SEASONAL ACCLIMATIZATION OF EELGRASS ZOSTERA-MARINA GROWTH TO LIGHT

Eelgrass Zostera marina L. was collected in March (7-degrees-C), Augus t (21-degrees-C) and October (15-degrees-C) in a Danish embayment. Gro wth and acclimatization of the plants were measured in the laboratory dt ambient temperature and different photon flux density. Weight loss in the dark increased with temperature whereas the slope of growth ver sus low light intensities was highest at 15-degrees-C. The light compe nsation point for zero growth was, therefore, lower (18.5 mumol m-2 s- 1) for October plants than for March (28.3 mumol m-2 S-1) and August ( 47.3 mumol m-2 s-1) plants. Biomass allocation from rhizomes to leaves increased with reduced light availability and rhizomes required more light than leaves to Maintain the biomass. Leaf weight normalized to a rea declined at low light, which together with allocation from rhizome s caused sustained leaf elongation for several weeks, despite severe s hading and loss of plant weight. Hence, common determinations of leaf turnover of seagrasses by tagging techniques will always remain positi ve and cannot be applied to determine the growth energetics at low lig ht under non-steady-state conditions. The experimental light demands f or 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 in crease at the depth limit during summer and to be expended for surviva l at low light during winter.