Although seagrass detritus is exposed to high solar rabation in its natural environment, the impact of solar radiation has not yet been taken into account in degradation of particulate detritus We exposed sterilised detrital leaves of eelgrass Zostera marina L. to solar radiation. Under solar radiation the chlorophyll in leaves bleached in < 5 d and other absorbing compounds within 1 mo. During a 1 mo period the organic matter content of leaves (32 g m-2 of leaves) decreased, mainly via leaching, 3 to 9%) In darkness and 23 to 36';' under solar radiation. First order kinetlcs described the loss of organic matter as a function of cumulative global radiation well. The decay coefficient was 0.00038 MJ-' ,m2 Solar radiation-induced changes in the structure of leaves increased the bioavailability of detritus to bacteria 2to 3-fold. Our results show that solar radiation is one of the major factors in the decomposition of seagrasses and has to be taken into account in degradat~on models of particulate detritus.