Ecologically engineered seawall revetments for enhancing marine biodiversity: elevation and design options

Hardened, flat concrete seawalls generally cause the degradation of coastal environment and habitat quality, resulting in low marine biodiversity. Therefore, it's timely to mitigate the adverse impacts of traditional seawalls and enhance the ecological function of coast construction. In Eastern China, we investigated the effects of tidal elevation levels and eco-engineered seawall revetments, i.e., surface-treated (surface-roughened, hole-punched, and shell-embedded), on the richness and biodiversity of marine organisms. During the observation period, Mollusca, Arthropoda, and algae were the dominant species attaching on the experimental seawall blocks. In total, 8, 18, and 10 species of marine organisms were found colonizing the blocks at high, mid, and low tidal levels, respectively, exhibiting a convex binomial trend. Due to high evaporation rate of seawater remaining on the blocks at high-tide level, the lowest species number, organism abundance, and biodiversity were observed. The taxon richness and biodiversity indices were the highest at mid-tide level, whereas their values decreased at low-tide level because of intensive flooding and sediment deposition. Furthermore, the surface-treated blocks enhanced species numbers, organism abundance, and biodiversity indices of attached marine organisms relative to the untreated flat blocks. The hole-punched and shell-embedded blocks supported the greatest values of taxon richness of attaching organisms, while the effect of simple roughening treatments was relatively weak. At low tidal elevation, the biodiversity indices on the shell-embedded blocks were higher than those on the hole-punched blocks. This in situ experiment revealed that tidal elevation, surface habitat heterogeneity and water-holding ability are the crucial factors controlling marine biodiversity. As oyster shells are a safe and plentiful bioresource in China, we recommend using them for constructing nature-based seawall revetments. The results obtained in this study provide experimental evidence for optimizing the ecological functions of seawalls to ensure the synergistic benefits of coastal protection and biodiversity maintenance.