Phytoplankton Changes in Response to Marked Water Quality Changes in Dokai Bay, Japan, from Heavy Industrial Pollution through Extreme Eutrophication to Eutrophication

Abstract

To forecast the future phytoplankton community in a water environment, it is necessary to clarify the past water quality and phytoplankton composition. Dokai Bay in northern Kyushu, Japan has undergone marked water quality improvement, from a period of industrial pollution−during which fish were unable to survive because of toxic effects from the inflow of untreated industrial wastewater−through a period of extreme eutrophication, including concentrations of ammonia harmful to phytoplankton in some instances, to the current eutrophic period. As the vegetative stages of the phytoplankton community in Dokai Bay have been studied since 1980, the phytoplankton community composition during the period of industrial pollution is unclear. We investigated the past phytoplankton community in Dokai Bay by germinating resting cells from a sediment core. The age profile of the core was determined by 210Pb-dating. The top10cm of the core（ca.1990－2015）was considered to be the disturbed layer, and the next 10cm was dated as ca.1960－90. Phytoplankton communities over the past ca.60 years were dominated by a diatom, Skeletonema spp., along with a mixture of three other diatom genera, Cyclotella, Chaetoceros and Thalassiosira, which consistently germinated in relative proportions similar to those of vegetative cells during the extremely eutrophic and eutrophic periods, excluding Thalassiosira. These results suggest that when the phytoplankton community is dominated by diatoms, such as Skeletonema, different species in the same genus or the same species with variations in gene structure can adapt to changes in water quality from industrial pollution and eutrophication.