Seagrass ecosystems are undergoing rapid degradation around the world. In the Banc d’Arguin, off the coast of Mauritania, seagrass beds experience episodic die-off events related to sulfide poisoning and desiccation stress due to dust deposition from Saharan storms combined with heat waves. These external stressors on seagrass may be impacted by two more biotic interactions, which could either oxygenate the sediment or, conversely, cause further sedimentation and suffocation. In a series of manipulative field experiments, we investigated the effects of both (1) the probing by foraging shorebirds and (2) the presence of the voluminous African bloody cockles (Senilia senilis) on intertidal seagrasses in this world heritage protected site. We found that excluding probing shorebirds for three months negatively impacted redox conditions in the sediment and resulted in a significant decline in seagrass cover and biomass at the intermediate tidal elevations. With increasing densities of Senilia, seagrass cover also significantly decreased, suggesting that these huge bivalves and seagrass compete for space. However, the mechanism of this interaction was not clear, as Senilia density did not correlate with a significant change in sediment accretion, redox potential, or total benthic biomass. In Senilia removal treatments, the lucinid bivalve Loripes orbiculatus, the most common benthic species in the system and a facilitator of seagrass, showed an increase, although the increase was not significant at the 5% confidence level. Together our results demonstrate the important roles played by both probing shorebirds and a dominant bivalve on the oxygenation of the intertidal sediments and thus the growth of seagrass beds.