A Nature-Based Solution for Oyster Reef Restoration: Evaluating Biodegradable Polylactic Acid Materials for Oyster and Macroinvertebrate Enhancement in a Subtropical Bay

The current approach to coastal oyster reef restoration currently relies on conventional plastics, raising concerns about plastic pollution. Therefore, developing biodegradable alternatives, such as nano-montmorillonite-modified polylactic acid materials (PLA), has become a priority. This study compared oyster recruitment on PLA substrates with that on four conventional plastic substrates (polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polyvinylidene chloride (PVDC)) through field experiments, examining how PLA substrate thickness and surface roughness influence oyster recruitment. Additionally, we evaluated the responses of oyster populations and associated macroinvertebrate communities after ten months of restoration using PLA-based versus polyethylene (PE) shell-bag reefs. The results showed no significant difference in oyster recruitment between PLA and conventional plastic substrates (p > 0.05). However, increasing the thickness and surface roughness of the PLA substrates significantly enhanced the recruitment of juvenile oysters (p < 0.05). After ten months, there was no significant difference in oyster abundance between PLA and PE shell bag reefs; however, there was a significant difference in resident macroinvertebrate abundance, with abundances markedly higher on PLA reefs (1372 ± 220 ind./m2 vs. 545 ± 90 ind./m2; p < 0.05). This study highlights the potential of PLA as a promising alternative to conventional plastics. However, its rapid degradation limits its applicability in high-energy environments. Furthermore, given that a comprehensive assessment of the microplastic risks associated with its degradation has not yet been conducted, large-scale application is not currently recommended.