Loss of wetlands in the last 50 years

Wetlands are among the most productive ecosystems on our planet. The richness of these transitional ecosystems relates mostly to the diversity of ecological niches created by the variability of seasonal and interannual cycles. Modifications in the hydrologic regime that disturb these cycles have been found to be the main stress factor threatening shoreline wetlands in all the world's major rivers (Mustow 1995). The regulation of water levels has also caused the shrinkage of wetlands, and a concomitant reduction in the diversity of plant communities and the number of plant species (Keddy and Reznicek 1986). Owing to the difficulty of distinguishing between natural and anthropogenic factors and, more importantly, the scope of the analysis that would be required, we were not able to develop a numerical index for the stress associated with the control of water levels on the St. Lawrence.

The percentage of wetland losses resulting from anthropogenic disturbances can be assessed, however, by comparing the data on historical wetland losses and the data on remaining wetland habitat. These two datasets are available for roughly a kilometre-wide band on either side of the river.

Wetland losses recorded between 1945 and 1978 in the Cornwall–Île d'Orléans section (Le Groupe Dryade Ltée. 1981) are attributable mainly to the filling and draining of wetlands to develop housing and transportation infrastructures and to agricultural lands. A total of 36 km^² of wetland was lost during this period, with the Montreal physiographic unit sustaining the greatest losses at 9.4 km^².

In the Cornwall–Île d'Orléans section, wetlands lost historically represent 5% of the wetlands surveyed in the entire section (MEIS images, 1990-91, and Grenier 1991). Although it appears unlikely that such losses could have exerted significant pressure on biodiversity on a regional scale, on a local scale, the available remaining habitat may have been reduced to the extent that a decline in biodiversity could be ascertained within the associated biological communities.

On the scale of physiographic units, the relationship between the loss of wetlands and remaining wetlands is more significant. For example, historical wetland losses in the Quebec City physiographic unit represent roughly 30% of the area of current wetlands. Even if accumulated historical losses are less significant in Quebec City than in Montreal (3.4 km^² versus 9.4 km^² for Montreal), the area of remaining wetlands in Quebec City is significantly smaller (11.6 km^² versus 66.2 km^² for Montreal). Although a direct link has not been proven between the extent of wetland losses and current observed biodiversity, it is crucial to protect the wetlands in the Quebec City physiographic unit to maintain biodiversity.

In addition, MEIS images was analyzed to determine the distribution of the various categories of wetlands along the St. Lawrence. Physiographic units in the upstream portion of the river (from Lake St. Francis to Lake St. Pierre) are characterized mainly by beds of submergent vegetation dominated by water-celery and European water-milfoil, which are in turn associated with marshes of emergent plants such as bulrushes, arrowhead, bur-reed and cattails along with wet meadows, the latter being typical of the area's floodplains. In the fluvial section east of Lake St. Pierre, which is subject to freshwater tides, the abundant beds of submergent vegetation are associated with bulrush (Scirpus americanus) marshes, which take on increasing important farther downstream, as one enters the fluvial (freshwater) estuary. The influence of saltwater is felt beginning in the brackish-water estuary (Lower St. Lawrence region) with its cord-grass marshes, and increases further downstream in the lower estuary, dominated by rockweed and kelp beds. Since 1950, the greatest losses of wetland habitats have occurred in beds of aquatic vegetation and cord-grass marshes. The loss of cord-grass marshes, caused mainly by draining of land for agricultural use, has been fairly recent (after 1978).

Enlargements of the Quebec City and Montreal physiographic units are also shown. As the map shows, the main wetland categories affected in the Quebec City physiographic unit are bulrush marshes and wet meadows, which have been filled in along the Beauport flats and drained in the Côte de Beaupré region.

In the Montreal physiographic unit, losses mainly involve beds of submergent vegetation that have been filled in along the St. Lawrence Seaway and drained in the portion southwest of Nuns' Island.

Modifications in natural habitats resulting from bank erosion and lower water levels

Given the fact that close to one third (1,400 km) of the shoreline in the river, estuary and Gulf have been modified by human activities, it is not surprising that that there are only forty or so sites along the shoreline with undisturbed hydroseres representative of their respective ecoregion (Picard et al. 1997). Shoreline development has been particularly destructive in the upper part of hydroseres, consisting mainly of swamps. Close to twenty tree associations and a dozen shrub associations have almost disappeared from the fluvial section of the St. Lawrence between Cornwall and Sorel, where more than 60% of the shoreline has been developed. In addition, several species of rare plants have disappeared from Nuns' Island and around fifty priority species around Lake St. Louis and the La Prairie Basin are currently listed as threatened (Gratton and Dubreuil 1990).

Bank erosion

The loss of riparian habitat has been caused not only by intentional shoreline modifications but also by various forms of erosion. The St. Lawrence is a major waterway giving ships access to the Great Lakes. Every year, there are more than 10,000 ship transits in the shipping channel, which was developed in the 1950s. All shoreline located less than 600 m from the channel is subjected to the wake generated by passing ships and hence to wake erosion (D'agnolo 1978). Commercial shipping has been estimated to be responsible for close to 20% of shoreline erosion on the St. Lawrence, with 5% attributable to pleasure craft, which are particularly numerous around the Berthier-Sorel Islands (Dauphin, in prep.). Other factors contributing to shoreline erosion include water level variations, wind-generated waves, ice and currents. Between Cornwall and Quebec City, it is estimated that roughly 400 km of shoreline suffers from erosion, or 25% of all the shoreline in this section of the river (Argus 1996). The effects are the greatest between Montreal and Sorel, with 50% (270 km) of the shoreline affected by erosion. The next most affected sections are between Lake St. Pierre and Quebec City, with 20% of the shoreline affected, and upstream of Montreal, with 10% affected. Erosion has caused particularly severe damage to the many islands between Montreal and Sorel, where it has been estimated, based on aerial photographs taken in 1964 and 1983, that the average annual losses are 3 m along 70 km of the shoreline, 1.25 m along 80 km of shoreline and less than 1 m a year along 100 km of shoreline (Argus 1991). Therefore, close to 1,500 ha of island habitat has probably been lost in this section of the river since the opening of the St. Lawrence Seaway in 1959.

Map – Erosion of islands in the fluvial section (coming soon)

Many of the islands in question provide protection for swampy habitats further inland on the islands by shielding them from the impact of the waves and currents. Swamps serve as crucial spawning grounds for several species of fish (Shooner et Associés 1990) and also provides ideal breeding habitat for waterfowl (Bélanger 1989). The loss of 1,500 ha of island wetland habitat over the last 40 years may have prevented the production of close to 40,000 broods of dabbling ducks and thus resulted in the loss of nearly 50,000 ducks in this part of the fluvial section alone.

Water levels

The hydrological regime—and more specifically the frequency, duration and seasonal fluctuations in water levels in the river corridor—exert a pervasive influence on the ecology of many plant and animal species. Water level variations along the St. Lawrence are influenced by multiple factors (Bergeron 1995). For example, water levels on Lake St. Francis are affected by the regulation of flow from the Great Lakes and the associated water control infrastructures. Water levels in the section between Lake St. Louis and Quebec City are affected by both the flow from the Great Lakes and from the Ottawa River. The Great Lakes supply 83% of the water in the St. Lawrence near Montreal and 60% of that near Quebec City. In the estuary and Gulf, water levels are also influenced by very strong winds and tides, while precipitation, strong winds, ice jams and tides have more local effects.

Several studies have shown that the flow in the St. Lawrence at Montreal could decrease by 40% in the next 50 years (see Robichaud et al. 1998 for a synthesis). If this prediction turns out to be accurate, major effects on St. Lawrence ecosystems and their living components are to be feared. Waterfowl could be especially hard hit since, during spring migration, over 600,000 aquatic birds stage on the floodplains of Lake St. Pierre (see Use of the St. Lawrence in spring by dabbling ducks).

Fortunately, low water levels would only affect around 5% of the birds since most of them concentrate in areas managed for wildlife (Dombroski and Dolan, in prep.), where water levels are controlled by the adjacent watershed or a pumping system.

Map – Sectors most vulnerable to a drop in water levels (coming soon)

An appreciable decrease in water levels could also have serious repercussions on breeding waterfowl by modifying the plant cover on many islands. A high water table (at least 20 cm from the surface in summer) is required to support the herbaceous vegetation dominated by rushes; annual flooding of at least two weeks during the growing season is also required (Marsan et Associés 1986). The absence of flooding, a drop in the water table or a significant long-term decrease in the duration of flooding could promote the growth of more terrestrial species such as shrubs and trees. According to the Groupe Dryade (1985), the regeneration in past decades of tree species on Lake St. Louis may be linked to a period of exceptionally low water levels during the 1930s. If this occurs again, as seems to be the case now, close to 4,000 ha of wet meadow could be modified over the medium term in the island habitats between Montreal and Sorel. This would also affect roughly 70% of duck broods in this area, which corresponds to around 1000 nests, or roughly 25% of the annual production of dabbling ducks for the entire St. Lawrence.

Fish communities in the freshwater section could also be impacted by lower water levels. In particular, seasonal periods of low flow could significantly decrease the area of floodplains, which serve as spawning and nursery grounds for over twenty fish species (Arsenault 1995). Lower water levels could also affect fish communities in the estuary since the reduced flow of fresh water would move the salinity front upstream and with it, the larval retention zone for several species including rainbow smelt and Atlantic tomcod, two species whose populations are declining. The hatching and larval drift periods of these two species are perfectly timed so that, at exactly the right time, the fish are carried downstream to a part of the river that is particularly dynamic and nutrient rich. Should the mixing zone shift further upstream, this timing could be disturbed (Arsenault 1995).



Denis Lehoux and Luce Chamard