Ship biofouling is recognized as a major pathway for the introduction and spread of invasive organisms. In-water cleaning of ship hulls generates hull cleaning wastewater (HCW) containing antifouling paint residues, heavy metals, biocides, and detached biofouling organisms, which are inevitably released into surrounding marine environments. However, the ecological effects of HCW on microalgal communities remain poorly understood. To evaluate these effects, we conducted microcosm experiments by adding HCW collected from international commercial ships to natural seawater. HCW samples were obtained using two different cleaning methods (soft cleaning with a sponge and hard cleaning with a brush), and treatments consisted of 5% and 10% HCW additions to ambient seawater. Additional experiments examined the effects of HCW under different temperatures (15 and 20 °C) and nutrient conditions. Dissolved copper concentrations in the HCW treatments were approximately 5–10 times higher than those in the control. The growth of unattached microalgae in the water column was significantly suppressed following HCW addition, with chlorophyll a concentrations substantially lower in treatments than in the control (e.g., 34.1 ± 0.8 μg L⁻¹ in control vs. 12.6 ± 4.3 μg L⁻¹ in treatments), and photosynthetic efficiency (Fv/Fm) also significantly reduced (p < 0.05). In contrast, the biomass of attached microalgae (periphyton) on artificial substrates increased with HCW dose and maintained relatively high photosynthetic capacity compared to planktonic microalgae. The release of copper and other antifouling-related compounds from HCW was considered a primary factor inhibiting phytoplankton growth, while attached microalgae appeared more tolerant to these conditions. These results indicate that HCW discharge can differentially affect planktonic and attached microalgae, potentially altering community structure and reducing primary productivity in coastal ecosystems. Our findings highlight the need for improved management of HCW discharge based on scientific assessments to minimize ecological risks associated with in-water hull cleaning operations

Hull cleaning wastewater (HCW),Ship biofouling,In-water hull cleaning,periphyton