Combined impact of water column oxygen and temperature on internal oxygen status and growth of Zostera marina seedlings and adult shoots

Eelgrass (Zostera marina L.) occasionally experiences severe die-offs during warm summer periods with variable water column oxygen partial pressures (pO2). Eelgrass is known to be very intolerant to tissue anoxia with reduced growth and increasing mortality after ≤ 12 h anoxia in the dark at temperatures of ≥ 25 °C. In the present study we experimentally examine the impact of combined water column oxygen and temperature on oxygen dynamics in leaf meristems of seedlings and adult shoots to better understand how stressful environmental conditions affect eelgrass oxygen dynamics and subsequent growth and mortality. There was a strong interaction between water column oxygen and temperature on meristem pO2 implying that eelgrass is rather resistant to unfavorable oxygen conditions in winter but becomes increasingly vulnerable in summer, especially at high temperatures. At 25 °C meristems became anoxic at a water column pO2 of 12 kPa corresponding to 60% of air saturation, and leaf growth was negatively affected after 24 h exposure to water column pO2 below 7 kPa. Temperature had in itself a substantial impact on meristem pO2 both in light and in the dark as a result of increasing tissue respiration. Seedlings were less prone to meristem anoxia than adult shoots suggesting that seedlings would better tolerate unfavorable combinations of water column oxygen conditions and temperature. The results suggest that eelgrass die-offs will occur more frequently in eutrophied coastal areas with the more frequent periods of unusually high temperatures expected in the future as a result of global warming.

► Meristematic anoxia has negative impacts on eelgrass growth and survival.
► Temperature affects internal oxygen status of eelgrass in both light and darkness.
► Eelgrass meristems become anoxic at water column hypoxia and high temperature.
► Seedlings seem less prone to anoxia than adult shoots.
► Global warming will make eelgrass more sensitive to eutrophication.