Photosynthetic pigments of co-occurring Northeast Atlantic Laminaria spp. are unaffected by decomposition

Abstract

Along Northeast Atlantic coasts, the genus Laminaria dominates kelp forests. Two species, L. digitata and L. hyperborea, are cold temperate whilst the third, L. ochroleuca, is warm temperate. In northern France and the southern British Isles, all 3 species now co-occur, with a gradual rise in predominance of L. ochroleuca evident over recent decades due to climate-driven range shifts. Despite recent focus on the export of photosynthetically viable macroalgal detritus, the effect of decomposition on detrital pigment concentration remains unknown for northern hemisphere kelps. Furthermore, notwithstanding their importance in determining habitat suitability, the photosynthetic pigments of all 3 species have not yet been assessed comparatively within the same forest. Here, we present results of a field experiment aimed to test potential changes in the pigments of decomposing kelp blade fragments. Using spectrophotometry and Gaussian peak spectral deconvolution to quantify pigments, we found that shallow benthic decomposition over 1 mo did not affect major photosynthetic pigment concentrations in any of the examined species. Moreover, the 2 boreal species were similar in their sporophyte pigment stoichiometry and had more chlorophyll a, chlorophyll c, fucoxanthin and minor carotenoids (β,β-carotene and zeaxanthin) than L. ochroleuca. This resulted in total pigment content that was 82 and 74% higher in L. digitata and L. hyperborea than in L. ochroleuca. These differences correspond approximately to each species’ latitudinal distribution and photosynthetic performance. Our results suggest photosynthetic viability of Northeast Atlantic kelp in the initial detrital phase and a potential reduction of forest-scale photosynthetic capacity under continued ocean warming.