Efficacy of removing marine debris and the effects of sustained exposure on a Zostera marina meadow

Marine debris is an environmental pollutant that can lead to the degradation of seagrass meadows. Two field experiments were run to examine the impact of marine debris on the coverage and area of a Zostera marina meadow in Arikawa Bay, Nagasaki, Japan. Experiment 1, which was run from May 2021 to November 2024, demonstrates how marine debris removal affected coverage and area. Experiment 2, which was run from March 2024 to January 2025, demonstrates the effect of marine debris (i.e., fishing nets) emplacement on seagrass shoot density. Due to the nature of experiment 1, a mean normalized difference index (NDI) was used to evaluate the effects of marine debris removal (where high values indicate relatively greater positive effects). The NDI of coverage and area in the section where marine debris removal ceased was 0.189 and 0.263, respectively. Whereas the NDI of coverage and area where marine debris was continuously removed was 0.230 and 0.287, respectively. In experiment 2, shoot density fluctuated naturally with a mean rate of -0.004 ind. m^-2 d^-1 when marine debris was absent. However, prolonged exposure to debris (67 to 252 days) led to a mean rate of shoot loss that decreased from -0.045 to -0.108 ind. m^-2 d^-1. In conclusion, we demonstrate that marine debris accumulation in seagrass beds affects the quantity of seagrass and that removal of marine debris can contribute to seagrass restoration. Finally, removing the historical load of marine debris appeared to have led to a natural recovery in this ecosystem.