By Jay T. Cullen
The purpose of this post is to report on a newly published, peer-reviewed study in the open access journal Scientific Reports that uses field observations to determine how intertidal species abundance and diversity were affected by the 2011 earthquake, tsunami and Fukushima Dai-ichi Nuclear Power Plant (FDNPP) disaster. This post is part of an ongoing series dedicated to summarizing the results of scientific studies aimed at understanding the impact of the FDNPP disaster on ecosystem and public health. Horiguchi and colleagues surveyed intertidal marine organisms and made measurements of artificial radionuclides in specimens in 2011, 2012 and 2013. They found that in 2012 the number of intertidal organisms was lower closer to the FDNPP than farther away and that the sea snail (Thais clavigera) was absent from sampling locations <30 km from the FDNPP. Because sea snails were found in other rocky habitats affected by the tsunami in 2011 the absence of these organisms in 2012 near the plant might be related to the FDNPP disaster. In 2013 both the numbers of organisms and diversity of species were found to be lower at sites within several kilometers south of the FDNPP site. While, according to the authors, there is no clear explanation for the findings at present it is clear that the intertidal biota has been impacted close to the FDNPP since the disaster. The authors conclude that:
- it is unlikely that the tsunami was solely responsible for changes in the intertidal communities given the distribution of sea snails
- other causes might include acute or sub-acute toxicities from the largest leaks from the FDNPP site in March-April 2011 containing artificial radionuclides, boric acid and hydrazine (and other chemicals)
- most significant impacts to the intertidal community occurred along the coast south and proximal to FDNPP which likely reflects predominant local water currents
The changes noted by Horiguchi and colleagues in the intertidal community contrast with the lack of significant changes in benthic organisms along the Japanese coast by Sohtome and colleagues that was summarized here.
Details of the sampling locations where Horiguchi and colleagues conducted surveys in 2011, 2012 and 2013 can be found by accessing the supplementary information tables here. A map showing the locations along with the numbers of intertidal species found and relative densities of the sea snails in 2012 are shown below:
Note the absence of the rock shell sea snail within the 20 km radius of the plant and to 30 km to the south of FDNPP. Taken together with the 2013 monitoring data and the pre-disaster data it is clear there were declines in the numbers of species and density of intertidal organisms close to the FDNPP site after the disaster.
What Role Might Have Radionuclide Contamination Played?
In 2012 the activity of the artificial radionuclides 137-Cs (137Cs half life ~30 years), 134-Cs (134Cs half life ~ 2 years) and 110-Ag (110mAg half life ~250 days) were measured in the soft tissue of some limpets and sea snails. The activity of these FDNPP associated radionuclides are reported in Bq kg-1 wet weight relative to their collection site and distance from the FDNPP in the figure below:
These results show that in 2012 organisms near and to the south of the FDNPP had the highest tissue content of radioactive contaminants as indicated by radiosilver and radiocesium activities. The potential negative impacts of ionizing radiation on these intertidal species will scale with the dose experienced by these organisms. In the first weeks following the meltdowns measured seawater concentrations of 131-Iodine (131I half life ~8 days) and 137Cs were on the order of 105 Bq L-1 while 90-Strontium (90Sr half life ~29 years) was about 10-100 times less. These seawater levels are about 10 to 100 million times higher than levels in seawater now being detected off the coast of North American from the disaster. Estimates of the dose experienced by marine organisms in 2011 range from those that could cause reproductive effects and even mortality in highly sensitive organisms to doses much less than these threshold levels. According to the authors, with available information it is not possible to unequivocally attribute changes in community structure and abundance to radionuclide contamination. For example, other harmful substances released during the accident (most notably boric acid and hydrazine used during the emergency) could have induced toxicity in the local intertidal communities.
The authors suggest the impact of ionizing radiation on community structure could be to allow competitive advantage between radio-tolerant and radio-sensitive organisms when the dose experienced in a location is high such as immediately after the disaster in 2011. When doses are lower ecological factors and variability might be more important than radiation effects. Further studies are required in order to determine the main cause(s) for declining intertidal biota near the FDNPP after the disaster. Such studies might include the determination of acute and sub-acute toxicity response to radionuclides, other contaminants and physical factors in the laboratory. Continued monitoring of the communities in time and space will be important to determine the time for full community recovery in the wake of the earthquake-tsunami-FDNPP triple disaster. The authors stress that longer term studies will be important to determine how the low levels of ionizing radiation now present in the intertidal and other environmental insults post-disaster might have multi-generational effects on these marine communities going forward.