Recent distribution, inventories and temporal trends of suspended particulate matter in the St. Lawrence Estuary, Canada

Suspended particulate matter (SPM) is considered to play important biogeochemical roles in aquatic systems, particularly in estuaries, and the assessment of their temporal trends is essential in the current period of climate change. Concentrations of SPM and particulate inorganic matter (PIM) in the St. Lawrence Estuary (SLE), downstream of the maximum turbidity zone (MTZ), were measured at 21 stations during four time periods, related to hydrological or biological events, between 2014 and 2016. SPM concentrations decreased seaward with no lateral difference observed. At the deepest stations, SPM vertical profiles followed a typical ‘C’ shape whereas PIM content increased from surface to bottom. The inventories of SPM in the study area varied from 1.29 ×106 tons during the period of low river runoff to 2.14 ×106 tons during the spring freshet. On average, 80% of SPM was made of PIM. The amount of PIM (SPM) entering the Lower SLE during the 8 to 9 ice-free months corresponded roughly to its inventory in the study area suggesting that PIM (SPM) stays in the water column for several months before being removed by sedimentation. Our results support the hypothesis that most SPM is retained in the MTZ during the summer and released with drifting ice during the winter–spring periods. Strong relationships were observed between water discharges at Quebec City and SPM (PIM) inventories in the study area (R2>0.94). The variability of SPM (PIM) inventories, estimated from water discharge proxy, was highest during the period of spring freshet and lowest during the period of low river runoff. When corrected for water discharges, a significant decrease in SPM and PIM inventories was observed over the last 15 years.