We used laboratory and field studies to determine whether forest composition influences aquatic bryophyte primary productivity, oxygen release, and carbon dioxide (CO2) uptake in forested blackwater swamps. To model total bryophyte oxygen release and CO2 uptake, we measured Porella pinnata biomass within a range of heights supporting bryophyte growth on tree trunks and knees. We used photosynthesis-irradiance (P-I) curves to estimate oxygen production and CO2 uptake across varying temperatures and light levels. These estimates informed four modeling scenarios of riverine oxygen dynamics and bryophyte-mediated CO2 flux: 1) current forest composition, with baldcypress (Taxodium distichum) knee surface area included, 2) current composition with knee surface area removed, and 3) forests composed entirely of gum/tupelo (Nyssa spp) or 4) entirely of baldcypress. At moderate discharge and temperature, aquatic bryophytes may exert strong influence on ecosystem metabolism, counteracting ~10-25 percent of the oxygen demand and CO2 released from decomposing leaf litter. Modelling results suggest that forest composition influences aquatic bryophyte primary production: buttressed trunks and abundant knees in baldcypress-dominated swamps produce greater surface area for bryophyte growth, which delays seasonal onset of hypoxia. This study demonstrates the importance of aquatic primary production in forested blackwater rivers and shows how baldcypress, a foundation tree species, can significantly modify aquatic ecosystem function.