Seasonal dynamics of plastisphere microalgal assemblages in aquaculture systems: An integrated assessment from...

Aquaculture infrastructures create persistent artificial habitats that support complex plastisphere communities shaped by both environmental variability and farming-derived enrichment. This study investigated the seasonal dynamics, vertical structuring, and ecological organization of microbial assemblages colonizing breeding nets, floating tubes, and anchor ropes at a commercial fish farm in Monastir Bay (Tunisia) across a one-year sampling cycle. Seasonal monitoring of physicochemical parameters, nutrient concentrations, trace metals, and microbial communities revealed strong temporal fluctuations linked to hydrodynamic and biogeochemical forcing. Temperature (T°) and nutrient availability were identified as major environmental factors strongly correlated with assemblage succession, while zinc and copper enrichment reflected sustained anthropogenic inputs associated with aquaculture activity. Although total microbial biomass remained relatively similar among substrates, taxonomic composition exhibited pronounced seasonal and vertical differentiation. Diatoms and cyanobacteria dominated during cooler nutrient-enriched periods, whereas dinoflagellates progressively increased during warmer and more stratified conditions. Deep anchor ropes preferentially accumulated harmful benthic taxa including Prorocentrum lima, Ostreopsis siamensis, and Coolia monotis, suggesting that stable submerged structures can act as potential physical reservoirs for toxigenic species. Multivariate analyses confirmed that seasonal turnover and depth-related gradients exerted stronger control on plastisphere organization than substrate composition alone. While providing valuable site-specific insights into net-cage biofouling in Monastir Bay, this study demonstrates that local aquaculture plastisphere communities function as dynamic and vertically stratified ecological systems associated with microbial succession, contaminant retention, and harmful algal presence in coastal bay environments.