We investigated temperature effects on eelgrass Zostera marina L. growing close to its southern distribution limit along the eastern coast of North America in Virginia, USA. We combined growth and survival experiments with microelectrode measurements of internal meristematic oxygen and analyses of metabolic compounds. Eelgrass shoots were grown at 3 different temperatures (22, 26 and 30°C) and field equivalent light levels (23% of sea surface insolation) for 28 d while water column oxygen concentration was kept at air saturation. Meristematic oxygen concentrations did not vary significantly with temperature, and meristems maintained a relatively high oxygen concentration (average: 38% air saturation) during dark hours. Despite high meristematic oxygen concentrations, shoots growing at 30°C exhibited increased mortality, reduced growth and reduced leaf production compared to shoots growing at 22 and 26°C. The leaf metabolome was significantly altered at 30°C, indicating an increase of reactive oxygen species. In addition, total nitrogen and metabolites related to the nitrogen cycle (amino acids, urea and γ-aminobutyric acid [GABA]) were low in the heat-stressed shoots, whereas soluble sugars increased. In conclusion, high temperature (30°C) has strong negative effects on eelgrass in the lower Chesapeake Bay, affecting growth, tissue integrity, nitrogen metabolism and protein/enzyme synthesis. Future global warming may likely deteriorate populations of eelgrass at its present southern distribution limit.