable to that of white croaker and walleye pollack. On the basis of the phylogenetic analysis of myosin heavy chains, and the properties of the heat-induced surimi gel, we conclude that Japanese codling is a promising raw material for the production of surimi-based products.

Estimating the change in regional scale distribution of seagrass and macroalgal beds using discrete local distribution data analyzed from aerial images

AbstractThe sensitivity of vegetated habitats to climate change and catastrophic disturbance, as well as their benefits to human beings, motivate efforts to estimate regional changes in distribution. This study examined the regional changes in the distribution of submerged macrophyte vegetation using aerial and satellite images from before and after the 2011 tsunami disturbance along the Pacific coast of northeastern Japan. We conducted an estimation of the change in the distribution of seagrass and macroalgal beds after the tsunami along the whole coastline (1,635 km). This was based on vegetation change in the 13 local areas (each &lt;40 km) spanning 27% of the coastline and included 28% of the macroalgal area and 59% of the seagrass area documented in satellite imagery before the tsunami, using generalized linear modeling (GLM). We found that in the 13 local areas 716.8 and 1,990.3 ha of macroalgal beds and seagrass beds had disappeared, respectively, suggesting that seagrass beds were more impacted by the tsunami than macroalgal beds. Significant predictors in GLM indicated that more seagrass beds disappeared in geographically complex coastal areas, whereas macroalgal beds were only marginally affected in such areas. It was estimated that the total decline in seagrass beds was at least 76% along the whole coastline, whereas macroalgal beds decreased only by about 20%. The magnitude and predictors of losses in these two vegetated habitats were similar to earlier estimates, suggesting that our simplified method with GLM is sufficient to extrapolate monitoring data to a regional scale.

Prevalence and species richness of trematode parasites only partially recovers after the 2011 Tohoku, Japan, earthquake tsunami

Trematode parasites have complex life cycles and use a variety of host species across different trophic levels. Thus, they can be used as indicators of disturbance and recovery of coastal ecosystems. Estuaries on the Pacific coast of northeastern Japan were heavily affected by the 2011 Tohoku earthquake tsunami. To evaluate the effect of the tsunami on the trematode community, we examined trematodes in the mud snail, Batillaria attramentaria, at five study sites (three sites severely exposed to the tsunami and two sites sheltered from the tsunami) in Sendai Bay for 2 years prior to and 8 years after the tsunami. While the trematode prevalence decreased at all study sites, the species richness decreased only at the sites exposed to the tsunami. Although parasitism increased over the study period post-tsunami, the community had not fully recovered 8 years after the event. Trematode community structure has changed every year since the tsunami and has not stabilised. This could be explained by the alteration of first and second intermediate host communities. Our study suggests that it will take more time for the recovery of the trematode community and the associated coastal ecosystem in the Tohoku region.

Occurrence of Pacific bluefin tuna Thunnus orientalis larvae off the Pacific coast of Tohoku area, northeastern Japan: Possibility of the discovery of the third spawning ground

AbstractRecently, based on the histological studies it is suggested that Pacific bluefin tuna (PBF) spawns in the Kuroshio–Oyashio transition, off the Pacific coast of northeastern Japan, in addition to the previously identified two spawning areas of the northwestern Pacific around the Nansei Islands and the Sea of Japan. Distributional surveys for PBF larvae have not been conducted in the Kuroshio–Oyashio transition, and thus, we conducted PBF larval surveys at twenty stations in this area in early August 2018 to obtain evidence for the spawning of PBF. Twelve PBF larvae (3.9–7.2 mm in body length) were collected at six stations where sea surface temperature ranged from 27.2 to 28.2°C. Otolith microstructure analysis indicated that age of these larvae ranged from 4 to 11 days after hatching and the larvae hatched in late July. Growth of the collected larvae was comparable to those in the other two spawning grounds. Therefore, PBF spawns, hatches, and at least survives to the postflexion stage, 11 days after hatching in the Kuroshio–Oyashio transition. This fact potentially has a large impact on recruitment processes of PBF if they survive to recruitment in this third spawning ground.

Factors shaping the macrobenthic animal assemblages in a tidal flat newly-formed by the Great East Japan Earthquake

In 2011, the Great East Japan Earthquake and tsunami struck the Pacific coast of northeastern Japan and created new intertidal habitats, which are ideal places to examine the roles of spatial and environmental variables shaping community structure since historical effects caused by resident species are precluded. In this study, therefore, we examined the relative importance of environmental and spatial variables in determining the community structure of the benthic macrofauna in the Higashi-Yachi tidal flat. During the study period, from July 2015 to June 2016, a total of 36 macrobenthic invertebrate taxa were found in the tidal flat. Among these, 14 taxa were abundant and in general significantly different in the abundance among the stations, but not among seasons. Redundancy analysis showed that most polychaetae such as Heteromastus spp. and Notomastus spp. were abundant in stations near the destroyed points of embankments, while the sand bubbler crab Scopimera globosa was abundant at stations with higher elevations. In addition, the analysis showed that 39% of the spatial and temporal variations in the community structures was explained by the examined environmental and spatial variables. The effects of spatial variables such as the elevation of ground level and distance from the destroyed parts of the embankment explained 18.7% of the variations, while the effects of temporally changing variables such as temperature and nutrient contents in the sediment and overlapping effects of these variables explained 7.4 and 12.7% of the variation, respectively. These results suggest that the macrobenthic animal community in this tidal flat was assembled by not only the environmental conditions but also the spatial configuration of the seawater flow and/or bottom elevation in the tidal flats.

Ontogenetic habitat shift of age-0 Japanese flounder Paralichthys olivaceus on the Pacific coast of northeastern Japan: differences in timing of the shift among areas and potential effects on recruitment success

Ontogenetic habitat shift and feeding habits in the shallow (< 15 m) and deep (30–80 m) habitats of age-0 Japanese flounder Paralichthys olivaceus in Sendai Bay, the Pacific coast of northeastern Japan, were examined to understand the mechanisms that potentially enable area-specific high recruitment success. The flounder was able to use the shallow nursery habitat for about 1 year, until the next summer of their settlement (June–August) when they had reached 250 mm total length (TL). In addition, age-0 flounder between 150 and 250 mm TL used both shallow and deep habitats from winter to the next summer of their settlement, where species, size, and availability of potential food and susceptibility to predators are considerably different. These area-specific characteristics in Sendai Bay are clearly different from other areas around Japan that have been described in the literature: shorter residence in the shallow habitat and smaller size at emigration to the deep habitat. The characteristics in Sendai Bay seem to be enabled by prolonged good feeding conditions in the shallow habitat, which result from an abundant food supply and relatively lower temperature that does not exceed the uppermost temperature (25 °C) for maximum growth of the age-0 flounder. We consider that the prolonged better feeding conditions in the shallow habitat in the study area for ca. 1 year after settlement contribute to higher recruitment success.

Position-dependent radiocarbon content of the macroalgae Undaria pinnatifida as an indicator of oceanographic conditions during algal growth

The radiocarbon content (∆14C) of the large brown macroalga Undaria pinnatifida was investigated as an indicator of oceanographic conditions during algal growth in Otsuchi Bay on the Pacific coast of northeastern Japan. In this region, warm boundary currents characterized by high dissolved inorganic carbon (DIC) ∆14C and cold subarctic Oyashio Current (low ∆14C-DIC) flow into the bay seasonally. We hypothesized that ∆14C values in U. pinnatifida pinnae (leaflet-like structures arising sequentially along the vertical axis of the blade) reflect ∆14C-DIC values in bay water during photosynthetic production. During the U. pinnatifida cultivation period (November 2013–April 2014), the Oyashio intrusion occurred from early March to early April, accompanied by a decrease in bay water ∆14C-DIC, from 15.7 ± 8.6‰ to − 57.1 ± 20.7‰. The ∆14C of the pinnae reflected ∆14C-DIC, as pinnae that formed in early March had significantly higher ∆14C (3.0 ± 5.4‰) than those that formed in early April (− 23.6 ± 7.5‰). These results indicate that the ∆14C of U. pinnatifida pinnae can serve as a record of Oyashio intrusion into the bay during algal growth.

A record of utilization as a spawning bed for the invasive ascidian Ascidiella aspersa (Müller, 1776) newly introduced in the Pacific coast of northeastern Japan

A record of utilization as a spawning bed for the invasive ascidian Ascidiella aspersa (Müller, 1776) newly introduced in the Pacific coast of northeastern Japan