To assess the effects of shading by epiphytes on the carbon balance of the seagrass Posidonia oceanica, we examined the patterns of within-shoot epiphyte abundance on leaves, and their variability with season and depth. Epiphyte biomass was found to depend on leaf age (larger epiphyte load on older tissues), leaf side (more epiphytes on the inner face than on the outer) and depth (more epiphytes in deep meadows). Depth differences were maximum in spring and disappeared in late summer. Percent light absorbed (absorptance) by epiphytes was measured; light absorptance followed an exponential- saturated model with epiphyte biomass. Combining these data of percent light absorbed, within-shoot epiphyte biomass distribution and an existing carbon balance model, we conclude that reduction in carbon gains caused by epiphyte shading is relatively small, and greater in deep meadows (8.8% on average, with values up to 14.2% in May) than that in shallow water (4.7% on average with maximum values of 7% in August). This is mainly due to the accumulation of epiphytes on old tissues, which contribute only marginally to the carbon gain of the plant. Using the same procedure, we modelled the effects of a doubling in epiphyte biomass, conserving the observed within-shoot distribution. The result was a very small additional carbon loss. However, using the same biomass but modifying the distribution (shifting the same abundance towards younger leaf age classes), the reduction in carbon gains was dramatic, particularly in deep meadows (between 21% and 41%, depending on the shift used). Therefore, it is the epiphyte growth timing rate rather than the final biomass reached which seems to be a key control for Posidonia oceanica survival, especially near the deep limit of its distribution.Financial support was provided by a grant from CICYT (REN2002-04020).Peer reviewed