Marine Ecology Enhancement Fund
Shore up with shells: enhancing marine biodiversity on artificial rip-rap seawalls with a nature-based approach
(MEEF2022013)
While Hong Kong’s diverse marine ecosystems host rich biodiversity with over 6000 species, it also faces a myriad of human-induced threats. In at North Lantau and the western waters Land reclamation and coastal developments have caused the loss of valuable natural marine ecosystems, and hardening of nature shores into artificial sea walls (such as rip-raps) which resulted a loss of structurally complex coastal habitats suitable for marine species. Both leads to a loss of marine biodiversity in the area.
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While restoring the hardened shores back to their natural state could be challenging and sometimes impossible, various ecological engineering approaches, such as adding elements to existing structures or developing new, ecologically sound materials and designs, have been developed to increase structural complexity and suitability of artificial shorelines.
More recently, the use of natural materials, processes and species (the “soft” ecological engineering approaches) have also been explored. Oyster shell is one of the main nature-based ecological engineering option being investigated because of their suitability as a substrate for other marine species.
The discarded oyster shells from Oyster farms in Hong Kong and nearby regions can be hence utilised as a cost effective natural material for shore enhancement.
Project objectives
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To explore and assess the effectiveness of using oyster shells as a nature- based approach to add complex microhabitats and enhance marine biodiversity on artificial riprap seawalls in North Lantau;
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To use this rehabilitation trial as a platform for general public education and engagement in local coastal restoration and conservation.
What we did
Between November 2022 to December 2023 we have conducted a total of 14 oyster shell string fabrication workshops in primary and secondary schools in Hong Kong, where participants attended educational talks on marine ecology and habitat restoration, and had hands-on experiences assembling oyster shells into a shore enhancement unit. Additionally, two public seminars, one talk, and an oyster shell string fabrication workshop were conducted.
The oyster shell strings were then used by HKMEA to create bundles of oyster shell strings (oyster shell clusters) used in the trial stage in this project.
In the trials we have deployed oyster shell clusters on a selected section of rip-rap sloping seawall at North Lantau. Regular ecological monitoring was conducted during a nine-month period to assess the effectiveness of the oyster shell clusters on enhancing marine biodiversity on the seawall. At the end of the trial, sample shell strings were bought back to laboratory for measuring biofiltration rate and inorganic carbon content of the filter feeders settled on the shell cluster.
Results
The results demonstrated the effectiveness of the shell cluster units in enhancing intertidal biodiversity on the rip-rap seawall.
An increase in species richness was recorded for sessile and mobile species including filter-feeders and non-filter feeders, as early as three months after shell unit deployment. During the nine-month trial period a total of 55 species/taxa were recorded from experimental plots with oyster shell cluster units deployed, compared to 35 species/taxa recorded in control plots.
Biofiltration rate inorganic carbon suggested that the oyster shell clusters were able to provide some ecosystem services including water purification and carbon storage to rip-rap seawalls, although the magnitude is expected to increase with mature communities.