Beyond Greening: Biodiversity Outcomes of Wastewater-Fed Ecosystem Restoration: A Systematic Comparative Review of Iran and Turkey

Introduction: Biodiversity is declining globally, particularly in arid and semi-arid regions where water scarcity and anthropogenic pressures increasingly threaten ecosystem integrity. Iran and Turkey experience comparable climatic aridity and increasing water stress but differ in wastewater governance, restoration practices, and environmental monitoring. Although both countries increasingly employ treated wastewater and nature-based solutions, biodiversity outcomes of wastewater-supported ecosystem restoration have not been systematically compared. This study synthesizes available evidence to evaluate restoration trajectories and biodiversity responses.

Methods: Following PRISMA 2020 guidelines, a systematic review was conducted using Web of Science, Scopus, and Persian databases for studies published between 2000 and 2026. After identifying 646 records, removing duplicates, and assessing full-text articles, 27 studies met the inclusion criteria. Eligible studies reported quantitative biodiversity indicators, including species richness, diversity indices, taxon abundance, community composition, or functional ecological responses.

Results: The reviewed studies revealed contrasting restoration trajectories. Iranian studies predominantly focused on wastewater treatment wetlands and revegetation, emphasizing vegetation cover and pollutant removal rather than comprehensive biodiversity assessment. For example, the Segzi restoration project reported increased vegetation cover but limited improvements in plant diversity, indicating that ecosystem greening does not necessarily translate into biodiversity recovery. In Urmia Lake, wastewater inflows were associated with nutrient enrichment and cyanobacterial proliferation, potentially altering aquatic food-web dynamics and threatening native biota.Turkish studies more frequently incorporated multi-taxa ecological monitoring within restoration frameworks, particularly in wetland restoration and constructed wetland systems. Nevertheless, nutrient enrichment, emerging contaminants, and insufficient long-term monitoring remain common challenges in both countries.

Conclusion: Vegetative greening alone is an inadequate indicator of restoration success. Biodiversity outcomes of wastewater-supported restoration are strongly context-dependent and cannot be predicted solely by wastewater reuse intensity. Standardized long-term monitoring encompassing multiple taxonomic groups and functional diversity metrics is essential to ensure biodiversity-positive restoration in water-scarce regions.