Cumulative impacts of hydrology and human activities on water quality in the St. Lawrence River (Lake Saint-Pierre, Quebec, Canada)

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Date: June 2008
Publisher: NRC Research Press
Document Type: Report
Length: 8,449 words
Lexile Measure: 1620L

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Abstract: The cumulative impacts of discharge and human activities on the spatial variations of water quality were assessed from monthly measurements at 80-110 stations in a ~300 [km.sup.2] widening of the St. Lawrence River (Lake Saint-Pierre, Quebec). Water quality was poorest under high discharge conditions and in shallow riparian areas under the influence of small tributaries draining farmlands; spatial variability was amplified by aquatic macrophytes. In 2004, over 40% of the study area exceeded the provincial water quality criterion (total phosphorus = 30 [micro]g P x [L.sup.-1]) to protect aquatic life in rivers. Nutrient retention occurred mostly during summer when macrophytes were abundant and current velocities were low. Erosion of dredged deposits near the navigation channel was observed during summer, when current velocities increased in the deep channel because of flow obstruction by macrophytes growing in shallow areas. Low discharge conditions and high macrophyte abundance sharpened the contrast between the Lake Ontario waters flowing rapidly through the central river channel and the slow-flowing, low-quality tributary waters along the shores. The cumulative effect of high nutrient inflow from urban areas and farmlands, the man-made navigation channel, and seasonal flow obstruction by macrophytes thus induce a marked spatial variability in water quality within Lake Saint-Pierre.

Resume : Les impacts cumulatifs du debit et des activites humaines sur les variations spatiales de la qualite de l'eau furent evalues a partir de mesures mensuelles faites a 80-110 stations situees dans un elargissement (~300 [km.sup.2]) du fleuve Saint-Laurent (Lac Saint-Pierre, Quebec). La qualite de l'eau etait la plus basse en conditions de fort debit et dans les milieux riverains pen profonds influences par de petits tributaires drainant des terres agricoles; la variability spatiale etait accentuee par les macrophytes aquatiques. En 2004, plus de 40 % du secteur d'etude etait audela du critere provincial de qualite des eaux (TP = 30 [micro]g P x [L.sup.-1]) visant a proteger la vie aquatique en eaux courantes. La retention des elements nutritifs se produisait principalement en ete, lorsque les macrophytes etaient abondants et les courants, faibles. L'erosion de depots de dragage pres du chenal de navigation a ete observee pendant l'ete, lorsque la vitesse du courant etait accrue dans le profond chenal en raison de l'obstruction causee par les macrophytes croissant dans les zones pen profondes. Les conditions de faible debit et l'abondance de macrophytes accroissaient le contraste entre les eaux du Lac Ontario coulant rapidement dans le chenal principal et les eaux de pietre qualite en provenance des tributaires coulant lentement le long desrives. Les effets cumulatifs des apports eleves d'elements nutritifs provenant zones urbaines et agricoles, du chenal excave pour la navigation et de l'obstruction saisonniere de la circulation des eaux par les macrophytes induisent ainsi une variabilite spatiale marquee de la qualite des eaux an Lac Saint-Pierre.

Introduction

With a mean annual discharge of 12 600 [m.sup.3] x [s.sup.-1] at Quebec City, the St. Lawrence River (SLR) is the river with the second largest discharge in North America, after the Mississippi. Over its 550 km course...

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Gale Document Number: GALE|A180592684