A knowledge gap exists regarding the ecological effects of antibiotics on bacterial communities in the sea surface microlayer of coastal waters. The effects of varying concentrations of ciprofloxacin on bacterial abundance in the sea surface microlayer and the corresponding underlying water were assessed using flow cytometry. The samples were amended with different concentrations of ciprofloxacin (0, 10, 50, and 100 ng mL⁻¹) and then incubated at 4 °C in the dark. Aliquots of 1.5 mL were taken at various time points and the cell numbers were subsequently measured using an Accuri C6-1 Flow Cytometer (BD Bioscience, USA).