Abstract
We observed the seasonal changes in eelgrass (Zostera marina L.) beds formed in the natural state off Miya, in Mikawa bay, Aichi Prefecture, Japan from 2009 to 2011. And we calculated Shields numbers on the sea floor from the sediment composition and wave heights and underwater light quantum density at the position of the sea floor and 0.5 m above the same point. This eelgrass bed had been maintained by nutrition shoots from winter 2009 but disappeared completely in winter 2010; it then reappeared from seedlings in summer 2011. The maximum Shields number observed was 0.23, in summer 2010—this was more than the limit at which eelgrass beds form, namely value. The cause of the high value was found to be the large waves. The light quantum density at noon was below the saturation value for eelgrass, namely 300 μmol m-2s-1, on 50% or less of the observation days. The cause of these low values was found to be water turbidity. These results suggest that any eelgrass-bed construction engineering to be performed in this area will need to be able to stabilize the bottom sediments and increase the amount of light reaching the bottom layers.
