Temporal and spatial variability in mudflat and mangrove foraminiferal eDNA communities in subtropical environments and their implication for sea-level reconstruction

Reconstructing relative sea level (RSL) is essential for understanding coastal evolution and mitigating impacts of climate change. Foraminiferal assemblages are established indicators for past sea levels, requiring modern training sets that link assemblage composition to known tidal elevations. However, seasonal and spatial variability in assemblages can affect reconstruction reliability. Foraminiferal environmental DNA (eDNA) has recently been validated as a robust proxy for reconstructing past sea levels. However, eDNA assemblages may be subject to similar spatiotemporal variability along with additional factors affecting DNA transport and preservation. The spatiotemporal stability of intertidal eDNA assemblages – and their influence on RSL reconstruction – remains uncertain. We conducted a 2-year eDNA monitoring study at three stations of varying tidal elevation, sampling across dry and wet seasons to assess variability in mangrove and mudflat environments of Hong Kong. Mid-mangrove eDNA communities exhibited temporal and spatial stability, whereas mudflat and upper-mangrove assemblages, particularly among monothalamous foraminifera, showed pronounced seasonal shifts. Despite this variability in the upper mangrove, eDNA-based elevation estimates in mangrove environments consistently aligned with observed elevations (within 95 % credible intervals), demonstrating reliable calibration for paleoenvironmental applications. However, mudflat samples exhibited an overprediction bias, especially during the wet season, reflecting their heightened sensitivity to exogenous eDNA inputs. These findings highlight the need to account for seasonal and environmental variability in eDNA-based RSL reconstruction. Stable mangroves are optimal for establishing elevation–assemblage relationships, while transitional or mudflat zones require caution due to higher variability. Our study provides guidance for applying foraminiferal eDNA as a proxy in past sea-level reconstruction in dynamic coastal settings.