Environmental factors structuring benthic primary producers at different spatial scales in the St. Lawrence River (Canada)

The influence of environmental factors controlling the biomass of submerged aquatic macrophytes, cyanobacterial mats, and epiphyton was examined at three nested spatial scales within the St. Lawrence River: (1) along a 250-km-long upstream-downstream river stretch, (2) among three fluvial lakes located within that river stretch and (3) within each fluvial lake at sites located upstream, at the mouth, and downstream of the St. Lawrence River tributaries. Over its 250-km-long course, large increases of water colour (fivefold), suspended matter (tenfold), dissolved organic carbon (DOC) (twofold) and dissolved N and P concentrations (2.5-fold) were observed in the St. Lawrence River, showing the cumulative effects of human activities on water quality. In contrast, biomass of submerged vascular macrophytes dropped tenfold along the sampled reach whereas biomass of epiphytes and cyanobacterial mats rose significantly. Biomass of the three benthic primary producers (PP) was explained (59 %) by the combined effects of conductivity, TP and spatial structure. Macrophyte biomass was related to changes in conductivity (+), biomass of epiphyton responded to DIN:TDP ratio (+) and light extinction coefficient (+) and cyanobacterial mats coincided with differences in DOC (+) and NH4 (+) (-). Within-lake structure was the most important spatial component for all benthic PP, suggesting that local effects, such as enrichment by the inflow of tributaries, rather than upstream-downstream gradients, determined the biomass of benthic PP. Our study shows that the sum of local tributary inflows exerts major overall pressures on benthic PP in the St. Lawrence River and that conversely, small-scale management of individual watersheds, can markedly improve local ecological condition of the river ecosystems.