Nutritional quality of biofilms with respect to light regime in Lake Saint-Pierre (Quebec, Canada)

1. In situ experiments were conducted using specialised incubation devices to grow biofilms under varying light regimes and grazing intensities (by excluding fish and large-sized zooplankton, >2 mm) both within and between two sites in Lake Saint-Pierre. 2. Biofilms growing under greater in situ UVR and light exposures found in the south water mass were characterised by a greater biomass and nutrient content, but their total fatty acid (FA) contents and ratios of elemental nutrients were not significantly different from the north. There was a relatively greater abundance of chlorophytes and cyanobacteria in the south water mass, along with a greater proportion of low nutritional quality saturated fatty acids (SAFA). Conversely, biofilms growing in the north had a greater relative abundance of diatoms, as well as greater eicosapentaneoic acid (20:5omega3) and docosahexaneoic acid (22:6omega3) concentrations (two FAs implicated in the physiological competency of grazers). 3. The prevailing community structures created differences in terms of nutritional status of the biofilms for benthic grazers and their predators at the two sites. The biofilms from the southern site were characterised by greater food quantity at the expense of quality, while biofilms from the northern site contained less food of a better quality. Despite this, the nutritional regime in the south supported a greater productivity at higher trophic levels. The secondary treatments (light and grazing by fish and macro-invertebrates) had lesser effects on food quality.