Annual variation in leaf canopy across a seagrass distribution range is modulated by local environmental variation

ABSTRACT Seagrasses create three-dimensional habitats (i.e. meadows) of paramount conservation relevance, which are distributed across large spatial scales, under varying environmental conditions. Cymodocea nodosa (Ucria) Ascherson is a warm-temperate seagrass, distributed along the entire Mediterranean and the adjacent Atlantic Ocean. As with many seagrasses, this species shows an annual (seasonal) pattern in canopy vitality, with maximums in leaf canopy structure (e.g. shoot density and leaf length) in spring-summer, while canopy leaf structure is reduced in winter. Since this seagrass experiences varying environmental (e.g. thermal) scenarios across its distribution range, we tested whether annual (seasonal) variation in the leaf canopy structure, in terms of shoot density, leaf length and above-ground biomass, are modulated by local environmental (here, thermal) variation. Annual variation in the shoot density of C. nodosa across its distribution range was not predicted by local environmental regimes (thermal regimes, in terms of variation in sea surface temperature, SST). However, annual variation in leaf length and biomass was significantly predicted by variation in local thermal regimes. A larger annual environmental variation, here in terms of SST, was connected with a more marked seasonal variation in leaf canopy structure, both in terms of leaf length and biomass. If a major effect of climate change is an increase in environmental variation, an increase in the annual leaf canopy variation for C. nodosa meadows along its distribution range might be expected.