Étude paléolimnologique de l'histoire trophique du lac Saint-Charles, réservoir d'eau potable de la Communauté Urbaine de Québec Paleolimnological analysis of changes in the trophic status of Lake Saint-Charles, a drinking water reservoir for the Québec urban community

Résumé

Cette étude utilise une approche paléolimnologique pour reconstituer l'histoire trophique du réservoir d'eau potable de la Communauté Urbaine de Québec (CUQ), le lac Saint-Charles. Ce lac manifeste présentement un manque d'oxygène près du fond à la fin de la stratification estivale et hivernale. L'étude révèle des changements dans la communauté diatomifère fossile depuis environ les 150 dernières années. L'événement ayant entraîné le plus de changements biologiques et physico-chimiques dans le bassin est la transformation hydrologique engendrée par la construction d'un barrage en 1934 qui éleva le niveau du lac d'environ 1,5 à 2 mètres. Par conséquent, il y eut des changements dans la structure des communautés de diatomées avec des effets sur le ratio espèces planctoniques / benthiques, sur la paléoproductivité et les caractéristiques physico-chimiques des sédiments suivant cette période. Les assemblages diatomifères indiquent que les conditions mésotrophes se sont maintenues pendant toute la période étudiée. L'analyse du phosphore total dans les sédiments et la reconstitution à partir des diatomées fossiles du phosphore total dans l'eau montrent une légère diminution de la concentration en phosphore avec le temps. Ces observations démontrent qu'il n'y a pas eu d'accélération du processus d'eutrophisation engendrée par les activités humaines. Par contre, l'analyse géochimique des sédiments révèle un apport plus important de métaux depuis la fin du 19e siècle, qui atteint un plateau vers la fin des années 70.

Mots-clés:

• Paléolimnologie,
• diatomées,
• phosphore total (PT),
• fonction de transfert,
• eutrophication,
• pollution,
• Québec

Abstract

This study adopted a combined paleolimnological-limnological approach towards evaluating the trophic history of Lake Saint-Charles, the drinking water reservoir for a population of 305,000 in the Québec City (Canada) region. Our limnological measurements indicate that the lake is currently in a state of advanced mesotrophy. Water column profiles during late summer stratification (September) showed that the bottom waters were anoxic, enriched in phosphorus (up to 17 µg total P L^-1) and had a pH almost 2 units lower than the surface waters. Surface phytoplankton concentrations were high at this time of year with Chlorophyll a concentrations of up to 12 µg·L^-1. At the end of winter stratification, oxygen concentrations were below saturation at all depths, ranging from 72% immediately under the ice to 4% at the bottom of the water column. This tendency towards eutrophic conditions was offset, however, by a rapid flushing rate (mean hydraulic residence time=23 days). Because there are concerns that the lake has experienced accelerated nutrient enrichment due to increased human activities in its drainage basin, the objectives of our paleolimnological approach were to document the recent trophic history of this lake, to estimate the extent of recent changes in trophic status, and to identify critical periods of past anthropogenic disturbances from the fossils of siliceous algae (diatoms; class Bacillariophyceae) preserved in its sediments. Quantitative estimates of past total phosphorus (TP) concentrations in the water column of Lake Saint-Charles were obtained by applying a diatom-TP reconstruction model developed for 54 lakes located in south-eastern Ontario to fossil diatom assemblages from a 28 cm long sediment core. The timing of changes in the fossil diatom record was estimated by²¹⁰ Pb dating. The study reveals changes in fossil diatom assemblage composition during the past ca. 150 years, with the most striking biological and physico-chemical changes occurring immediately after 1934. This date coincides with the construction of a dam, which raised the lake water level by 1.5-2 m. This modification was accompanied by significant shifts in diatom community structure, especially in the planktonic/benthic ratio (with increases in planktonic diatoms Cyclotella stelligera and Aulacoseira distans), and by changes in the physico-chemical characteristics of the sediments. Paleoproductivity increased at the same time, but remained more or less stable following conservation efforts between 1950 and 1970 (e.g., construction of a sewage treatment system). The organic matter content of the sediments showed an increase in the order of 20% between 1850 and 1950, after which it remained constant. Fossil diatom community structure indicates that mesotrophic conditions have prevailed during the recent history of Lake Saint-Charles, and that diatoms typical of eutrophic conditions never became established in the lake. The geochemical analysis of phosphorus in the sediments as well as the diatom-inferred quantitative reconstruction of lake water total phosphorus reveals a slight decrease in total phosphorus over time, from close to 17 µg·L^- prior to 1887 to about 13 µg·L^- 1 in recent times. These observations suggest that Lake Saint-Charles has not experienced significant recent changes in trophic status due to increased human activities in its drainage basin. However, our geochemical analyses show a sharp rise in metal concentrations (especially Fe, Mn, Cu, Pb and Zn), beginning in the late 19th century, reaching a plateau by the late 1970s, which may be attributed to increased atmospheric pollution since the beginning of intense human colonization in the lake's catchment and surrounding areas. This in combination with the advanced mesotrophic status of the lake indicates the ongoing need for careful management of the watershed to prevent further changes in this important urban water resource.

Keywords:

• Paleolimnology,
• diatoms,
• total phosphorus (TP),
• transfer function,
• eutrophication,
• pollution,
• Québec