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Spawning synchrony represents a common reproductive strategy in sessile marine organisms and for broadcast spawning corals, buoyancy of egg-sperm bundles is critical to maximise fertilisation at the ocean surface. Here we demonstrate a novel threat to coral reproduction whereby buoyant egg-sperm bundles intercept and are “ballasted” by sediment grains on their journey to the ocean surface, preventing them from reaching the ocean surface and greatly reducing egg-sperm encounter rates. Empirical observations of this mechanism are successfully captured by a mathematical model that predicts the reduction in ascent probability and egg-sperm encounters as a function of sediment load. When applied to 15 m deep reefs, the model predicts that 10% and 50% reductions in egg-sperm encounters occur at 35 mg L⁻¹ and 87 mg L⁻¹ suspended sediment concentrations, respectively and for a 5 m deep …