Suspended particulate matter acts as a co-stressor to amplify imidacloprid impacts in estuarine mussels.

Suspended particulate matter (SPM) in turbid estuaries redistributes pesticides between dissolved and particle-associated phases, potentially biasing dissolved-only risk metrics. Here, we examined how natural estuarine SPM alters imidacloprid (IMI) bioaccumulation and sublethal responses in the mussel Mytella strigata. IMI rapidly partitioned to SPM, reaching quasi-equilibrium within 12-24 h; however, mass-normalized sorption declined at higher particle loadings, constraining the particle-associated fraction to ∼10-13% across the tested SPM range (0.5-5 g L-1). The presence of SPM modestly enhanced IMI bioaccumulation in mussels, increasing tissue concentrations from 37 to 41 µg g-1 dry weight. This particle effect was captured by an added ingestion-associated pathway (kp = 2.00 ± 0.42 mL g-1 h-1), while elimination remained comparable to that under dissolved-only exposure (ke = 0.26 vs. 0.30 h-1). The addition of SPM caused limited changes in alpha diversity but further reorganized the mussel microbiome and increased within-group dispersion; IMI and SPM jointly explained 18% of variation in genus-level composition. IMI alone induced 304 DEGs versus controls (FDR ≤ 0.01), whereas high-SPM co-exposure was associated with 146 additional responsive genes. These results show that SPM alters exposure routes and sublethal responses even when dissolved IMI is comparable, supporting two-phase pesticide risk assessment in turbid estuaries.