Abstract
ABSTRACTTsunamis are huge disasters that can significantly damage benthic organisms and the sea-bottom environment in coastal areas. It is of great ecological importance to understand how benthic ecosystems respond to such destructive forces and how individual species are affected. Investigating the effect of such disasters on animals that are seldom caught alive is particularly difficult. Bivalve mollusks are especially suitable for investigating how a tsunami affects coastal benthic species because they preserve an environmental record in their shells that can be extended back in time by crossdating the records of multiple individuals. Here we studied dead shells of Mercenaria stimpsoni, a long-lived clam, and precisely determined the time of death by using nuclear bomb–induced radiocarbon (bomb-14C) and by counting annual growth increments. First, a quasi-continuous, regional bomb-14C record was created by analyzing the shells of 6 live-caught M. stimpsoni individuals. Then 27 dead shells collected from the seafloor of Funakoshi Bay were 14C-dated and analyzed. The results showed that the huge tsunami that struck northeastern Japan on 11 March 2011 caused mass mortality of this bivalve in Funakoshi Bay. Nine of the 27 clams died during the March 2011 tsunami, probably by starvation after burial by tsunami deposits or exposure above the seafloor as a result of sediment liquefaction during the earthquake. The dating method used in this study can help us understand how long-lived marine organisms with low population density are affected by huge natural disasters such as a tsunami.
Highlights
Natural disasters such as tsunamis, earthquakes, typhoons, and volcanic ash falls can substantially disturb benthic ecosystems in coastal areas (Lopez et al 2008; Jaramillo et al 2012; Seike et al 2013; Harris 2014)
We found that the correlation between an average of standardized shell growth indexes (SGI) of the 6 live-caught individuals and the SGIs of 10 dead shells was statistically significant in all combinations (r = 0.57–0.92, p < 0.01; see Online Supplementary Material)
We showed that mass mortality in M. stimpsoni (9 of 20 individuals that died in the post-bomb era) was likely related to the March 2011 tsunami, but other possible reasons for this mass mortality need to be carefully addressed
Summary
Natural disasters such as tsunamis, earthquakes, typhoons, and volcanic ash falls can substantially disturb benthic ecosystems in coastal areas (Lopez et al 2008; Jaramillo et al 2012; Seike et al 2013; Harris 2014). Tsunamis can cause either rapid sedimentation, burying benthic organisms deep under the seafloor, or rapid erosion, washing them out of the upper layer of substrate and exposing them to predation (Kranz 1974; Bromley 1996; Lopez et al 2008). In Funakoshi Bay, Iwate prefecture, the tsunami run-up height was 29.4 m (Ishimura and Yamada 2019), and largescale seafloor erosion and sedimentation occurred, followed by the re-establishment of bioturbation (Seike et al 2016, 2017, 2018, 2019). Investigating the population dynamics of animals with a large population density before (and after) the tsunami is possible if the data from before the event are available, investigating those with a small population density and rare catch opportunity
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