Period and Interval2014/04/01 - 2015/03/31
July and/or December in 2014
Research Area
CategoryField survey
OverviewWe will obtain data or samples of seaweeds, invertebrate animals, and environments in 7 sites in Onagawa Bay. At each site, survey will be conducted mainly at 1 m-intervals from 0 m water depth to maximum of 10 m water depth on a transect line. Survey on sea urchins and sargassum epifauna will be conducted at all sites in July and October, respectively. In the July survey, seaweed and topshell will also be investigated at site A.
Research Point Table
| Point | | Name | St. A |
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| Remarks | Base point of transect line |
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| Name | St. B |
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| Remarks | Base point of transect line |
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| Name | St. C |
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| Remarks | Base point of transect line |
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| Name | St. D |
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| Remarks | Base point of transect line |
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| Name | St. E |
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| Remarks | Base point of transect line |
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| Name | St. F |
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| Remarks | Base point of transect line |
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| Name | St. G |
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| Remarks | Base point of transect line |
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Results and Future Plan(1) Sea urchin population dynamics and temporal change in macroalgal community structure:
A monitoring survey at site A showed that the average density of the sea urchin Mesocentrotus nudus at this site has increased after September 2012, when its 2011 class cohort had appeared, and became 17-18 ind./m2 in 2014. The density boost was significant at lower depth (0-4 m in water depth), where about 15-35 ind./m2 urchins were observed in 2014. This is almost highest density of this species compared with previous knowledge.
Moreover, we studied the temporal change in macroalgal community structure at this site. Number of taxa and diversity index H' in 2013 and 2014 were lower than those in 2012. Principal component analysis using major 20 taxa indicated that macroalgal bed existed in 2012 became barren in 2013 and 2014. A large brown algal species, Sargassum yezoense was observed at 0-8 m in water depth in September 2012, but decreased its range and was finally observed only at 0 m depth in July 2014. These results suggest that algal bed at this site has collapsed by the feeding of sea urchins recruited after the earthquake.
(2) Epifaina on Sargassum seaweed:
We clarified temporal change in epifaunal community on brown algae, Sargassum since 2012 spring, when the recruitment of epifauna on Sargassum has occurred at fist time after the earth quake, by investigating specimens sampled from Onagawa Bay at March, September, December 2012, March, July 2013, and July 2014. Density at spring recruitment was higher in second year than first year after the earthquake. A pattern of increasing diversity index in summer was observed every year. We also compared the community data of Onagawa Bay and Shizugawa Bay after the earthquake with the data from several locations in Japan before the earthquake. Post-earthquake communities tend to have higher density and lower diversity.This tendency suggests that epifaunal communities had reduced its density by the earthquake and have been in a recovery phase in which opportunistic species recruited. Our study also showed a tendency of increasing diversity along water temperature, suggesting this might be a general tendency in Sargassum epifaunal community.
