Hydrodynamic control of the underwater light climate in fluvial Lac Saint-Pierre

We measured characteristics of the underwater light spectra (e.g., attenuation of ultraviolet [UV] radiation, photosynthetically active radiation) and select dissolved and particulate physicochemical properties (e.g., chromophoric dissolved organic carbon [CDOM], dissolved and particulate organic carbon, inorganic dry weights, beam attenuation coefficients, particulate absorption coefficients, and nutrients) in different water masses of fluvial Lac Saint-Pierre (Canada). We used these variables as tracers to reveal the extent and magnitude of spatial and temporal heterogeneity in this large, shallow, fluvial lake of the St. Lawrence River. We superimposed these tracer variables over radiance data obtained from satellite images to identify spatial and temporal changes in the distribution of different water masses and their bio-optical components. The underwater light environment showed strong horizontal (longitudinal and lateral) variability because of the strong connectivity between the terrestrial and aquatic environments in the lake’s tributaries and adjoining wetlands. Analyzing the downstream distribution of optical and chemical variables as a function of transport time rather than distance from source tributaries allowed us to demonstrate large differences in the age of the different water masses depending on the characteristics of the source tributary, in-stream processes, and distance from its source. CDOM explained most of the UV attenuation and allowed the greatest discrimination between water masses. Nature is organized along three spatial dimensions, a fact which contributes to its complexity. Traditionally the physical, chemical, and biological processes occurring within lake ecosystems have been described mostly in two dimensions, that is, with emphasis on bivariate combinations of physical, chemical, and biological variables over time, but especially with depth. Such variables include changes in light attenuation and vertical distributions of plankton and predators, factors that are strongly influenced by stratification (epi-, meta-, and hypolimnion), mixing regimes within the water column, or both. This approach has been applied successfully to the pelagic zone of lakes on the basis that this area is the most representative portion of the lake (in volume), despite the pre-eminent importance of the littoral zone in terms of productivity (Wetzel 2001). This might, in part, explain why shallow littoral stations are typically undersampled compared with deep stations, even though they are the most productive regions in lakes. Littoral zones are also the sites of terrestrial influxes of nutrients and of lightattenuating organic and inorganic matter, resulting in large differences in underwater light climate between inshore and offshore regions of lakes (Frenette and Vincent 2003). This horizontal (i.e., inshore–offshore) heterogeneity is further accentuated in fluvial lakes (especially in fluvial lakes with multiple inflows), which can result in the formation of chemically and therefore spectrally distinct water masses within the lake (Frenette et al. 2003). In fluvial lakes with large width/depth ratios, such water masses often persist and can exhibit reduced lateral mixing downstream (Frenette et al. 2003), conferring a unique pattern of Acknowledgments We thank Christine Barnard, Pierre-André Bordeleau, Olivier Champoux, Kim Huggins, Marianne Lefebvre, Marie-Audrey Livernoche, Sylvain Thélème, and Geneviève Trudel for their invaluable help in the field and in the lab. We thank captain Guy Morin of the ship Le Pêcheur of the Meteorological Service of Canada, Environment Canada, and captain Roger Gladu for sharing their experience and knowledge of Lac Saint-Pierre. Warwick Vincent critically reviewed this manuscript. This research was funded by the Natural Sciences Research Council of Canada (NSERC) and the Fonds Québéquois de la Recherche sur la Nature et les Technologies (FQRNT) to J.-J.F.; National Water Research Institute (NWRI), Environment Canada to M.T.A.; and Meteorological Service of Canada to J.M. This study is a contribution of the Groupe de Recherche Interuniversitaire en Limnologie (GRIL). Limnol. Oceanogr., 51(6), 2006, 2632–2645 E 2006, by the American Society of Limnology and Oceanography, Inc.