Harmful algal blooms (HABs) are a recurring phenomenon along all continents, posing a global threat, particularly due to shellfish poisoning and public health, and their timely monitoring is vital for safeguarding the blue economy. The ability to efficiently isolate species of interest, such as harmful dinoflagellates, is a key process for HABs monitoring which is particularly complex because of the typically high diversity of marine microalgae communities. This study introduces a simple spiral microchannel device using inertial microfluidics to separate and concentrate microalgae based on size. This device effectively concentrates microalgae, achieving up to 5.8-fold fluorescence increase after 3 cycles in the spiral and isolates target species with <6 % cell loss. A continuous enrichment method demonstrated a 2.85-fold increase in fluorescence signal for Alexandrium minutum after 80 min. This method proved suitable to enhance the sensitivity of devices designed to detect harmful phytoplankton species' early blooms.