Turning riprap into reefs: Integrating oyster shells into shoreline armouring

Boulder seawalls constructed with granite riprap for shoreline armouring lack habitat complexity, leading to lower marine biodiversity than natural rocky shores. Baskets of live oysters and cured oyster shells, and strings of cured shells laid on concrete blocks were installed on ripraps in Hong Kong, China with an aim to enhance biodiversity and ecosystem functioning towards that of a natural rocky shore. Inhabiting taxa were monitored for at least 18 months and biofiltration capacity of the emerging community was determined ex-situ. Despite high mortality of the live oysters, the baskets and shell reefs developed consistently greater biodiversity than control riprap, culminating in a mean 3.8 (±0.28, 95 % C.I.) times higher across sites. The baskets and shell reefs harboured suspension feeders, herbivores and carnivores generally absent from control riprap, demonstrating the potential for enhancing ecosystem functioning. Overall, baskets and shell reefs increased biodiversity through increased microhabitat availability for epibiota on ripraps.