Increased flux of carbon and nutrients from human activities in river basins were linked to acidification and deepwater hypoxia in estuaries and coastal areas worldwide. Annual loads (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) of suspended particulate matter (SPM), dissolved organic carbon (DOC), total nitrogen (TN) and total phosphorus (TP) were assessed at the Lake Ontario inlet of the St. Lawrence River (SLR) (7110 m 3 s -1 ) and its estuarine outlet at Québec City (12,090 m 3 s -1 ). Internal loads from the Ottawa River (1950 m 3 s -1 ), seventeen other tributaries, urban wastewaters, atmospheric deposition and erosion were also estimated. Erosion (65% of SPM, 29% of TP), inflow from Lake Ontario (42% of DOC, 47% of TN) and Ottawa River (28% of DOC) contributed important flux to the estuary. Loads from other tributaries (20 and 27% of TN and TP at Quebec City) largely exceeded municipal sources (6% of exported TN and TP) and require future remediation. Aquatic plants fixed 277,000 t of C, 49,000 t of N and 7000 t of P (May-Sept.), delaying the nutrient flux to the estuary and turning the SLR into a nutrient sink over summers of lowest discharge. Degradation of exported organic C could consume 5.4-7.1 million t O 2 year -1 in the estuary whereas SLR flux of N and P represent 31-47% and 7-14% of total annual estuarine flux, respectively. Carbon and Nitrogen flux from freshwaters partly explain the decline in pH and oxygen concentrations in deep estuarine waters thus highlighting the need to reduce diffuse sources of nutrients in the entire watershed.

According to the bacteriological and physicochemical water quality index IQBP5, the water quality of the St. Lawrence River in Québec (Canada) is good upstream of Montreal, but deteriorates downstream, both in the northern water mass and the navigation channel. This is due to fecal contamination from the Montreal, Longueuil and Repentigny waste water treatment plants. South of the navigation channel, water quality is good throughout the river corridor. In the Québec region, water quality at all sampling stations was either good or at the limit of this class. Between 2000 and 2014, the number of sites with bad or very bad water quality decreased, but the frequency of water quality exceedances for phosphorus and fecal coliforms showed no significant trend. The various tributaries of the St. Lawrence that drain agricultural land contained between 4 and 33 pesticides, and 20 of these were also detected in the Lac Saint-Pierre (Lake Saint Pierre). In some of the lake samples, the concentrations of clothianidin, thiamethoxam and atrazine exceeded water quality criteria for the protection of aquatic life. Emerging contaminants, such as nonylphenol ethoxylates, polybrominated diphenyl ethers (PBDEs), perfluorinated compounds and pharmaceutical and personal care products are also present in the St. Lawrence River. The concentrations of PBDEs found in fish exceed criteria for the protection of fish and fish-eating terrestrial wildlife. Concentrations of nonylphenol ethoxylates decreased as a result of government controls introduced during the period between 2000 and 2010. Following 2 decades of improvement, the water quality of the St. Lawrence River remained relatively stable between 2000 and 2014. However, new pesticides and contaminants of emerging concern are now being detected and, in many cases, the environmental significance of these new findings remains unclear.Selon l’indice de qualité bactériologique et physicochimique (IQBP5), l’eau du Saint-Laurent est de bonne qualité en amont de Montréal, mais elle se détériore en aval, dans le chenal de navigation et dans la masse d’eau au nord de celui-ci, à cause de la contamination bactériologique provenant des stations d’épuration de Montréal, Longueuil et Repentigny. Au sud du chenal, la qualité est bonne ou satisfaisante tout au long du corridor fluvial. Dans la région de Québec, toutes les stations d’échantillonnage présentent une eau de bonne qualité ou à la limite de cette classe. De 2000 à 2014, le pourcentage de stations de qualité mauvaise ou très mauvaise a diminué, mais la fréquence de dépassement des critères de qualité pour le phosphore et les coliformes fécaux n’a pas montré de tendance significative. De 4 à 33 pesticides ont été détectés dans les tributaires du Saint-Laurent, selon le tributaire, et 20 de ces produits ont été détectés dans le lac Saint-Pierre. Les concentrations de clothianidine, de thiaméthoxame et d’atrazine dans le lac ont parfois dépassé les critères de qualité de l’eau pour la protection des espèces aquatiques. Les contaminants d’intér...

The aim of this work is to evaluate and discuss river water quality trends over the last decade in ten watersheds where diffuse pollution represents more than half of the annual load of phosphorus (P) and nitrogen (N). Trend analyses taking into account flow data indicate a significant reduction of total P in eight rivers, of ammonia N in five rivers, of nitrate + nitrite in four rivers, of total filtered N in three rivers and of suspended solids in two rivers. An increase of turbidity was observed in four rivers and, for fecal coliforms, no trends. P decrease can be explained by reduced mineral P inputs on cropped lands related to means such as agro-environmental fertilization plans and addition of phytase in pig and poultry feed. However, for seven of them, median P concentrations remain at least two times greater than the Québec water quality guideline for protection of rivers against eutrophication. Concentrations of other parameters remain problematic in some rivers too. These results indicate the need to continue the efforts for further diffuse pollution reduction. Future work should better quantify actions taken at the watershed scale to reduce diffuse pollution.