Pseudo‐nitzschia is a genus of harmful algal bloom (HAB)‐forming diatoms that can produce domoic acid (DA), a compound known to cause death and disease in marine wildlife as well as amnesic shellfish poisoning in humans. Here, we show the effects of multiple cultivable seaweeds— Saccharina latissima (sugar kelp), Ulva spp., and Gracilaria spp.—on multiple toxic species of Pseudo‐nitzschia . Co‐culture growth assays of Pseudo‐nitzschia multiseries and Pseudo‐nitzschia australis together with environmentally realistic concentrations of each seaweed showed that all seaweeds except for Gracilaria caused significant reduction in Pseudo‐nitzschia cell density relative to control treatments of 13%–47% in 24‐to‐48 h and up to 74%–94% reduction at 72 h and later ( p < 0.05 for all assays). In almost all experiments, Pseudo‐nitzschia spp. in control treatments displayed exponential growth whereas populations in seaweed treatments did not. Bottle incubations of field‐collected bloom populations of Pseudo‐nitzschia spp. containing different Pseudo‐nitzschia species assemblies with aquaculture realistic concentrations of S. latissima led to a significant reduction in Pseudo‐nitzschia spp. cell density of 69%–81% by S. latissima at 2 g L −1 ( p < 0.05 for all assays). In toxin accumulation experiments, S. latissima significantly lessened ( p < 0.05) DA accumulation in blue mussels ( Mytilus edilus ) and razor clams ( Ensis leei ). Collectively, these results suggest that the integration of seaweeds (particularly S. latissima ) with shellfish aquaculture should be considered as a non‐invasive and potentially profit‐generating measure to mitigate the damage to that industry caused by the growing threat of Pseudo‐nitzschia blooms.
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