Seston fatty acid composition and copepod RNA:DNA ratio with respect to the underwater light climate in fluvial Lac Saint-Pierre

Abstract

The relationship between the underwater light availability at different wavelengths (from 351 to 700 nm) and the fatty acid (FA) composition of seston, as well as the trophic transfer of fatty acids from producers to consumers and its influence on copepod growth condition, were investigated throughout fluvial Lac Saint-Pierre (Quebec, Canada). Seston and zooplankton were collected at 11 sampling sites located within distinct water masses discriminated according to their underwater spectral characteristics. Diffuse light attenuation coefficients (Kd(λ)) varied among sampling sites and wavelengths (λ) and were negatively correlated to seston composition in some essential fatty acids. Particularly, the relationships between Kd(λ) and the seston concentration in 20:5n3 and 22:6n3 differed and were wavelength dependent, being stronger for λ close to the absorption maxima of chlorophyll a, suggesting a potential link with photosynthetic processes. The concentrations of 16:1n7, 18:3n3 and 20:5n3 in copepods were strongly correlated to those in the seston, which points towards the trophic transfer of these fatty acids between primary producers and herbivorous consumers. Moreover, the growth condition of copepods, as expressed by their RNA:DNA ratio, was correlated to the concentrations of 16:1n7, 18:3n3 and 20:5n3 in the seston and in copepods. Our field study sheds light on the potential importance, yet to be precised, of specific wavelengths as a driver of Lac Saint-Pierre’s productivity through their influence on fatty acids composition of seston and its nutritional quality for primary consumers.