Changes in the spectral and chemical properties of a water mass passing through extensive macrophyte beds in a large fluvial lake (Lake Saint-Pierre, Québec, Canada)

Abstract

Large fluvial lakes, as part of river corridors, are recognized as spatially heterogeneous ecosystems. Due to their shallowness, the littoral zone of these lakes occupies a large proportion of their surface and is extensively covered by macrophytes that are known to affect their physical, chemical and biological environments in various ways. This study documents the small-scale (5 km) bio-optical variations associated with the longitudinal passage of a water mass through macrophyte beds during their maximal growth season. The utilization of a 2D hydrodynamic model allowed us to establish hydrological connectivity between stations within the beds, and thus to identify longitudinal heterogeneity along the macrophyte beds. Significant changes in the inherent properties of the water were observed along the upAquat. Sci. 67 ( Aquatic Sciencesstream-downstream gradient. Due to their effects on hydrodynamics, macrophytes were responsible for a decrease in particles and dissolved organic carbon (DOC) resulting in an increasing penetration depth of ultraviolet (UV) and photosynthetically active radiation (PAR). Along the transect, chromophoric dissolved organic matter (CDOM) decreased more rapidly than DOC resulting in a decrease of the CDOM to DOC ratio. The observed changes in the DOC pool may be explained by the constant input of non-chromophoric DOC from macrophyte leachates and exudation within the bed and/or the photochemical and microbial degradation of CDOM. The implication of such small-scale heterogeneity on Lake Saint-Pierre productivity is discussed.

Effects of upstream lakes on dissolved organic matter in streams

“…The sources of DOM will likely also differ between streams with and without upstream lakes. Organic matter entering north temperate forested streams is often dominated by terrestrial inputs (Hinton et al 1998;Elder et al 2000) but lake algal or macrophyte production in upstream lakes may contribute significant quantities of DOM to outflowing streams (Martin et al 2005). Upstream algal blooms have previously been observed to contribute significant quantities of DOM to streams (Kaplan and Bott 1982), but we are unaware of any study that has linked upstream lake productivity with DOM quantity in outflowing streams.…”

“…Compared to free-flowing systems, water emanating from upstream lakes has experienced much greater residence time and, consequently, exposure to processing agents that can influence DOM properties. Autochthonous production of algal and macrophyte DOM also is likely to be greater in lakes because of their lack of vegetative canopies and long residence times (Martin et al 2005). Such DOM is likely to have lower molecular weight, be less absorptive of light, and have different biodegradability than terrestrially derived humic matter (Kreutzweiser and Capell 2003;Mash et al 2004).…”

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

“…UV photochemical (oxidative) reactions and microbial processes might increase UV radiation penetration in the water column because of photobleaching of allochthonous CDOM (Gibson et al 2000;Osburn et al 2001) and bacterial production of autotrophic DOM by degradation of recalcitrant CDOM (Wetzel et al 1995;Osburn et al 2001). For example, sampling along a 5-km longitudinal gradient in the SaintFrançois River water mass revealed a downstream decrease in the CDOM/DOC ratio, revealing a structural change in the DOM pool during a 9-h segment of the 3-5-d transit of the water mass through Lac Saint-Pierre (Martin et al 2005). Such changes in inherent and emergent properties were not observed in the St. Lawrence water mass.…”

Effects of macrophytes and terrestrial inputs on fluorescent dissolved organic matter in a large river system

“…Recent work has focused on the allochthonous FDOM originating from wetlands and terrestrial ecosystems (Hood et al, 2005;Mladenov et al, 2007) and on the autochthonous FDOM derived from microbial and phytoplankton sources (McKnight et al, 2001;Cory and McKnight, 2005;Hood et al, 2005). Macrophytes provide an important source of DOM in several environments (Martin et al, 2005;Fischer et al, 2006;Maie et al, 2006), yet little is known about the fluorescence properties of this DOM, especially in temperate aquatic ecosystems. Considering their abundant biomass in shallow, productive environments, macrophytes are likely to contribute significantly to the pool of FDOM that fuels the microbial loop.…”