In 2011, 2012 and 2013, Horiguchi et al. [34] investigated the ecological effect in the intertidal zone of eastern Japan affected by the FNPP accident that accompanied the Great East Japan Earthquake Disaster. They reported that the number of intertidal species decreased significantly with proximity to FNPP, and that no rock shell (T. clavigera) specimens were found near FNPP, from Hirono to Futaba Beach (a distance of approximately 30 km) in 2012. The presence of rock shell specimens at many other sites hit by the tsunami suggested that the absence of rock shells around FNPP in 2012 might have been caused by the FNPP accident. The quantitative 2013 surveys also showed that the number of species and population densities in the intertidal zones were much lower at sites near or within several kilometers south of FNPP than at other sites, and lower than in 1995, especially in the case of Arthropoda. There is no clear explanation for these findings. Although there are no estimates of radiation doses from environmental exposure in invertebrates in the intertidal zone near FNPP after the accident, clearly the intertidal biota around FNPP has been harmed since the nuclear accident.

The present study was carried out to investigate temporal changes in species composition and population densities in the intertidal zone of eastern Japan since 2014. Increases in species richness were evident at Tomioka Fishing Port (Tomioka) and Ottozawa (Okuma), which are located south of FNPP, from 2016, though this increase might have started at Ottozawa (Okuma) in 2015 (Fig. 6.2). Meanwhile, population densities have increased in the lower and middle intertidal zones at Ottozawa (Okuma) and Tomioka Fishing Port (Tomioka) since 2015 and 2016, respectively (Fig. 6.3). These increases were due to increases in small mussels (i.e., Mytilus galloprovincialis) at Ottozawa (Okuma) and barnacles (i.e., Semibalanus cariosus and Chthamalus challengeri) at Tomioka Fishing Port (Tomioka). In 1995, TEPCO conducted similar seasonal surveys using 30 × 30-cm quadrats at 20 sites in intertidal zone along the coast of Fukushima Prefecture, but only published a summary of their results [37]. In May 1995, they found an average of 7,158 individual sessile organisms/m2, consisting of Arthropoda (4,593, 64.2%), Annelida (179, 2.5%), Mollusca (2348, 32.8%), and other organisms (38, 0.5%) [37]; thus, although Arthropoda clearly predominated, many other invertebrates were also present in Fukushima Prefecture in 1995, before the FNPP accident. Compared with the TEPCO data, we observed similar or higher numbers of individuals/m2 in the lower and middle intertidal zones at Ottozawa (Okuma) in 2015 and Tomioka Fishing Port (Tomioka) in 2016 (Fig. 6.3). Because of their small sizes, however, increases in the total biomass at Ottozawa (Okuma) in 2015 and Tomioka Fishing Port (Tomioka) in 2016 were unclear (Fig. 6.4).

The species composition of sessile organisms at Kubo-yaji, approximately 1 km north of FNPP, differed markedly from that at other sites surveyed, in that the Arthropoda accounted for less than 1% of all sessile organisms collected (Fig. 6.3b–d). Although Arthropoda predominated in the intertidal zone of Fukushima Prefecture in 1995 [37], population densities seem to have decreased after the FNPP accident. The reduced population densities there have continued since 2013. Population densities of Arthropoda at sites near FNPP (Tomioka, Okuma, and Kubo-yaji) were significantly lower than those at other sites in 2014 and 2015 (t-test; P < 0.05), but not in 2016 (P = 0.66), possibly due to increases in barnacle population densities in the lower intertidal zone at Tomioka Fishing Port (Tomioka) in 2016 (Fig. 6.3d).

No T. clavigera specimens were observed near FNPP from Hirono to Futaba Beach (a distance of approximately 30 km) in 2012 [34]. In the present study conducted during 2014–2017, however, their distribution gradually expanded year by year from sites in the north and south to sites along the central Fukushima coast (Fig. 6.8), and the gap in the distributions, where no T. clavigera specimens were found, had closed by July 2016, approximately a year after an observation of increases in small mussels (i.e., Mytilus galloprovincialis) at Ottozawa (Okuma) in 2015; these mussels are prey for T. clavigera (Fig. 6.3c). The increase in prey organisms, such as small mussels (i.e., Mytilus galloprovincialis) at Ottozawa (Okuma) in 2015 might have been associated with the first observation of T. clavigera specimens there in July 2016, since our field survey started in December 2011, 9 months after the FNPP accident. The densities of T. clavigera populations seemed to increase from 2012 to 2016, with large variations (Fig. 6.8). Low densities in T. clavigera populations (around or less than 1 individual collected per minute), however, have persisted at sites in Okuma Town, within a few kilometers south of FNPP (Fig. 6.8d). In general, population densities of T. clavigera in Japan range from 1 to 4 individuals collected per minute (Horiguchi et al., unpublished data). Neither spawning behavior nor egg capsules spawned by T. clavigera populations were observed at sites near FNPP (from Tomioka Fishing Port to Kubo-yaji) through the summer of 2016 (Table 6.3). Egg capsules were first observed at Koirino, Ottozawa (Okuma Town), and Kubo-yaji (Futaba Town) in the summer of 2017, though the number of egg capsules spawned was limited (Table 6.3).

The number of species and population densities in the intertidal zone near FNPP did not begin to increase until at least 4–5 years after the FNPP accident. Even in 2017, 6 years after the disaster, densities of and reproductive performance by T. clavigera populations near FNPP remained below pre-disaster levels. Most invertebrates in the intertidal zone, including sessile organisms, have planktonic stages in their early life histories for certain periods: for example, T. clavigera has a veliger larva that lives a few months before settling [38]. In the coastal waters off Fukushima Prefecture, the generally weak currents and tides [39] are expected to effect the dispersion of larvae of most invertebrates in the intertidal zone, as well as dispersion of FNPP-derived radioactive Cs. The Kuroshio and Oyashio currents, which are the wind-driven boundary currents of the North Pacific, meet in the Kuroshio–Oyashio transition area off FNPP [40]. The Kuroshio is part of the subtropical gyre and transports warm saline water along the south coast of Japan and then eastward via the Kuroshio Extension (KE), whereas the Oyashio is part of the subarctic gyre and transports cold less saline water southward [41]. Actually, results on the bottom-sediment survey collected from a coastal strip (~30 × 120 km) off FNPP in October 2012 revealed that radioactive Cs concentration in the uppermost sediment layer were higher south of FNPP, though high activity concentration patches were also observed at a few sites north of FNPP [42]. Therefore, recruitment of their larvae could have been expected from remote areas to sites near FNPP. Nevertheless, data in the present study revealed that neither juvenile nor adult T. clavigera were observed at Ottozawa (Okuma), approximately 1 km south of FNPP, until at least 2016; thus, for invertebrates including sessile organisms in the intertidal zone near FNPP, some environmental factors seem to have inhibited reproduction, recruitment, or both.

43 44 3 of bentonite and cement were used for the construction from March 14 to July 5, 2012 [ 43 3 of bentonite, sand, and cement were also used for the construction from July 17, 2014 to April 23, 2015 [ 44 3 [ 45 6.4 46 47 48 6.3 Metals Composition (mg/kg) Estimated total amount (t) Total Cr 98 9.8 Hexavalent Cr 10.8 1.1 Cu 140 14.0 Zn 511 51.0 As 18.9 1.9 Se <1 <0.1 Cd 2.0 0.2 Total Hg 0.023 0.002 Pb 111 11.1 Among the factors that might have inhibited recovery of invertebrates are strong waves; sustainability or suitability of substrate (e.g., tetrapods or similar concrete structures set along the coast for wave protection) for larvae to settle on; harmful substances such as radionuclides; heavy metals; turbidity or suspended material; insufficient amounts of prey organisms; and excess of predators. Before the FNPP accident, there were strong waves, substrate (e.g., tetrapods or similar concrete structures set along the coast for wave protection) for larvae to settle on, and various kinds of intertidal predator and prey invertebrates in the coastal waters off Fukushima Prefecture, similar to those after the FNPP accident. Although the possibility of changes in predators or prey affecting invertebrate populations cannot be ruled out, strong waves and substrate are unlikely to be those that inhibited reproduction or recruitment of invertebrates including sessile organisms in the intertidal zone near FNPP. Regarding the possible impacts by harmful substances, such as radionuclides, heavy metals and turbidity or suspended material, we need to carefully examine the effect of construction to cover the radionuclide-contaminated bottom sediments of the FNPP port; this construction was conducted between March 2012 and April 2015 []. Approximately 33,000 mof bentonite and cement were used for the construction from March 14 to July 5, 2012 []. Another approximately 32,500 mof bentonite, sand, and cement were also used for the construction from July 17, 2014 to April 23, 2015 []. Cement is known to include several heavy metals, and the specific gravity of cement is 3050 kg/m]. We tried to estimate the total amount of heavy metals, which had been deposited into the marine environment during the construction. Assuming that the half of total amount of material (i.e., bentonite, sand and cement) used for the construction was cement, then the total weight of cement used for construction to cover the radionuclide-contaminated bottom sediments of the FNPP port was approximately 100,000 metric tons. This cement included various amounts of chromium (Cr), copper (Cu), zinc (Zn) and lead (Pb) and other heavy metals, which were intentionally deposited into the marine environment adjacent to FNPP; consequently, the coastal waters of Fukushima Prefecture, especially close to FNPP, might have been contaminated (Table). Thus, these heavy metals, as well as turbidity or suspended material from the construction activity, may have had ecological effects (i.e., adverse effects on larval, juvenile, and adult invertebrates in the intertidal zone) that should be elucidated in the near future. Similar impacts on the number of species and population densities by the restoration work of coastal structures were also likely at Tomioka Fishing Port (Tomioka) (since 2015), Urajiri (since 2013) and Ishinomaki (since 2012) []. The lack of observed egg capsules spawned by rock shells at Urajiri in 2015 (Table) might also have been associated with the restoration of coastal structures.

There have been many scientific researches and review papers on the toxicities of heavy metals to marine and estuarine organisms [49, 50, 51, 52, 53, 54]. Although it is known that the toxicities of heavy metals to marine and estuarine organisms can vary according to their chemical speciation (e.g., free ions are more toxic than other chemical species) and pH [50, 53, 54], the embryos and larvae of marine or estuarine invertebrates are generally sensitive to heavy metals: for example, LC 50 values (the concentrations at which 50% of the tested organisms die) for the embryos of American oyster (Crassostrea virginica) were 5.6 ppb (µg/L), 5.8 ppb (µg/L), and 103 ppb (µg/L) for mercury (Hg), silver (Ag) and Cu, respectively. LC 50 values for nickel (Ni), Pb and cadmium (Cd) were 1.18 ppm (mg/L), 2.45 ppm (mg/L), and 3.80 ppm (mg/L), respectively [52]. Moreover, in the presence of multiple metals, the toxicities are reported to be additive; the presence of multiple metals may be more realistic than the presence of single metals in marine/estuarine environments [54].

The effect of turbidity and suspended material in aquatic environments has been well studied (e.g., Stern and Stickle [55]). Most studies on adult estuarine and marine bivalves (clams, oysters, and mussels) have indicated that the mortality rate among populations adjacent to dredging and disposal areas is low, except for individuals directly buried by the disposal operation; however, the percentage occurrence of normally developing eggs and larvae may decrease as the concentration of suspended solids increases in the range of concentrations normally resulting from dredging and disposal [55]. Thus, further experimental studies are necessary to elucidate the effect of construction to cover the radionuclide-contaminated bottom sediments of the FNPP port, as well as other restoration work of coastal structures.

The exposure situation for intertidal organisms in Fukushima Prefecture could be complex with many aspects, including various potential direct impacts such as physical harm from the tsunami, and toxicity from chemicals and radionuclides in the massive release immediately after the accident, potentially leading to acute effects [34]. Thereafter, there could have been continued releases to the sea of radionuclides and other harmful substances (i.e., heavy metals and turbidity or suspended material from the construction to cover the radionuclide-contaminated bottom sediments of the FNPP port); these releases could have had ecological effects, for example, on interspecific relationships (such as predator-prey relationships or competition for prey organisms and habitat). The effect could also have involved intraspecific relationships (competition for prey organisms, habitat and mating partners) [56, 57, 58].

At much higher dose rates, possibly immediately after the accident, differences among taxa in sensitivity to radiation could create competitive advantages for resistant organisms within a taxon and between populations of interacting taxa [23, 59]. Thus, in addition to differences in the direct radiosensitivity of individual organisms, life history traits including responses to changes in resources and generation times affect the consequences of radiation exposure. Exposures to radiation may have been high during or shortly after the accident [56]. In addition, note that particularly at lower doses, ecological factors and variability can be more important than direct radiation effects; therefore, a different conceptual methodology may be necessary to assess ecosystem-level effects, possibly including site-specific assessment of the effect of potential disturbances on ecosystems [56].

Further studies are needed to clarify the main causal factors for declining intertidal biota and subsequent slow recovery near FNPP, possibly through determining the acute, subacute and chronic toxicity of various radionuclides, chemicals and other factors in laboratory experiments. Continued field observations of spatiotemporal changes in the populations of sessile organisms near FNPP, including rock shell populations, are also necessary to evaluate their recovery in the future; these studies should consider the characteristics of habitats that may influence the distribution of sessile organisms. The focus should be on increasing population densities and reproductive success in terms of active behaviors such as mating and egg-laying and the subsequent successful recruitment of larvae and juveniles. Both field and laboratory studies will also be necessary to observe and evaluate possible multigenerational effects such as changes in reproductive success resulting from exposure to low-dose radiation and other environmental stressors.