Phytoplankton assemblages in the St. Lawrence River, downstream of ...

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Phytoplankton assemblages in the St. Lawrence River, downstream of its confluence with the Ottawa River, Quebec, Canada Christiane Hudon

Abstract: Consistent differences in physical, chemical, and biological characteristics were observed along a transversal river section located 2 km downstream of the confluence of the Ottawa and the St. Lawrence rivers. Phytoplankton sampled at stations subject to the influence of the St. Lawrence River had a lower biomass, smaller cell volume, lower chlorophyll a, and lower taxa richness than did phytoplankton at stations influenced by the Ottawa River. The stations influenced by St. Lawrence River waters showed regular seasonal changes in phytoplankton biomass and composition, reflecting the major impact of the stratification and mixing cycles observed in the Great Lakes. By comparison, at the stations influenced by Ottawa River waters, monthly variations were found in biomass and composition correlated with temperature and water clarity, suggesting the effects of the watershed’s morphology. A discharge reduction of 12% in the St. Lawrence River and 46% in the Ottawa River between summer 1994 and summer 1995 coincided, for stations in both water masses, with lower biomass and greater species richness and an increase in taxa that generate noxious smells and odours. Phytoplankton is recommended for use in monitoring the biological impacts of changes in water characteristics resulting from human activities and climate change in the Great Lakes watershed. Résumé : Des différences persistentes des caractéristiques physiques, chimiques et biologiques furent observées le long d’une section transversale du Saint-Laurent à 2 km en aval de sa confluence avec la rivière des Outaouais. Le phytoplancton prélevé aux stations influencées par les eaux du Saint-Laurent affichait des valeurs inférieures de biomasse, de volume cellulaire, de chlorophylle a et de richesse taxonomique que le phytoplancton aux stations influencées par les eaux de l’Outaouais. Les stations influencées par le Saint-Laurent montraient un patron saisonnier régulier de biomasse et de composition phytoplanctonique, montrant l’influence du cycle de stratification et de mélange observé dans les Grands Lacs. Par comparaison, la biomasse phytoplanctonique aux stations influencées par l’Outaouais variaient mensuellement selon la température et la clarté des eaux, suggérant l’influence indirecte de la morphologie du bassin versant. Une réduction du débit estival de l’ordre de 12% dans le Saint-Laurent et de 46% dans l’Outaouais entre les étés 1994 et 1995 coincidaient, dans les deux masses d’eau, avec une diminution de la biomasse phytoplanctonique mais un accroissement de la richesse taxonomique, particulièrement pour les taxons générant des goûts et des odeurs dans l’eau. Le phytoplancton est recommandé pour la surveillance des impacts biologiques induits par des changements des caractéristiques physico-chimiques des eaux, résultant des activités humaines et des changements climatiques dans le bassin des Grands Lacs. Hudon

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Introduction Phytoplankton dynamics in temperate ecosystems are largely determined by seasonal variations in light intensity and water temperature. In large temperate rivers (>100 m wide), light availability is largely unaffected by shoreline forest canopy, but is rather a function of average stream depth and of the concentration of suspended solids, which both vary seasonally with discharge (Relexans et al. 1988). Basin morphology and seasonal discharge patterns further determine current velocity and the retention/advection time of water, which affect phytoplankton development and accumulation (Søballe and Threlkeld 1985; Reynolds et al. 1994). Received September 30, 1998. Accepted August 18, 1999. J14809 C. Hudon. Environment Canada, St. Lawrence Centre, 105 McGill St., 7th floor, Montreal, QC H2Y 2E7, Canada. e-mail: [email protected] Can. J. Fish. Aquat. Sci. 57(Suppl. 1): 16–30 (2000)

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The dynamics of river algae in low-gradient rivers with long retention times were found to be similar to those of shallow, turbid lakes (Reynolds et al. 1994): phytoplankton biomass was inversely correlated with discharge (i.e., residence time) and light penetration (i.e., turbidity) and positively correlated with nutrients. In fast-flowing rivers, by contrast, there was little to no correlation between phytoplankton characteristics and local physicochemical parameters because the phytoplankton originated from a source located upstream, subject to different environmental conditions (Hudon et al. 1996). The phytoplankton composition in large rivers may nevertheless be altered locally, downstream of tributaries or point-source effluents. Depending on the specific morphological characteristics of the watershed, phytoplankton biomass may be produced in different areas of a given river: headbasin (Blue Nile River: Talling and Rzoska 1967), reservoirs (Asahi River Dam Reservoir: Kawara et al. 1998; different U.S. waterways: Søballe and Threlkeld 1985), fluvial lakes (Rideau © 2000 NRC Canada


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River: Basu and Pick 1997; St. Lawrence River: Hudon et al. 1996), old branches and harbour basin (Elbe and Spree rivers: Schorler 1900, cited in Köhler 1994), or resuspension of algae sedimented on the bottom (Murray River: Hötzel and Croome 1996). The phytoplankton composition downstream of these various productive reaches most likely reflects specific habitat characteristics resulting from the movement of water through submerged macrophytes (epiphytic species), resuspended muddy bottom (epipelic species), or slow-flowing pelagic (truly planktonic species) environments. The complex linkages between particular river watershed characteristics, hydrology, chemistry, and phytoplankton dynamics account largely for the difficulties in developing a generalized model of river plankton. This study assesses the seasonal variations in phytoplankton composition and abundance downstream of the confluence of the St. Lawrence and Ottawa rivers. The St. Lawrence River is a clear (2–10 m Secchi depth), low-nutrient river originating in the Great Lakes, whereas the Ottawa River is more turbid (1–2 m Secchi depth), nutrient rich, and drains a multitude of small rivers and lakes carved into the Precambrian Canadian Shield. We hypothesized that such major disparities in water quality and watershed characteristics would make it possible to identify and compare the mechanisms controlling their respective phytoplankton populations. In addition, the occurrence of a drought period in the summer of 1995 provided the opportunity to compare phytoplankton populations under widely divergent summer discharge conditions. This study’s ultimate aim is to assess the potential of phytoplankton as a tool for long-term monitoring of the biological effects of humaninduced or natural ecosystemic changes in the Great Lakes – St. Lawrence watershed.

Material and methods Description of the study area The study area is located 2 km downstream of the confluence of the St. Lawrence River (average annual discharge at Beauharnois 7000 m3·s–1) with the Ottawa River (average annual discharge at Carillon 1980 m3·s–1) in the greater Montreal area (3 million inhabitants) (Fig. 1). Water from the Ottawa River enters the St. Lawrence in part through Lake Saint-Louis and flows alongside the south shore of Montreal Island. The rest of the Ottawa River’s discharge enters the St. Lawrence downstream of Montreal Island via the des Mille Îles and des Prairies rivers. Downstream of the confluence, St. Lawrence River waters flow along the south shore, whereas water from the Ottawa River transits along the north shore, with a median zone of mixed water whose location and dimension vary seasonally with discharge. Owing to their particular physical and chemical characteristics, St. Lawrence waters are readily distinguished from those of the Ottawa River (see Results section below). Phytoplankton and water quality sampling was carried out at 10 stations located along a cross section of the St. Lawrence River. The 2-km-wide river cross section was located 2 km downstream of the confluence, extending from the town of Varennes on the south shore to Repentigny on the north, in order to sample waters subject to the influence of the St. Lawrence River (stations closest to the south shore), the Ottawa River (stations closest to the north shore), and mixed river waters (median stations). Sampling of stations 3, 4, and 9 was interrupted in May 1995 owing to the redundancy of information with adjacent stations. The analysis of the

17 complete 2-year sequence presented here therefore focuses on the remaining stations. Turbulent water mixing in both rivers ensures the vertical homogeneity of the water column, except during the spring freshet, in the case of the Ottawa River, at which time, its waters flow atop those of the St. Lawrence when they meet in the centre of the river (Proulx 1996; Hudon and Sylvestre 1998). Each station was sampled at weekly to monthly intervals (depending on the parameter being investigated) from May 1994 to May 1996. To identify the water masses at the study site, the waters of the St. Lawrence and Ottawa rivers were also sampled at Cornwall and Carillon, respectively, 140 and 70 km upstream of their confluence, respectively (Fig. 1). Physical, chemical, and biological parameters were sampled at each upstream station at fortnightly to monthly intervals (depending on the parameter) from May 1995 to September 1996. Detailed analyses of the results can be found in Cossa et al. (1998) and Hudon and Sylvestre (1998). Water temperature (degrees Celsius) and conductivity (microsiemens per centimetre) profiles were drawn at each station with a HydrolabTM multiprobe to assess the vertical homogeneity of the water. Water samples were pumped (March Meg, model LC-3CP-MD) from 0.2 × total depth or drawn manually from the surface (in winter and early spring) and partitioned for various chemical and biological analyses. Suspended solids were filtered (Whatman GF/F) from a 0.5-L water sample (Environment Canada 1993). Water transparency was estimated using a 20-cm-wide Secchi disk. The light extinction coefficient (K, per metre) was calculated from the slope of the ln-transformed ratio of light intensity at depth z to surface light intensity using LI-COR probes (SA-LI188B and SA-LI192SA) and a LI-COR logger (Li-1000). This value was used to estimate the depth of the euphotic zone (i.e., 1% of incident light = Zeu) and to provide information on the light conditions in relation to total depth (Zt), obtained by depth sounding (Furuno FMV603) at every sampling station. Nutrient (NH4, NO2–NO3, total phosphorus, total dissolved phosphorus, PO4, SiO2) concentrations (milligrams per litre) were determined colorimetrically (TechniconTM autoanalyzer) (Environment Canada 1993).

Phytoplankton composition and abundance Phytoplankton cells were preserved with Lugol acid and then identified and enumerated using the Utermöhl method (Lund et al. 1958; Hamilton et al. 1997). Aliquots of 50 mL were left to settle and examined under a Zeiss inverted phase-contrast microscope; enumeration and identification were carried out on random transects scanned across the settling chamber at 200 and 800×. The entire bottom area of the chamber was scanned for large and rare plankton using a low-power objective (100×). Diatom identification was verified on peroxide-cleaned frustules permanently mounted in HyraxTM and with scanning electron microscopy. Taxa were identified using the monographs of Hüber-Pestalozzi (1938, 1961, 1968), Prescott (1951), Findlay and Kling (1979), Germain (1981), Krammer and Lange-Bertalot (1986–1991), Anagostidis and Komarek (1988), Ettl and Gärtner (1988), and Compère (1989), among others (for a complete list, see Hamilton et al. 1997). All taxa enumerated are catalogued in a photomicrograph collection associated with this study held at the Canadian Museum of Nature, Ottawa, Ontario. A voucher collection from this study is archived in the Canadian Museum of Nature (CANA 53825–53965). Plankton abundance measurements included chlorophyll a, cell density, and biomass. Chlorophyll a was extracted with 90% acetone and measured spectrophotometrically before and after acidification following the procedures of Lorenzen (1967). The total volume of each taxon was calculated from the average dimensions measured on about five specimens using one of seven geometrical shapes (Vollenweider 1969; Wetzel and Likens 1991). The total biovolume was converted to wet weight assuming a specific gravity © 2000 NRC Canada



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Can. J. Fish. Aquat. Sci. Vol. 57(Suppl. 1), 2000

Fig. 1. Study area. Location of the 10 stations across the St. Lawrence River, downstream of its confluence with the Ottawa River in the Montreal archipelago. Municipalities pumping their drinking water supply from the St. Lawrence River are indicated. Sampling points located upstream of the confluence in Cornwall and Carillon are indicated. The bathymetric profile of the St. Lawrence River cross section at the study site is shown (inset).

of 1. Seasonal changes in the phytoplankton in each water mass could then be characterized by total biomass as well as by changes in the representation of taxonomical groups and size-classes.

Statistical analyses The degree of influence of the St. Lawrence and Ottawa rivers and the mixed waters on each station along the Repentigny–Varennes transect throughout the study period was assessed using a principal components analysis carried out on 11 selected variables (suspended solids, temperature, conductivity, NO2–NO3, NH4, total phosphorus, total dissolved phosphorus, PO4, SiO2, chlorophyll a, and log10 fecal coliforms). The position of each of the 11 variables with respect to the first two principal components axes was plotted to assess the explanatory power of each axis and of the position of groups of stations under the influence of the same water mass. The relationships between phytoplankton characteristics and river discharge and physical and chemical water parameters were assessed using parametric (Pearson r) correlation coefficients. Groups of stations under the influence of St. Lawrence and Ottawa River waters were compared in terms of discharge and physical, chemical, and biological characteristics over the summers of 1994 and 1995 (July, August, September samples) using one-way ANOVAs and after verification of the homogeneity of variances between groups.

Results Physicochemical characteristics of the St. Lawrence and Ottawa rivers, upstream and downstream of their confluence Upstream of their confluence, the physical and chemical characteristics of the St. Lawrence and Ottawa rivers differed markedly (Table 1). Owing to the origin of the St. Lawrence River in the Laurentian Great Lakes, waters from this basin were characterized by their extremely low suspended solids concentrations (250 mS·cm–1), and relatively low nutrient concentrations (Table 1). In contrast, the Ottawa River drains the Precambrian Canadian Shield; this river, as well as other tributaries discharging on the north shore of the St. Lawrence, exhibited higher suspended solids concentrations (³ 8 mg·L–1), lower clarity (K = 1.3·m–1) and conductivity (900 and >6500 m3·s–1) never dropped below the threshold fostering the hydraulic retention of algae. It may seem paradoxical that higher plankton biomass should be consistently found in the more turbid waters of the Ottawa River, in the presence of nonlimiting nutrient concentrations for both water masses. This apparent contradiction can be explained by the combination of a number of factors also related to watershed morphology. First, high phytoplankton production could only be maintained in turbid (K = 1.3–1.6·m–1) (Table 1) Ottawa River waters if its average depth were on the order of 3 m to ensure sufficient illumination through the water column. This possibility is unlikely, given the 10–15 m depth of the main channel of the Ottawa River and the comparatively smaller surface area (151 km2) of shallow (2–3 m) Lake des Deux Montagnes (Table 7). In the clear waters (K = 0.3–0.6·m–1) (Table 1) of © 2000 NRC Canada



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the St. Lawrence River, by contrast, the ratio Zeu·Z t–1 decreases from 1 upstream to 0.5 downstream of Montreal in the 15-m-deep main channel, thus indicating generally favourable light conditions for phytoplankton. Second, periphyton biomass and production could be higher in the Ottawa River, since it is supported by higher nutrient concentrations and is concentrated in the shallow portions colonized by macrophytes, such as Lake des Deux Montagnes. This hypothesis should be examined in greater detail in the future. Third (and most likely), the Ottawa River is fed by a very large number of small streams rich in phosphorus (10–100 mg·L–1: Basu and Pick 1996; Primeau 1996) that likely behave in ways similar to those described in the literature (see previous paragraph), producing high phytoplankton biomass (2–27 mg·L–1: Basu and Pick 1996, 1997) under low discharge conditions, only to be flushed out into the larger Ottawa River under rainy conditions. Implications for monitoring long-term changes in water quality and quantity The Great Lakes Basin has been the focus of a phosphorus load reduction program (International Joint Commission 1969). This program led to a reduction in phosphorus and chlorophyll a concentrations in spring (Stevens and Neilson 1987) and to a shift in the relative abundance of major taxonomical groups of phytoplankton, lowering the contribution of Cyanobacteria and Chrysophyta from 20–30% in 1970 to