PARENT SESSION
Contributed Oral Session 43: Landscape Ecology
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 518 A, Level 5, Palais des congrès de Montréal

Spatial patterns in macrophyte beds in Lake Saint-François (St.Lawrence River, Québec, Canada).

Léonard, Rosalie^*,1, Jean, Martin², Bouchard, André¹, ¹ Institut de recherche en biologie végétale, Montréal, Québec, Canada² Centre Saint-Laurent, Montréal, Québec, Canada

ABSTRACT- Landscape analysis approaches are used in ecosystem studies to investigate the interrelationships between spatial patterns and ecological processes. In freshwater environments, the assemblage of diverse macrophyte species (having different propagation, competition and resource utilization strategies) can result in a variety of responses to the heterogeneity patterns inherent in the landscape. The aim of this study was to investigate spatial patterns in the macrophyte communities in Lake Saint-François, a fluvial lake of the St.Lawrence River. Two-hundred thirty-three quadrats distributed over 24 transects perpendicular to shore were sampled to identify the distribution and abundance of macrophyte species. Environmental, landscape, geographical and temporal data were also obtained. Cluster analysis discriminated two plant communities in the study zone: a first community (A) represented by Chara sp. and a second (B) dominated by Butomus umbellatus f. valisneriifolius, Vallisneria americana, Potamogeton richardsonii and Myriophyllum sp. Variation partitioning of the species abundance matrix, using redundancy analysis, identified a significant spatial structure explained by landscape elements. Furthermore, the spatial analysis utilizing geographical coordinates revealed a strong southwest/northeast gradient in the vegetation and environmental variables. The two contrasted macrophyte groups identified by the cluster analysis are positioned on either side of the confluence of the La Guerre River with Lake Saint-François: community A is located southwest (upstream) and community B is located northeast (downstream) of the confluence. The same gradient was observed in water pH values, in sediment phosphorus content, and in sediment sand and silt fractions. The results suggest that the degradation of the littoral zone downstream from the La Guerre River, due to inputs of fine particles and nutrients from the surrounding agricultural lands, could be at the origin of the southwest/northeast changes observed in the data. However, water depth northeast of the La Guerre River confluence tends to be greater. Thus mineral and fine particle accumulations in the sediments could be attributed to sedimentation, which is known to occur in Lake Saint-François in deep areas unaffected by wave action.

Key words: spatial structure, macrophytes, Lake Saint-François, landscape