<p>Understory plants have long been considered rare in mangrove forests, leaving their ecological roles poorly understood. This limits our understanding of how below-canopy vegetation contributes to sediment carbon cycling and benthic habitat dynamics in mangrove ecosystems. We examined the widespread mangrove fern <i>Acrostichum aureum</i> in a neotropical mangrove system to test whether fern-derived organic matter influences sediment properties and benthic macrofaunal assemblages. We established a litter-manipulation experiment (fern litter exclusion, control, procedural control, and mangrove-litter plots) in Bahía Málaga, Colombia. Sediments were sampled in October 2016 and March 2017 for pH, total dissolved solids, carbon:nitrogen ratio, organic matter content, and organic carbon, while macrofauna (crabs, molluscs, and polychaetes) were sampled after five months of manipulation. Macrofaunal species composition did not differ significantly among stations or treatments, whereas multivariate sediment characteristics differed significantly among treatments. Organic matter showed a treatment-dependent temporal response, increasing in plots receiving litter inputs and remaining lower in fern-litter exclusion plots by March. In contrast, organic carbon and other sediment variables showed no significant treatment effects over the experimental period. Overall, fern-litter exclusion was associated with reduced maintenance of sediment organic matter but did not restructure macrofaunal assemblages over five months, indicating that longer timescales or broader environmental gradients may be required for detectable faunal responses to changes in understory-derived litter. Together, these findings highlight the contribution of mangrove understory vegetation to sediment organic matter dynamics and the importance of longer-term perspectives when evaluating faunal responses to understory change.</p>