Abstract
Pseudo‐nitzschia H. Peragallo, a marine planktonic diatom genus containing some species capable of producing the neurotoxin domoic acid, is often documented in extremely high concentrations in the northern Gulf of Mexico in the plume of the Mississippi River, especially when river flow and nutrient inputs are high. Limited historical data suggest that Pseudo‐nitzschia abundance has increased in the northern Gulf of Mexico since the 1950s. Five sediment cores were collected and analyzed to test whether Pseudo‐nitzschia increases coincided with increasing nutrient concentrations in the Mississippi River, thereby suggesting a cause‐effect relationship. Pseudo‐nitzschia abundance increased in all five cores, correlating significantly with increasing nitrate fluxes and decreasing silicate to nitrate ratios. A diatom dissolution index, based partly on scanning electron microscopic analysis of the fine structure of Pseudo‐nitzschia and other lightly silicified diatom valves preserved in the sediment, indicates that the increase in Pseudo‐nitzschia abundance appears to reflect a response to eutrophication rather than diagenesis. This study provides evidence for a possible link between coastal eutrophication and harmful algal blooms.