Sub-canopy light conditions only allow low annual net productivity of epiphytic algae on kelp Laminaria hyperborea

The stipes of older Laminaria hyperborea individuals are heavily covered by epiphytic assemblages that are dominated by macroalgae, and we hypothesized that the production of these algae may contribute significantly to total primary production of the kelp forest ecosystem. The epiphytic assemblages on the stipes of Laminaria were dominated by potentially fast-growing red algae with total biomass ranging from 25 to 120 g dry weight (DW) m−2 seafloor depending on season and site. Sub-canopy light conditions were poor and averaged only ~10% of the surface irradiance in summer. Photosynthetic profiles of the epiphytic assemblages indicated that they were acclimated to shade, and the poor sub-canopy light conditions were nevertheless sufficient to ensure positive net photosynthesis for at least 2 to 3 h daily throughout the year. Photo synthetic efficiency at low light and dark respiration varied seasonally, which led to a threefold increase in minimum light requirements in early fall, simultaneously with high water temperatures and declining surface irradiance. This resulted in low productivity and net carbon loss from September throughout winter. Net productivity became positive in February–March and rose through spring as surface irradiance increased. Annual net productivity was relatively low, ranging from 42 to 96 g DW m−2 seafloor depending on site. We conclude that the net productivity of these macroalgal epiphytes is insignificant relative to that of kelp itself, and that the large observed biomass needs several years to accumulate.