Effects of hull cleaning wastewater on coastal plankton community: a mesocosm experiment

In-water hull cleaning, widely used to manage biofouling on ship hulls, releases wastewater (HCW) that contains high levels of heavy metals and suspended solids. These contaminants can directly alter water chemistry and disrupt planktonic community dynamics, yet their ecological effects in coastal environments remain poorly understood. In this study, a mesocosm experiment was conducted to evaluate the effects of increasing HCW concentrations (1%, 5%, and 10%) on coastal marine planktonic ecosystems, including phytoplankton, zooplankton, and periphyton. Exposure to ≥ 5% HCW resulted in sharp declines in phytoplankton abundance (e.g., from 144 × 10⁴ cells L⁻¹ to 2 × 10⁴ cells L⁻¹), coinciding with elevated dissolved copper (Cu) and zinc (Zn) levels, while zooplankton abundance declined rapidly even at 1% exposure. In contrast, periphytic diatoms (e.g., Cylindrotheca closterium, Navicula spp.) exhibited relative resilience, with abundances increasing up to 101 × 10³ cells cm⁻² during the later stages, particularly under nutrient-enriched conditions. Community composition analyses revealed a shift toward metal-tolerant, attached microalgal taxa at higher HCW concentrations. Trait-based functional diversity analyses indicated reduced trait dispersion and redundancy under high HCW exposure, while ordination analysis indicated that changes in trait composition were linked to environmental gradients. These findings demonstrate that HCW acts as a complex ecological stressor that alters plankton community structure and reduces functional capacity, and emphasize the importance of integrating ecological response indicators into future management strategies for robotic hull cleaning discharges.