LONG-TERM ECOLOGICAL FUNCTIONALITY OF POROUS ARTIFICIAL REEF STRUCTURES FOR MACROALGAL BED RESTORATION IN OLIGOTROPHIC COASTAL WATERS

Abstract

Seaweed bed ecosystems in semi-enclosed coastal seas such as the Seto Inland Sea have experienced significant degradation due to land reclamation, oligotrophication, and rising sea temperatures. To address these challenges, we developed and field-tested porous artificial reef structures (Marine Mash) designed to promote macroalgal colonization and associated ecosystem functions under nutrient-limited conditions. Long-term monitoring (>13 years) at two coastal sites evaluated three ecological functions: prey organism biomass on porous substrates, macroalgal growth, and epiphytic community development. Porous substrates supported significantly higher biomass of preferred prey organisms compared with stone, concrete, and steel controls (F=161, p<0.01), while the Marine Mash structure promoted greater macroalgal biomass than stone reefs (t (12) =12.475, p=3.16×10⁻⁸) and enhanced epiphytic organism biomass on algal surfaces (t (9) =9.3, p=6.51×10⁻⁶). These results suggest that the integrated structural design—combining hydrodynamic sediment control via the sloped roof geometry, microhabitat complexity within the graded porous substrate (mean particle diameter 5 mm, void ratio 25%), and strategic deployment on sandy mud substrate to minimize sea urchin grazing—may foster multiple trophic functions simultaneously, offering a potentially scalable Nature-based Solution for oligotrophic coastal restoration.