This study evaluated the abihty ot Zostera marma L (eelgrass) to balance the daily photosynthetic deflc~t by mobilization of carbon reserves stored In below-ground tlssues durlng a period of extreme w n t e r llght lln~itation A quantitative understand~ng of the mobllizat~on process and its hmltatlons is essential to the development of robust models predict~ng mnimuni llght levels r e q u~r e d to nialntaln healthy seagrass populatlons Plants were grown m runnlng seawater tanks under 2 llght reglrnes One treatment was provlded wlth 2 h irradiance-saturated photosynthes~s (H,,,) to produce severe llght limitation, whlle control plants were grown under 7 h H,,, simulating the typical w~n t e rtune condition In Monterey Bay, California, USA Although plants maintained under 2 h H,,, were more severely carbon h m t e d than plants grown under 3 h H,,, whole-plant carbon balance calculated from metabohc needs and growth rates was negative for both Q,, treatments The eelgrass studied here responded to negative carbon balances by suppressing the product~on of new roots, depleting sucrose reserves and effecting a gradual decrease In growth rate a n d a n Increase In the activlty of sucrose synthase (SS, E C 2 4 1 13) in s~n k tlssues in the termnal stages of carbon stress The 3 h H,,, plants survlved the 45 d course of the expenment while the plants grown under 2 h H,,, died within 30 d even though one-thlrd of the11 carbon reserves remalned lmmobdized In the rhizome Thus extreme l~g h t llmitatlon can prevent full mob~llzation of carbon reserves stored in below-ground tissues probably through the effects of anoxla on translocation Metabolic rates, particularly photosynthesls and resplration of the shoot, were unaffected by prolonged carbon hrmtation in both treatments The patterns observed here can provide useful indices for assess~ng the state and fate of seagrass ecosystems in advance of catastrophic declines KEY WORDS: Seagrass Carbon balance . Resource a l l~c d l i~~n .Photosynthesis . Light