Implications of sulphide toxicity for seed-based seagrass restoration

Seagrass restoration is a critical strategy for reversing habitat loss, yet the mechanistic drivers of early recruitment failure remain poorly understood. Seed-based approaches offer a scalable and cost-effective solution for large-scale restoration, but low germination and establishment success in situ continue to limit their effectiveness. This study investigates the effects of sulphide-rich sediments on the germination of the temperate seagrass Zostera muelleri (syn. Nanozostera muelleri) as a model species and explores whether the presence of native infaunal bivalves can mitigate sulphide stress. A multi-factorial mesocosm experiment reflecting observed ranges in temperate seagrass systems was conducted manipulating sediment sulphide levels and bivalve presence. Germination success, sediment sulphide accumulation, and subsequent plant growth were monitored to investigate biotic and abiotic interactions affecting seedling establishment. High sediment sulphide concentrations significantly reduced Z. muelleri germination and the presence of native bivalve S. trigonella did not reduce sediment sulphide accumulation. At low sulphide levels, bivalve presence improved germination; however, under high sediment sulphide conditions, bivalve presence further suppressed germination, likely due to synergistic effects of biodeposition and organic enrichment. These findings demonstrate that elevated sediment sulphide concentrations can act as a bottleneck to seed germination, with implications for both natural recruitment and seed-based restoration across seagrass systems. By evaluating generalizable mechanisms, this work positions Z. muelleri as a model species for understanding the impact of sediment geochemistry on restoration outcomes.