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Forested headwater streams disproportionately rely on inputs of organic matter to fuel their food webs, and characterizing the breakdown of organic matter offers insights into ecosystem function. Organic matter breakdown rates can be influenced by the availability of limiting nutrients, and describing patterns of breakdown rates across nutrient gradients is increasingly relevant as inland waters undergo eutrophication. Here, we determined the breakdown rates of coarse woody debris (kwood) across 5 streams located at La Selva Biological Station, Costa Rica, that receive a gradient of interbasin modified groundwater inputs, creating a gradient in P concentration (6–134 lg/L soluble reactive P [SRP]). The fastest breakdown rate (kwood 50.77/y) occurred in the stream with the highest SRP, and kwood was positively correlated with SRP across the 5 streams. Further, we characterized the assemblage of macroinvertebrates from wood packs. Macroinvertebrate assemblages were different between the 5 streams, with more dense and diverse assemblages in streams with higher SRP and faster breakdown rates. Our results contribute to a growing field of study on the effect of nutrients on organic matter dynamics in inland waters by characterizing the effect of SRP on breakdown rates of wood in tropical streams.