Distinct pesticide profiles in sediments and red mangrove crabs (Ucides occidentalis): Impact of land use in a tropical estuary.

Tropical estuaries are increasingly subjected to pesticide contamination from agricultural and urban sources, yet the relationship between land use and pesticide contamination within these ecosystems remains insufficiently understood. This study investigated the occurrence and retention of pesticides in surface sediments and red mangrove crab (Ucides occidentalis) tissues across two sub-estuaries in Guayas Estuary (Ecuador) under the influence of various land-use patterns. Twelve pesticide compounds were detected in sediment samples, mainly from agriculture-influenced sites, while crab tissues contained 21 compounds, including 18 not detected in sediments, with more pesticide detections in crabs from urban-influenced sites. These divergent contamination profiles suggest that crabs may be exposed to pesticides through additional pathways beyond sediment contact, such as waterborne exposure and dietary uptake. Notably, the concentrations of diuron, a widely used herbicide, in agricultural sediments (0.43-5.7 μg kg-1), exceeded sediment quality guidelines (SQC EQSsed: 0.39 μg kg-1) but showed limited bioaccumulation. Conversely, azoxystrobin and cadusafos, commonly used fungicide and insecticide, respectively, were accumulated in crab tissues despite their negligible sediment levels. Land use analysis within a 1000 m buffer indicated that cropland dominated in pesticide detections in agriculture-influenced sub-estuary, while urban-influenced sub-estuary showed a balanced pattern of land use proportions across detected categories. These findings highlight the value of incorporating contaminant profiles from biota, alongside land-use context, into pesticide monitoring strategies to better characterize contamination pathways in tropical estuarine ecosystems.