Ecosystem Services of the Endangered Fan Mussel Pinna nobilis in Greek Coastal Waters: Implications of Population Collapse for Coastal Ecosystem Functioning

The protected fan mussel Pinna nobilis (Linnaeus, 1758) is a key ecosystem-engineering species in Mediterranean coastal ecosystems, historically providing important regulating ecosystem services—particularly nutrient removal and shell-based carbon storage—as well as the ecological function of large-scale biofiltration. Prior to the mass mortality event in 2016, the Greek population was estimated at approximately 2.66 × 109 individuals. This is associated with an annual gross shell carbon storage of 13,689–108,139 t “C”, together with 304.7–2988.3 t “N” and 49.7–450.5 t “P” retained in shell material, valued at EUR 8.14–76.6 million yr−1 using replacement-cost proxies. These values represent a gross shell carbon storage rather than a net ecosystem carbon sequestration, due to the fact that metabolic fluxes and remineralization may partially offset long-term retention. Furthermore, P. nobilis performed a substantial biofiltration function, filtering 6.07 × 1011–6.85 × 1012 m3 of seawater per year (2.89–32.53 turnovers of the 30 m coastal zone). The mass mortality event led the population to a decline by >81%, reducing the value of nutrient retention and gross shell carbon storage to EUR 1.31–13.47 million yr−1 and filtration capacity to 1.15 × 1011–1.30 × 1012 m3 yr−1. Using an illustrative order-of-magnitude replacement-cost approach, we estimate that by reproducing the lost biofiltration function through engineered systems, we would require approximately EUR 1–5.2 billion annually which indicates the magnitude of natural capital lost and the utmost importance of P. nobilis recovery for Greek coastal ecosystems.