The tolerance of eelgrass (Zostera marina L.) to anoxia was assessed experimentally to evaluate the potential role of short-term anoxia on eelgrass performance. Eelgrass ramets (terminal leaf bundles with rhizomes and roots) were submerged in anoxic seawater for variable periods of time (0.5 to 48 h) at three temperatures (20, 25 and 30 °C) in darkness. Photosynthetic efficiency (Fv/Fm), leaf growth and shoot mortality were assessed as response parameters.

Photosynthetic efficiency and leaf growth of ramets exposed to anoxia were closely coupled and declined with both increasing exposure time and temperature. At 20 °C, negative effects of anoxia occurred after 12 h (Fv/Fm) and 24 h (leaf growth). Shoot mortality appeared after 24 h. The negative impacts of anoxia on photosynthesis and growth increased markedly with increasing temperature. At 30 °C, Fv/Fm declined after 1 h, leaf growth after 2 h and no plants survived 8 h of anoxia.

The results show that eelgrass is surprisingly intolerant to anoxia and support the hypotheses that anoxia may induce sudden eelgrass die-off and that high temperature strengthens the negative impact of anoxia. The study documents that anoxia in itself may have strong negative impacts on eelgrass performance within ecologically relevant time scales.