Interannual variation in freshwater and marine phycotoxins quantified from oysters cultured across diverse coastal sites

The expanding footprint of global aquaculture has pushed production systems into regions with unknown or limited baseline information about associated hazards and risks. Oyster ( Crassostrea virginica ) aquaculture, for example, has expanded throughout euhaline coastal sounds across the southeastern United States yet there exists limited information on phycotoxin levels in oysters cultured in these environments. Here we investigated domoic acid and microcystin accumulation in cultured oysters at three coastal sites in North Carolina, with all sites varying in surrounding land use. Oyster deployments throughout 2021 and 2023 were complemented by harmful algal bloom monitoring methods including the use of solid-phase adsorption toxin tracking devices and microscopy analyses to determine microalgal cell concentrations from water samples. Domoic acid was detected in 20% and 38%, and microcystin in 48% and 11%, of the tested oysters during 2021 and 2023 ( N = 178 total oysters tested), with both toxins co-occurring in 7% of the animals. While domoic acid concentrations remained far below seafood safety action levels, maximum microcystin levels at some sites approached tolerable daily intake guidelines in 2021 but were nearly absent across all sites in 2023. This latter observation implicated environmental drivers of cyanotoxin production, transport, and accumulation in oysters operating at regional scales greater than individual oyster farms. Fully understanding the environmental drivers of phycotoxin production within a landscape context, including how and when these toxins accumulate in shellfish, therefore represents a critical research gap. Given the regional synchrony in phycotoxin concentrations observed in oysters, there holds considerable promise for developing adaptive sanitation and control programs in response to conditions when perceived toxin risks are high. • Field deployment to characterize marine and freshwater phycotoxin levels associated with diverse coastal oyster aquaculture sites in North Carolina. • Domoic acid was detected in 20% and 38%, and microcystin in 48% and 11%, of the tested oysters during 2021 and 2023 ( N = 178 total oysters tested), with both toxins co-occurring in 7% of the animals. • Domoic acid concentrations remained far below seafood safety action levels, maximum microcystin levels at some sites approached tolerable daily intake guidelines in 2021 but were nearly absent across all sites in 2023. • Fully understanding the environmental drivers of phycotoxin production within a landscape context, including how and when these toxins accumulate in shellfish, therefore represents a critical research gap.