Bottom-up effects of variable winter weather conditions on phytoplankton dynamics in an enclosed bay: implications for ecological responses to climate change

To investigate phytoplankton dynamics in response to variable winter weather conditions along the Sea of Japan, we conducted daily sampling in enclosed Maizuru Bay in five winters and found distinctive differences between 2016/2017 and 2018/2019. A clear diatom bloom occurred through the water column in moderately cold/snowy 2016/2017, whereas in exceptionally warm/snow-free 2018/2019 a small peak of dinoflagellates occurred only in the subsurface. Nutrient concentrations changed drastically depending on freshwater discharge and diatom growth in 2016/2017, showing a contrast with constant nutrient concentrations in 2018/2019. Copepod densities were similar to 10 times higher in 2016/2017 than in 2018/2019, indicating bottom-up effects in 2016/2017. As the absence of a diatom bloom from 2018/2019 can be attributed neither to nutrient limitation nor to grazing pressure, we hypothesize that wind mixing is a key factor triggering a diatom bloom. Within the five winters, clear diatom blooms were found only after strong winds and heavy precipitation including snowfall. Wind mixing would possibly promote the germination/rejuvenation of viable resting stage cells as inocula for a diatom bloom, while the growth and maintenance are supported by nutrient supply through freshwater discharge. In Maizuru Bay, winter production is likely activated by harsh weather conditions characteristic of this region.