Field survey of rocky shore ecosystem in Onagawa Bay

Representative Organization: Tohoku University

2013/07/01 - 2013/12/31
July, September, and November, in 2013

Onagawa Bay

Field survey

We obtained data or samples of seaweeds, invertebrate animals, and environments in 7 sites in Onagawa Bay. At each site, survey was conducted at 1 m-intervals from 0 m water depth to maximum of 10 m water depth, or was conducted along the transect line. Survey interval depends on survey item: for most items surveys were conducted only at July, but some at July, September, and November.

0028_R01_map01.pdf, 0028_R01_map02.pdf

In almost all study sites, a population of sea urchin Mesocentrotus nudus was dominated by the 2011 year-class, which was born after half a year of tsunami. At a certain site, this year-class was remarkably abundant, with maximum density of 40 individuals m-2 and average density of 14 individuals m-2. In this site, coverage of a brown algae Sargassum yezoense declined with increasing sea urchin density. Probably feeding of those sea urchins is causing fall of seaweed bed. On the other hand, it is showed that growth of 1-3 year-old S. nudus is faster after tsunami, when compared with that of before tsunami. This possibly indicates changes in environment after tsunami made S. nudus growth facilitate. Densities of hernivorous Tegulidae gastropods were less than 1 individual m-2 in average and 5-13 individuals m-2 in maximum. Their density is not high relative to other regions in Pacific coasts of Tohoku. Epifaunal communities on Sargassum horneri showed low value in the diversity index. Possibly this may habe been caused by the tsunami, and wheather it will increase or not will ber monitored next year, too. Thickness of sediment tend to slightly decrease at some water depth zone at two sheltered sites.

FY2013 Field survey of rocky shore ecosystem in Onagawa Bay

CTD Onagawa (St A-G) 1307