Assessment of the Trophic Index (TRIX) of Sea‐Cage Aquaculture Farms in the Southern Black Sea: Influence of Water‐Column Depth Gradients

OpenAlex · İngilizce

Marine sea‐cage aquaculture continues to expand, increasing the need for monitoring tools capable of distinguishing farm‐related nutrient enrichment from natural coastal variability. This study evaluated the trophic status of 18 sea‐cage aquaculture farms along the southern Black Sea coast using the Trophic Index (TRIX), which integrates chlorophyl‐a, oxygen saturation deviation, dissolved inorganic nitrogen, and total phosphorus. A nested hierarchical sampling design was used to compare cage and reference site types across three depth strata (surface, midwater, and bottom). A total of 131 TRIX observations were analyzed using linear mixed‐effects models, with farm identity included as a random intercept to distinguish cage‐proximity effects from farm‐level background conditions. TRIX values fell predominantly within the good (4–<5) to moderate (5–<6) trophic classes, with the highest mean values observed in Zone 3 during spring. Model‐adjusted means were similar between site types (reference: 4.98; cage: 4.89), indicating no systematic evidence of water‐column enrichment associated with cage proximity at the examined spatial scales. In contrast, trophic conditions exhibited significant vertical structuring. Variance partitioning showed that fixed effects explained 29.6% of TRIX variability ( R 2 m = 0.296), whereas the full model explained 66.1% ( R 2 c = 0.661), with substantial among‐farm heterogeneity (ICC = 0.518). Multivariate ordination further indicated a strong overlap between the cage and reference samples, with the first two PCA axes explaining 60.0% of the total variance. In sum, these findings suggest that the pelagic trophic variability in this region is governed primarily by bathymetry and local hydrographic context, supporting the spatial planning strategies that prioritize deeper, well‐flushed sites.