Improvement and application of qPCR assay revealed new insight on early warning of Phaeocystis globosa bloom

•DNA barcode design by screening SNP is essential for accurate quantitation.•High-quality DNA extraction prevents qPCR inhibition.•Spiking H. pluvialis genomic DNA is suitable to quantify DNA recovery efficiency.•Plunge in solitary-cells abundance before bloom conflicts with lab-based findings.•Blooms can be forecasted earlier from the perspective of early-stage solitary cells.