Response of natural estuarine microphytobenthic biofilms to multiple anthropogenic stressors

Estuarine ecosystems are subject to multiple anthropogenic stressors, yet their combined effects on sediment microbial communities remain poorly understood. Microphytobenthic biofilms (MPB) play a critical role in primary production and sediment stabilisation, influencing ecosystem functions and underpinning estuarine food webs. In this field study, we examined the interactive effects of glyphosate and titanium dioxide nanoparticles (TiO₂ NP) on MPB in a temperate estuary. We assessed MPB biomass (chlorophyll a), photosynthetic efficiency (F_v/F_m), extracellular polymeric substances (EPS), and sediment stability over 13 days. Glyphosate reduced MPB biomass, but its effect on photosynthetic function was inconsistent, with evidence of both reductions and transient increases in F_v/F_m. TiO₂ NP had a small positive effect on MPB biomass but did not significantly impact EPS or sediment stability. No strong interactions between the two stressors were observed. Additionally, macrofaunal communities and sediment erosion thresholds were largely unaffected. Under acute, patch-scale pulse exposure in a macrotidal estuary, responses were limited and variable, and interaction effects were generally inconclusive. Our findings highlight the importance of field-based studies in assessing multiple stressor impacts, where environment variability may buffer or amplify stressor effects in ways not observed in controlled mesocosm experiments. These findings should not be extrapolated to chronic or spatially extensive contamination scenarios.