Introduction
Oyster habitats are recognised as important biogenic ecosystems that enhance biodiversity, ecosystem functioning and coastal resilience through habitat provision, nutrient cycling and benthic–pelagic coupling (Beck et al., 2011). Native European oyster Ostrea edulis restoration and Pacific oyster Crassostrea gigas aquaculture are increasingly promoted as nature-based approaches to support ecosystem recovery and sustainable food production, yet comparative ecological assessments of these managed systems within Irish estuaries remain limited (zu Ermgassen et al., 2012). Oyster habitats enhance biodiversity by increasing habitat heterogeneity and providing settlement substrate and refuge for diverse faunal and floral assemblages (Theuerkauf et al., 2021). This study investigated biodiversity, trophic structure and functional trait composition associated with commercially managed subtidal O. edulis beds and intertidal and subtidal C. gigas culture systems in Cork Harbour, Ireland, to evaluate their contribution to ecosystem functioning and nature-based coastal management.
Materials and Methods
The study was conducted at Rossmore Oysters, Cork Harbour, a temperate estuarine system supporting long-standing shellfish aquaculture. Native O. edulis habitats included a fallowed post-harvest bed and actively seeded 2+ and 4+ year subtidal beds, while C. gigas habitats comprised intertidal bag-and-trestle systems and subtidal beds of differing ages at Jamestown. Biodiversity surveys were conducted using timed visual encounter surveys and replicated dredge sampling. Sampling encompassed multiple habitat types representing different cultivation methods and successional stages. Species were identified to the lowest practical taxonomic level and classified according to trophic and functional traits including feeding mode, mobility, habitat association and structural role. Functional diversity and trophic structure were assessed to examine how biodiversity and ecological functioning developed across differing management and successional stages.
Results
Managed O. edulis habitats supported 67 associated taxa comprising 46 faunal species and 21 macroalgal species, while C. gigas habitats supported 58 taxa including 46 faunal species and 12 macroalgal species. Oyster-associated habitats consistently supported substantially greater biodiversity than oyster- and shell-free sediments, demonstrating the importance of shell-associated habitat complexity. Functional analyses revealed strong representation of filter feeders, predators, grazers and detritivores across both oyster systems, indicating high functional redundancy and resilient benthic–pelagic coupling. Functional trait composition remained broadly consistent across management stages despite differences in species richness, indicating maintenance of key ecological functions. Older managed habitats generally supported greater trophic complexity and faunal richness; however, younger seeded beds also demonstrated rapid development of diverse assemblages, indicating early establishment of structurally complex communities. Several species of high conservation value were associated with oyster habitats, including the critically endangered European eel Anguilla anguilla and egg cases of the endangered spotted ray Raja montagui, highlighting the wider ecological value of managed oyster habitats. Commercially and ecologically important taxa including queen scallop Aequipecten opercularis, common cockle Cerastoderma edule, common prawn Palaemon serratus and diverse Rhodophyta assemblages were also recorded.
Discussion
Managed oyster aquaculture and restoration habitats function as biodiversity hotspots that support resilient ecological communities within temperate estuaries. Shell-associated habitats appear to support diverse assemblages even following harvest or in the absence of living oysters, highlighting the ecological importance of retained shell substrate and habitat complexity (Forrest et al., 2009; Commito et al., 2008). Rotational seeding practices and prolonged in situ grow-out allow oyster habitats to develop structurally and functionally diverse biological communities, demonstrating that commercial oyster production and biodiversity conservation can be mutually supportive objectives within ecosystem-based coastal management. These findings demonstrate that well-managed oyster culture can simultaneously support seafood production, biodiversity conservation, ecosystem functioning within working coastal landscapes and contribute to EU restoration policy objectives.
Acknowledgement
This work was funded by the Department of Agriculture, Food and the Marine (DAFM) gov.ie through the BIVALVE Project number 2021R609.
References
Beck, M.W., Brumbaugh, R.D., Airoldi, L. et al., (2011) Oyster reefs at risk and recommendations for conservation, restoration and management. BioScience 61:107–116. https://doi.org/10.1525/bio.2011.61.2.5
Commito, J.A., Como, S., Grupe, B.M., & Dow, W.E. (2008). Species diversity in the soft-bottom intertidal zone: Biogenic structure, sediment, and macrofauna associated with mussel beds. Journal of Experimental Marine Biology and Ecology, 366 (1-2), 70–81. https://doi.org/10.1016/j.jembe.2008.07.010
Forrest, B.M., Keeley, N.B., Hopkins, G.A., Webb, S.C. & Clement, D.M. (2009) Bivalve aquaculture and benthic ecosystem effects. Aquaculture, 298: 1-15. http://dx.doi.org/10.1016/j.aquaculture.2009.09.032
Theuerkauf, S.J., Barrett, L.T., Alleyway, H.K., Costa-Pierce, B.A., St. Gelais, A., & Jones, R.C. (2021) Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture, 00, 1-19. https://doi.org/10.1111/raq.12584 zu Ermgassen, P.S.E., Spalding, M.D., Blake, B., et al., (2012). Historical ecology with real numbers: Past and present extent and biomass of an imperilled estuarine habitat. Proceedings of the Royal Society B, 279, 3393-400. 10.1098/rspb.2012.0313