Influence of the vertical structure of macrophyte stands on epiphyte community metabolism

The physical structure of submerged aquatic plant communities differentially influences the availability of light and substratum in the water column and, thus, the functional role of epiphytes growing on macrophytes. We examined the depth distribution of photosynthesis and respiration of epiphyte communities within macrophyte stands of contrasting growth forms over a 2-year period in Lake Saint-Pierre (St. Lawrence River). To do so, we used a modelling approach, combining laboratory measurements of photosynthesis and respiration with field data of macrophyte and epiphyte biomass and vertical light attenuation. In stands dominated by canopy-forming macrophytes, shading resulted in strong vertical gradients in epiphyte metabolism, with a positive net oxygen balance in the canopy and a negative net oxygen balance in the bottom portion of the stand. In low-growing macrophyte stands, the net oxygen balance of epiphytes was either positive or negative, depending on water transparency and depth. Epiphyte communities had a daily negative net oxygen balance under light conditions below ~10% of surface light intensity. Areal production simulations demonstrated that neglecting variations in the vertical distribution of epiphytes, macrophytes, and light within macrophyte stands can result in errors in areal production estimates of >100%.