Influence of Water Temperature Anomalies on the Growth of Zostera marina Plants Held Under High and Low Irradiance Levels

Coastal water temperature anomalies are expected to intensify during the current period of global climate change. Sudden water temperature elevation (e.g., heat waves) or decrease (e.g., summer upwelling) can have a far-reaching influence on seagrasses in temperate regions. We examined the influence of anomalously elevated and decreased water temperatures on the growth of Zostera marina plants held under two different levels of irradiance. Core samples of wild Z. marina shoots were cultured under either suddenly elevated (from the in situ level of 20 to 27 degrees C during the fall experiment) or decreased (from the in situ level of 27 to 20 degrees C during the summer experiment) temperature. We included light as an experimental factor because the light requirements of seagrasses increase when plants are under thermal stress. The growth of Z. marina shoots under anomalously elevated water temperature conditions, which simulated marine heat waves, was significantly declined, whereas the plant growth under suddenly decreased water temperature conditions, which simulated summer upwelling events, was maintained or slightly increased. In the fall 2014 experimental period, Z. marina shoots under the suddenly elevated water temperature condition (27 degrees C) showed significant decreases in shoot density, biomass, and leaf productivity by ca. 40-80%, whereas plants held under the ambient water temperature condition (20 degrees C) maintained or slightly increased their growths. In the summer 2015 experimental period, Z. marina under the sudden temperature reduction conditions (20 degrees C) maintained relatively high shoot density and leaf productivity, whereas plants under the in situ high water temperature condition (27 degrees C) showed significantly reduced growths. The growth of Z. marina shoots, which were cultured in the optimal temperature condition (20 degrees C), was enhanced by increasing underwater irradiance. However, the adverse effects of high water temperature on Z. marina growth could not be reversed by increasing irradiance levels at the end of the experiments. According to the results of this study, the intensified sea surface water temperature anomalies induced by global climate change will alter the growth and distribution of temperate seagrasses and, consequently, the structure and function of the coastal seagrass ecosystems in the temperate region.