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

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 between Kingston, at the outlet of Lake Ontario, and Quebec City, the SLR drains a watershed of over 1 million [km.sup.2]. The presence of the Laurentian Great Lakes (watershed: 737 400 [km.sup.2]), which are large (25 700 - 82 170 [km.sup.2]), deep lakes with very long residence times (2.6-191 years) at the head of its watershed, confers to the SLR properties that are unique among the large rivers of the world (Meybeck and Ragu 1996). For example, upper SLR waters are characterized by exceptionally low suspended particulate matter (SPM), dissolved organic carbon (DOC), and total phosphorus (TP) concentrations.

In the SLR headwaters, reduction of phosphorus (P) loads following the Great Lakes Water Quality Agreement (International Joint Commission (IJC) 1978; Stevens and Neilson 1987) induced a decline in total P (0.9 [micro]g x [L.sup.-1] x [year.sup.-1]) between 1976 and 1999 (Nicholls et al. 2001), resulting in concentrations below 10 [micro]g x [L.sup.-1] in recent years over the first 250 km of its course. Between Lake Ontario (Kingston) and Quebec City, the river drains a watershed inhabited by 7 million people, half of whom live along its shores and discharge their wastewaters into the river following varying levels of treatment. SLR tributaries draining the fertile St. Lawrence Valley, such as the Ottawa River (watershed: 146 334 [km.sup.2]) and southern Quebec rivers (50 000 [km.sup.2]), also represent important additional sources of suspended solids and nutrients. Between 1951 and 2001, agriculture shifted from extensive pasture and dairy production to intensive corn production (14-fold rise in area) to support chicken (3-fold rise) and hog (4-fold rise) production (Statistics Canada 2001; Ministere de l'environnement (MENV) 2003).

[FIGURE 1 OMITTED]

Downstream of the Greater Montreal area (Fig. 1a), the SLR receives the primary-treated municipal effluents of over 3 million inhabitants, as well as large N and P loads from several tributaries (Ottawa, Richelieu, Saint-Francois, Yamaska, Nicolet, and l'Assomption rivers) draining farmlands and small urbanized areas. As a result, the mean annual TP concentration in the main river stem increases from 8 [micro]g P x [L.sup.-1] at Cornwall to 14 [micro]g P x [L.sup.-1] downstream of Montreal (Hudon 2000) and to 36 [micro]g x [L.sup.-1] at Quebec City (B. Rondeau, Environment Canada, Water Quality Monitoring and Surveillance Division, 400 Place d'Youville, Montreal, QC H2Y 2E7, personal communication).

The Lake Saint-Pierre (LSP; Fig. 1b) segment of the SLR, where most urban and agricultural effluents converge, is particularly vulnerable to the impacts of such substances owing to its large expanses of slow-flowing riparian areas. This shallow and spatially complex broadening of the SLR is the last major freshwater area for wetlands and wildlife before the estuary and was recently designated as a UNESCO World Biosphere Reserve (in 2000) and Ramsar site (in 1998). Although transport of carbon (Pocklington 1982), sediment (Carignan and Lorrain 2000; Rondeau et al. 2000), and various contaminants (Carignan et al. 1994; Pham et al. 2000; Rondeau et al. 2005) have been documented, little is known of the effects of human activities on water quality and nutrient concentrations in SLR, particularly in Lake Saint-Pierre. The present study was therefore undertaken to quantify present loadings of nutrients, DOC, and SPM and to establish the seasonal evolution and spatial distribution of water quality variables within this river segment. We hypothesized that significant water quality degradation in this river segment resulted from the inputs of high nutrient and SPM loads from the small tributaries and the limited lateral mixing of water masses.

Materials and methods

Study area

Lake Saint-Pierre (46[degrees]12'N, 72[degrees]49'W) is a 30 km long, ~300 [km.sup.2] widening of the SLR located 65 km downstream of the Greater Montreal urban area. In the upstream portion of LSP, river flow is distributed among the numerous channels of the Berthier-Sorel archipelago. The river broadens to 10-12 km in LSP proper; this section of the river is relatively shallow (mean depth ~3 m) and slow-flowing (<0.5 m x [s.sup.-1]), with the exception of a 250 m wide, man-made (1854-2001) central navigation channel (>11 m deep) in which a significant portion of the flow (0.5-1 m x [s.sup.-1]) is now concentrated (Fig. 1). The excavated channel is bordered by eroding shallow banks produced by deposits of dredged material and by a series of emergent rock piles acting as pillars to stabilize the ice cover outside the navigation channel.

Lake Saint-Pierre occupies the St. Lawrence lowlands, which extend to the Great Lakes and are composed mainly of Ordovician and Silurian limestone. Tributaries draining the north (l'Assomption, Chicot, Maskinonge, du Loup, and Yamachiche rivers) and south (Richelieu, Yamaska, and Saint-Francois rivers) shores of the SLR flow over Precambrian rocks and Appalachian metamorphic rocks, respectively (Douglas 1970).

This fluvial lake is characterized by large, sheltered bays, and shallow sloping shoreline supports large emergent marshes and extensive beds of submerged aquatic vegetation to a depth of about 3 m (Hudon 1997). During mid- to late summer, submerged aquatic plants cover 260 [km.sup.2] (85%) of the lake's surface area (mean biomass 54 g dry mass x [m.sup.-2]) and represent an annual production of 8700 tonnes (t) of carbon (Vis et al. 2007). The upstream third of the lake (transects 1 and 2, T1 and T2, respectively; Fig. 1) is incised by several natural and man-made channels, whereas the middle part (T3 and T4) is flat and features a regular bottom sloping to the central navigation channel. At the downstream end of the lake (T5), a shallow (<3 m) sill occupies 65% of the river section, thus further concentrating the flow towards the centre of the river.

Water discharge

Daily water discharge data were obtained for the St. Lawrence River at the Beauharnois and Les Cedres dams, the Ottawa River at the Carillon Dam (Hydro-Quebec 2006), and near the mouth of the l'Assomption, Richelieu, Yamaska, and Saint-Francois rivers (Ministere du developpement durable, de l'environnement et des parcs (MDDEP) 2006a). Discharge data for tributaries were corrected for total watershed surface area. The sum of the above-mentioned contributions yielded a conservative estimate of water discharge and loads entering the study area, as it did not include minor tributaries draining the north and south shores. Daily water levels in LSP (Courbe no. 2 gauging station 15975), expressed in metres above sea level (asl; International Great Lakes Datum of 1985), were obtained from the Department of Fisheries and Oceans (DFO 2006).

Bathymetric data and interpolation

Raw bathymetric and elevation data were obtained from the Canadian Hydrographic Service (~85 000 echo soundings), from Environment Canada and the International Joint Commission for the nearshore area (~102 000 echo soundings; Fortin 2002), and from the floodplain (~1 000 000 LIDAR elevation points; Fortin et al. 2002). These data sets were converted to the same datum and combined to produce a digital elevation model of bottom topography (25 m x 25 m pixel size, 0.1 m vertical resolution) by triangulation and linear interpolation (MapInfo V. 6.5 and Vertical Mapper V. 2.6; MapInfo Limited, Troy, New York).

Sampling

Spatial variation in water quality in LSP was assessed during the ice-free season based on a total of nine, 1-day surveys carried out in 2003 (13 August 2003 only) and at monthly intervals in 2004 (eight dates: 20 April, 12 May, 15 June, 13 July, 10 August, 14 September, 13 October, and 16 November). On each date, physical and chemical data were acquired at 80 to 110 stations distributed along five transects (T1 to …