Seasonal acclimatization of eelgrass Zostera marina growth to light

Eelgrass Zostera marina L, was collected in March (7 "C), August (21 'C) and October (15 "C) in a Danish embayment.Growth and acclimat~zation of the plants were measured in the laboratory at ambient temperature and different photon flux density.W e ~g h t loss in the dark increased with temperature whereas the slope of growth versus low light intensities was highest at 15 "C.The light compensation point for zero growth was, therefore, lower (18.5 pm01 m-2 S-') for October plants than for March (28.3 pm01 m-' S-') and August (47.3pm01 m-' S -' ) plants.Biomass allocation from rhizomes to leaves increased with reduced light availability and rhizomes r e q u ~r e d more light than leaves to maintain the biomass.Leaf weight normalized to area declined at low l ~g h t , which together with allocation from rhizomes caused sustained leaf elongation for several weeks, despite severe shading and loss of plant weight.Hence, common determinations of leaf turnover of seagrasses by tagging techniques will always remain positive and cannot be applied to determine the growth energetics at low light under non-steady-state conditions.The experimental light demands for zero growth for March and October plants were equivalent to 11 % of in situ surface irradiance, which is close to estimated light levels at eelgrass depth limits.Eelgrass biomass, however, is expected to increase at the depth limit during summer and to b e expended for survival at low light during winter.