Lake primary producer community responses to anthropogenic use

Anthropogenic activity around a lake is usually accompanied by eutrophication of its waters. While the relation between eutrophication and dissolved nutrients is established, direct relationships between anthropogenic activity and biotic variables related to lake eutrophication are less well studied, especially in early stages. We evaluated the effect of what is still a relatively light human presence on primary producer communities in lac Lusignan, a north temperate lake (Quebec, Canada). Human recreational activities around the lake have increased in recent years, primarily towards the lake outflow. To account for varying degrees of human activity and vegetation type along the lake's shoreline, we classified sites into four types: human housing (Inhabited) and forest (East-forest) on the eastern bank and peat bogs (Bog) or forests (West-forest) along the western bank. We measured dissolved nutrients (nitrates and phosphorus) in the water column, as well as four primary producer responses: cyanobacterial density, littoral periphyton thickness, and macrophyte biomass and diversity. We expected that more intensively used areas of the lake (Inhabited East bank sites) would have thicker periphyton, higher macrophyte biomass and diversity, and greater cyanobacteria density; all characteristics related to eutrophication and anthropogenic influence. Nitrate was higher along the western shore sites and phosphorus concentrations tended to be higher in Bog and Inhabited areas. Periphyton was thickest in Inhabited sites, while macrophyte biomass and diversity were reduced. Cyanobacterial communities were particularly dominated by the potentially toxic and nuisance genus Dolichospermum in Inhabited sites. Our study demonstrates symptoms of eutrophication in this historically unperturbed lake and indicates that attention should be paid to regulating the densification and spread of human activity to preserve water quality.