Estuarine and Coastal l~r Science (x979) 9, 785-795

Hydrochemistry of Dissolved Inorganic Carbon in the St. LawrenceEstuary (Canada)

E. PeUetier and J. Lebel Dgpartement d'Oc~anographie, Universit~ du Quebec ~ Rimouski, Ra'mouski, Qudbec, Canada GSL 3AI

Received 22 ffanuary x979 and in revised form 7 ffuly x979

Keywords: alkalinity; seasonal variations; chemical balance; St. Lawrence Estuary; Canada east coast

The concentration and the distribution of dissolved inorganic carbon were studied in the Upper St. Lawrence Estuary during three seasons in t976. The data collected have permitted us to bring into prominence that the dilution area directly influenced by the St. Lawrence River (mean annual discharge of x'x3 X xo ~ m ~ s -t) is demarcated by the isohaline of 32"57,~ The seasonal variations of the alkalinity of freshwater can attain 40% but the quantity of bicarbonate ions brought by the river varies less than 8% from season to season. Budget calculations of dissolved CaCOs indicate that 45 • xo' tons per year is provided by the St. Lawrence River. This figure is almost xz times more than all the suspended matter transported from the s ~ n e s o u r c e .

Introduction

The dilution of seawater with freshwater causes many chemical modifications of the species present according to the existing conditions; these modifications can be influenced by the nature of those chemicals implied, the temperature and the pH of the environment, the river ionic contribution, the nature and the concentration of suspended particulate matter, the tidal action and the topography of the region where it mixes, the biological activity and the direct action of man by the pollution and the shore management (Goldberg, x971 ). The complexity and the diversity of the factors that can act on the chemical composition of the waters of such a large zone of mixing as the St. Lawrence Estuary (Canada) caused us to undertake systematic testing of major species in the solution. Among these species, the dissolved inorganic carboff as carbonates, bicarbonates and carbonic acid constitute an important group susceptible of influencing the ionic composition of the mixture. We initiated this study by measuring the alkalinity of several water samples taken during three seasons of 1976. The results of these analyses can contribute to the advancement of the studies related to the influence of the variations of the ionic composition in the salinity-conductivity relation- ship (Poisson, i978; Lewis & Perkins, x978 ) and furthermore they fillow us to specify certain thermodynamic models of seawater (Morel et al., x976; Whitfield, x974). On the other hand, marine environmental chemists and geochemists cart better understand the pH influence on the speciation and precipitation of several ions during the mixing processes (Long & Angino,

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IIydrochemistry of dissolved inorganic carbon 787

z977; Millero, x977); the application of such models to a given estuary necessitates the most precise estimates of the ionic contribution of the river water according to the season.

The aim of this paper is then to give a good description of the seasonal fluctuations of the alkalinity of the river water and the saline water of the St. Lawrence Estuary in order to establish the terrigeneous contribution of the river in dissolved calcium carbonate.

Sampling and analytical methods

Area of study The region chosen for our study is the upper part of the St. Lawrence Estuary, stretching about x7o km from the Ile d'Orlfiarts (near Qu6bec City) to the mouth of the Saguenay River (Figure z). The main source of freshwater (approximately 99% of the total) is the St. Lawrence River with an average annual flow rate of z.z 3 • 4 m a s -1 (Llamas, z974). The fjord of the Saguenay which flows out at the north-east extremity of the area under study pours into the lower St. Lawrence Estuary waters already heavily mixed with seawater. Figure 2 shows the bathymetric details of the area of study. The islands and the shallowness near the south shore of the Upper Estuary limit the accessibility to this area and restrict sampling to the North Navigation Channel which follows approximately the dashed line on

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Figure =. t]athymetric map and profile of the Upper St. Lawrence Estuary in z976.

788 E. Pelletier ~ ft. Lebel

TABLE L Cruises in the Upper St. Lawrence Estuary in z976

Seasons Dates Ships Stations

Spring 3 ~ April to 4 l~lay CSS Dawson s3 Summer 3I July to 2 August Techno-Canada z3 Fall =4 November to =6 November CSS Dawson z6

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the bathymetrie map. The bathymetrie profile of the North Channel (Figure 2) shows three basins varying in depth between 5 ~ and i5o m. This accentuated topography continually forces the deepest saline waters to rise to the surface. Table z furnishes the details of each cruise in the Upper St. Lawrence Estuary during the year x976.

Measurement of the alkalinity The samples used in the total alkalinity measurements are filtered on Millipore | 0"45 lam on board and poisoned with HgCI= to stop biological activity. In the laboratory, at 25 ~ each sample is titrated with HCI o.zoo N; the endpoint is precisely determined by the Gran method (Gran, z95z ) applied to the total alkalinity (TA) determination by Dyrssen and Sillen

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Figure 3. Isohalines and isotherms in the Upper St. Lawrence Estuary in z976.

Hydroehemistry of dissolved inorganic carbon 789

0967). The value obtained is expressed in equivalent in kilogram of water sample (meq kg-X); the carbonate alkalinity (CA) is then calculated according to:

CA = TA--BA--5- 5 X ro-3AT (z)

where BA is the borate alkalinity [KB'ZB/(KB'WaH)] and 5.SXIo-3AT represents the alkalinity contribution of other weak acids. The borate alkalinity can be estimated knowing that the ratio EB/CI(~oo) is constant on the whole salinity range in the St. Lawrence Estuary (Pelletier & Lebel, z978 ). The equilibrium constant of boric acid, Ks' , was computed according to Edmond & Gieskes (z97o) at z 5 ~ and an was calculated from initial pH of each alkalinity titration. The experimental error on CA, determined by duplicate analyse of 38 samples, does not exceed -t-o-oo6 meq kg -1, or d-o'z5% for a sample of salinity 34~oo-

Measurement of the salinity The salinity of all the samples is calculated from the electrical conductivity ratio measured with a Guildline | salinometer. In this paper the chlorinity of a sample corresponds to the one defined by the relation of Cox et al. (x967):

Cl(7oo) = s( oo)/,.8o655

R e s u l t s

Hydrographic data One of the important oceanographic features of the area under study is the weak thermo- haline stratification that can be observed all year long (Figure 3). The main factors which control circulation in this estuary are the tidal movements, the seasonal variations of the freshwater flow rate and the accentuated topography of the bottom of the estuary which causes rising to the surface of the always cold and more saline water coming from the

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Figure 4. Temperature-chlorinity Estuary in x976.

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relationships in the Upper St. Lawrence

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Gulf of the St. Lawrence (Lauzier & Trites, 1958 ). The tidal movements, with an average amplitude of 5 m between the Ile d'Orl6ans and the Ile-aux-Coudres, bring about a reversible tidal current which can attain a speed of seven to eight knots near the Ile-aux-Coudres and move the salting wedge several kilometers between high and low tide (Soucy, 1975). On the other hand, according to the results of Llamas (x974) , the average monthly flow rate of the St. Lawrence River can attain 1.65 • to 4 m 3 s -x at the spring in April and diminishes to less than 1.o • lO 4 s-1 at the end of the summer in August. These factors make the Upper St. Law- rence Estuary a very active area of mixing, with the saline waters constantly renewed, making impossible a detailed interpretation of the salinity and temperature profiles (Figure 3)-

Figure 4 shows that the temperature-chlorinity relationships are almost linearforthethree seasons under study. This observation is a sign that the mixing happens quickly and thatthe mixture does not remain in the estuary very long; the temperature of the water during the mixing is hardly influenced by the air temperature but it is influenced above all by the masses and the temperatures of the surrounding fresh and seawaters.

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Figure 5. Carbonate alkalinity (CA)-chlorinity relationships in the Upper St. Lawrence Estuary in 1976. (Open circles are used for chlorinities higher than 18~o).

Hydrochemlstry of dissolved inorganic carbon 79z

Analytical data The values obtained for the carbonate alkalinity are shown with their relations to the ehlor- inity in Figure 5. For all those samples of chlorinity smaller than 18~oo, the relations obtained are linear and the least squares regression method allows us to write the following equations; in spring:

CA (meq kg-1) = 0.0645 Ci(~oo)+O.942 (2)

in summer:

in fall:

CA (meq kg-1) = o.o4o 5 Cl(~oo) + 1"382 (3)

CA (meq kg -1) = o.o 351 Cl(~oo) + 1.47 ~ (4)

The constants on the right indicate the carbonate alkalinity for the freshwaters entering the estuary. This non-negligible contribution makes the carbonate alkalinity/chlorinity ratio variable not only on the whole salinity range but also on a seasonal scale.

Discussion

The study of the alkalinity-ehlorinity relationship in estuaries and coastal areas was of little interest until recently. In his study of the coastal waters of Oregon (U.S.A.), Park (i966, 1968 ) showed that the alkalinity-salinity ratio for waters of depths of more than 200 m with a salinity greater than 34~oo is linear and has no seasonal variation. On the other hand, for the waters near the coast and near the surface, an important seasonal variation, due to the combined action of the rain and of the Columbia River, has been observed.

As in this example, the alkalinity of the St. Lawrence River shows important seasonal variations (Figure 5). Most of these variations are brought on by the flood in the spring and the rain in the fall. Moreover it can be observed that the saline waters diluted by the St.

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CI (%o) Figure 6. DJscontinuitT of the carbonate alkalinity-ehlorinity relationships: I , data from the Upper St. Lawrence Estuary in the fall x976; ", data from the North Atlantic Ocean in the summer z977; m, data from the GEOSECS program in the North Atlantic Ocean (Millero et al., t976).

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Lawrence River are distinguished from the seawater coming from the St. Lawrence Gulf and the North Atlantic by a discontinuity in the alkalinity-ehlorinity ratio of the kind described by Park et al. (i969). Such a discontinuity is shown in Figure 6 where we first observe that the three straight lines of dilution for the Upper St. Lawrence Estuary converge at an average value of 2.xo 5 meq kg -1 for a chlorinlty of *8~oo. The value of the carbonate alkalinity of the St. Lawrence water at this chlorinity is independent of the seasonal fluctuation of the run-off and of the concentration of bicarbonates of freshwater. This observation allows us to suggest that the isohaline of 32"5~oo (CI = I8~oo) is the line which shows the approximate boundary of the dilution area directly influenced by the St. Lawrence River (Figure 3, Fall, shaded zone). This hypothesis is confirmed by the alkalinity results obtained from samples of chlorinity greater than X8~oo taken from the Lower Estuary and the North Atlantic Ocean (Figure 6). The data shown in this figure were picked up from our own measurements and from the results of the GEOSECS program in the North Atlantic Ocean as reported by iMillero et al. (I976). These carbonate alkalinity values for waters of chlorinity higher than x8~oo are clearly observed above the prolongation of the three lines of dilution calculated for the Upper St. Lawrence Estuary. The weak dilution of the oceanic water coming into the Gulf and Lower Estuary is brought about by a slow process of diffusion betweentheupper and middle layers (Lauzier & Trites, x958 ) and has no direct relation with the more turbulent dilution of the Upper Estuary. So, this observation is important when the ionic concentration is calculated in estuaries in order to evaluate the effects of the variations of composition on the conductivity-salinity-density relationships (Poisson, x978; lX{illero, x975). In those calculations, for the St. Lawrence Estuary, the dilution equation will be linear up to the chlorinity limit of x8~oo (Figure 6). Beyond this value the freshwater's ionic contribution becomes negligible. We note that the extrapolation of the straight line for chlorinities higher than ,8~oo reaches the origin. So the carbonate alkalinity/chlorinity ratio of these results is constant (mean--o.xI56-t-o.ooo6meq/~oo) and equal to the value observed in oceanic waters. These carbonate alkalinity values are the result of the dilution of seawater by fresh- water of salinity zero and are independent of the estuarine mixing.

TABLE 2. Carbonate contribution of the St Lawrence River

Freshwater Dissolved runoff at inorganic

CA HCO,- iMontmagny carbon discharge Seasons meq kg-* mg kg-* to ~ m' s -* kg s-*

Spring 0"942 57"3 x5"3 874 Summer x.382 84"0 9'9 829 Fall x "47o 89"4 to '7 953 Seasonal mean x'z65 76"9 x2.o 886

Bicarbonate ions tn the freshwater of the St. Lawrence River (Table 2) were calculated from: (a) our carbonate alkalinities of freshwater; (b) the apparent dissociation constants (K x' and Kz' ) of carbonic acid in freshwater, furnished by ~,Iook & Koene (i975); and (c) the pH values of the St. Lawrence River waters'(Subramanian & D'Anglejan, x976 ). We first observe that the value of 84.o mg kg -x for the summer of x976 is identical with that furnished by Livingstone (1963) for a measurement taken in August ,958. The similarity of the two results in a x 6 years' interval indicates the p6rmanence of the erosion area of the St. Lawrence River on one hand and that the increasing human activity has little or no influence on the dissolved inorganic carbon. The Livingstone's (x963) data show that the bicarbonate ions are

Hydrochemistry of dissolved inorganic carbon 793

70% of the total anionic concentration in the freshwater of the St. Lawrence River. If we suppose that this relative concentration of HCO 3- is constant due to the permanence of the erosion, the seasonal variations by 3o--4o% observed for the bicarbonate ions imply a non negligible variation of the total ionic contribution which must be considered in the application of models describing the dilution process.

The average seasonal variation of the freshwater runoff (Table 2) measured at Montmagny (Figure 2) attains 35% in the spring in relation to the annual average of I 'I 3 • lO 4 m3s -1 The variation of the freshwater runoff is inversely proportional to the variation of the concentration of bicarbonate ions; the combination of the two results for the calculation of the discharge of inorganic carbon dissolved under the form of HCO a- (Table 2) shows that the seasonal variation hardly rises above 7.6% in relation to the average of the three seasons and that the greatest difference is in the fall. It is noteworthy that the quantity of dissolved inorganic carbon (principally HCO~-) carried by the river varies little from season to season and that consequently the erosion of the carbonate rocks of the drainage basin is practically constant. During the spring flood, the time of contact between the freshwater and the sedimentary rocks of the St. Lawrence basin is very short. At this period, the flow rate is at its maximum but it appears that no more dissolution of calcareous is observed during this period compared to the longer residence time taking place in the summer or fall. An identical phenomenon was demonstrated for the Mackenzie River and its tributaries (Reeder et al., 1972 ) . Considering that the flow rate of freshwater is weak in winter like in the summer and that dissolution takes place approximately in the same way during the two seasons, we can estimate that the winter concentration of bicarbonate is of the order of 85+ 5 mg kg-X. This concentration coupled with art average monthly freshwater run-off of 9"3 • m3 s - t (Llamas, x974) gives a flow rate of dissolved inorganic carbon in the order of 7904-50 kg s-1 for the winter. Our results and this last estimation allow us to conclude that the quantity of dissolved inorganic carbon (under the form of HCO~-) brought annually by the St. Lawrence River is in the order of 274-4 million metric tons. According to the steady state model of the world ocean, the inorganic carbon is withdrawn from the ocean water solution in the form of biological CaCO.~, and about 45 million tons of calcareous sediment must be deposited on the ocean floor. As a comparison, we remind that Loring & Nota (1973) estimated the annual total weight of the suspended particulate matter brought by the same source to be 3"9 million tons.

Conclusion

Our work on the hydrochemistry of inorganic carbon of the St. Lawrence Estuary allowed us to bring into prominence the dilution zone directly influenced by the St. Lawrence River, limited by an isohaline of 32-5~o which is usually found at the height of the Saguenay sill. The seasonal variations of the alkalinity of freshwater can attain as much as 4o% but considering the flow rate of the river, the quantity of bicarbonate ions brought by the river varies less than 8~o from one season to another. We have estimated that the St. Lawrence River provides annually to the ocean approximately 45 million tons of calcium carbonate under a dissolved form, being almost z2 times more than all the suspended particulate matter brought from the same source.

In the future, these results on the alkalinity variations in the St. Lawrence Estuary will be correlated with measurements of the pH in situ together with the calcium concentrations in order to evaluate the freshwater contribution to the calcite saturation of some estuarine waters.

794 E. Pelletler & ft. Lebel

Acknowledgements

The authors would like to thank the crew and the captain of the 'Techno-Canada ' and of the 'CSS Dawson' for their contribution to the success of their cruises in the estuary. The authors are indebted to the Bedford Institute of Oceanography and the Minis t ry of Environ- ment of Ottawa who allowed them free use of the CSS Dawson for the spring and fall cruises of I976. The financial support necessary to carry out this work was assumed by the National Research Council of Canada (grant A-4265) , by the Direction de l 'enseignement Supdrieur du Qurbec (gram FCAC-EQ-Ioo9) and by the Fonds Insti tutionnel de l 'Universit6 du Qudbcc ~t Rimouski. The authors wish to express their sincere gratitude to Dr M. Khalil. member of their department, for critically reading the manuscript.

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Hydrochemlstry of dissolved inorganic carbon 795

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