Photosynthetic capacity of co-occurring kelp species revealed by in situ measurements

Abstract

Accurate estimates of productivity of marine macrophytes are crucial to predict the consequences of invasive species expansions and climate-related range shifts on coastal ecosystem functioning and carbon cycling. We examined the potential impacts of climate-driven shifts in the relative abundances of foundation species within a temperate reef system on net primary productivity. Specifically, we estimated productivity rates of 3 co-occurring kelp species (Laminaria hyperborea, L. ochroleuca and Undaria pinnatifida) in contrasting regions of their ranges (centre, leading-edge and invasive, respectively) via in situ photorespirometry methods in Brittany, France. Lamina tissues of cold-temperate L. hyperborea and warm-temperate L. ochroleuca had similar rates of net productivity. The stipes of L. hyperborea, however, displayed much higher rates of productivity and respiration than L. ochroleuca, likely due to the dense macroalgal epiphytes colonising their surface. Our results suggest that replacement of L. hyperborea by L. ochroleuca in the NE Atlantic due to increased dominance or further range expansion of this warm-temperate species might lead to reduced net rates of productivity as a result of increased lamina respiration and reduced photosynthetic capacity of stipes. The invasive kelp U. pinnatifida had high rates of net productivity at saturating light levels (294 µmol O2 g-1 dry weight h-1), 3 times higher than any kelp species ever incubated in situ, helping to explain its high success as an invader. We show that shifts in the distributions and relative abundances of seemingly similar foundation species may lead to subsequent changes in net metabolic balance.