Differentiation of pseudo-nitzschia species (Baccillariophyceae) in seawater samples from the Salish Sea using the compound microscope

Taxonomic analysis using the compound microscope remains an efficient and commonly used method of monitoring phytoplankton in seawater samples. Certain diatoms in the Pseudo-nitzschia genus produce the toxin domoic acid, and thereby cause amnesic shellfish poisoning or domoic acid poisoning. Because differentiation between Pseudo-nitzschia species using the compound microscope is challenging, often routine sample analysis in the past has only noted these algae to genus level. This makes it difficult to assess if there is a range shift in the more toxic species, such as Pseudo-nitzschia australis, that may be linked to alterations in environmental conditions in the Salish Sea due to climate change. We present here a practical method for identifying P. australis in the light microscope, and suggest groupings that can be commonly used to monitor other local Pseudo-nitzschia species