Giant tsunamis can disturb marine sediments, leading to muddy water in nearshore areas. Marine sediments can be resuspended and transported by tsunamis, as well as by tidal, wind-forced, and density currents in coastal oceans. Marine sediment in coastal oceans around urban areas contains heavy metals and cysts of harmful algae. The resuspension of marine sediments can induce multiple forms of marine pollution, including harmful red tides and heavy metal contamination. This study evaluated heavy metal pollution caused by transport of resuspended sediment with adsorbed zinc in a pilot sea, Osaka Bay, based on a tsunamigenic earthquake scenario along the Nankai Trough, which is predicted to occur within 30 years with approximately 70% probability. Tsunami and three-dimensional ocean simulations were conducted to calculate sediment transport using a particle-tracking simulation based on tsunami-induced sediment resuspension. The simulation results suggest that particles would upwell vertically from the seabed in sediment resuspension areas that would form locally in nearshore regions, and then be transported southward offshore by estuarine circulation. Sediment transported by tidal and wind currents would gradually migrate to the southern offshore region. The remaining sediment carrying zinc in the bay would decrease to less than half of its initial volume over the 20 days following resuspension.

In this study, we investigated seasonal changes in chlorophyll a (Chl.a), microcystin, and the 16S rRNA gene-based cyanobacterial community structures using 16S rRNA gene amplicon analysis of the surface water of Lake Hachiro, Akita Prefecture, Japan. Moreover, we examined the growth characteristics of four Microcystis spp. strains (N1, N6, N7, and 0824) isolated from Lake Hachiro. In 2014, Chl.a and total microcystin concentrations (173.4 and 1.7 µg/L, respectively) were the highest in October, when the water temperature was less than 20°C. Although algal blooms were observed for a longer term in 2015, the concentrations of Chl.a and total microcystin also increased transiently at the end of September. The cyanobacterial compositions of algal blooms included the genera Aphanizomenon, Anabaena, Prochlorothrix, and Microcystis in 2014 and Aphanizomenon, Anabaena, and Microcystis in 2015. In October of both years, algal blooms were dominated by Microcystis spp., suggesting the presence of major producers of microcystins during this term. In laboratory cultures, Microcystis strain 0824 showed a high specific growth rate at 15°C (0.17 1/d). Low temperature -adapted Microcystis spp., such as strain 0824, may be responsible for the occurrence of harmful algal blooms during the low water temperature season in Lake Hachiro.