Aquaculture and biodiversity-ecosystem functioning: A path to lay the foundations of ecosystem-based approach

Marine aquaculture generates localized organic enrichment that alters benthic community composition and ecosystem functioning, particularly in nutrient-poor systems such as the Mediterranean Sea. Current monitoring relies predominantly on abiotic variables (e.g., dissolved oxygen, nutrient loads), failing to capture the biological mechanisms through which biodiversity change affects functional outcomes. Biodiversity-ecosystem functioning (BEF) theory links community composition to ecosystem processes but has rarely been operationalized in aquaculture contexts. Here we applied the extended Price equation to a historical dataset of hard-substrate fouling communities-sessile filter-feeding assemblages dominated by barnacles, bivalves and gastropods-along a gradient of organic enrichment from offshore fish cages in the Gulf of Castellammare (NW Sicily). Exploiting a hierarchical sampling design (six baseline plots × three post-loss plots per site), we obtained 18 replicated decompositions at each of four downstream sites, enabling formal inference on five additive components of functional change. Total community biomass (ash-free dry mass) increased significantly at all four sites relative to the upstream baseline (overall Cohen's d = 0.82). The species richness effect due to gain (SRE_G) was the largest positive component (d = 2.59), reflecting consistent species additions at enriched sites. However, these colonizers contributed less biomass per species than the community average (SCE_G strongly negative, d = -1.76), indicating that gains were numerically important but functionally dilute. The context-dependent effect (CDE), capturing enhanced performance of persistent filter-feeders under organic subsidies, was the second largest driver (d = 0.89), significantly positive at three of four sites. The species richness effect due to loss (SRE_L) was consistently negative but smaller in magnitude. Among environmental correlates, only the association between SCE_L and total suspended matter was statistically supported (bootstrap 95% CI excluding zero), while correlations between abiotic variables and total functional change were weak-highlighting the limited capacity of water-quality monitoring to predict biodiversity-function outcomes. This study demonstrates that the extended Price equation, applied to replicated field data, can formally partition the functional consequences of biodiversity change in aquaculture-impacted communities. The approach provides a practical, management-relevant diagnostic tool that complements abiotic monitoring and supports the Ecosystem Approach to Aquaculture.