A comparison between the FlowCam 8100, microscopy, and sandwich hybridization assay for quantifying abundances of the saxitoxin-producing dinoflagellate, Alexandrium catenella

Light microscopy, FlowCam, and sandwich hybridization assay (SHA) are three approaches that facilitate the monitoring of harmful algal bloom (HAB) forming phytoplankton. Yet, cross-comparisons among these tech-niques have not been conducted. This study addressed that gap using the saxitoxin-producing 'red tide' dino-flagellate Alexandrium catenella, a species responsible for blooms and paralytic shellfish poisoning worldwide. To achieve this goal, the dynamic ranges of each technique were compared using A. catenella cultures spanning low (pre-bloom), moderate (bloom), and high (dense bloom) levels. To assess field detection, water samples con-taining very low (<3 cells mL-1) A. catenella levels were collected from Long Island Sound, USA (Jun-Aug 2021) and evaluated using each method. Field samples were also spiked with A. catenella to high (160 cells mL-1) or low (40 cells mL-1) concentrations. In general, microscopy, FlowCam, and SHA returned comparable A. catenella cell concentrations for all tests. Mean cell concentrations from laboratory intercalibration experiments were not significantly different for any method or concentration (ANOVA, p > 0.05). However, relative to microscopy at times SHA produced non-detect signals <2 cells mL-1 in field samples and the FlowCam slightly underestimated cell concentrations when A. catenella abundances were high in laboratory and field samples. Mean cell con-centrations of spike experiments were not significantly different for any test date, sampling location, or method, despite variability among methods within the high concentration treatment (ANOVA, p > 0.05 for all treat-ments). Findings are relevant to HAB researchers, managers, and public health officials because they help reconcile disparate cell abundance datasets that inform numerical models and enhance HAB monitoring and prediction. Results are also likely broadly applicable to several HAB species.

Automated summary

Light microscopy, FlowCam, and sandwich hybridization assay (SHA) were compared for monitoring harmful algal bloom (HAB) forming phytoplankton. The results showed that these techniques returned comparable cell concentrations, but SHA produced non-detect signals <2 cells mL-1 in field samples and the FlowCam slightly underestimated cell concentrations when abundances were high. The findings are important for HAB researchers, managers, and public health officials in reconciling cell abundance datasets and enhancing HAB monitoring and prediction.