Functionality of photobiological traits of the giant kelp (Macrocystis pyrifera) as key determinant to thrive in contrasting habitats in a sub-Antarctic region

Because of its large size and foundational role, the form and function of the giant kelp Macrocystis pyrifera define key responses to the environmental shifts and ecosystem services. The present study compared several morphological, bio-optical and fluorescence-based photobiological traits as well biomass allocation patterns of the kelp in three sites with different environmental settings along the west coast of the sub-Antarctic strait of Magellan. The morpho-functional and bio-optical characteristics of the algae varied between the sites, following differences in underwater light and tidal range between Atlantic (Buque Quemado and San Gregorio) and Pacific (Bahía Buzos) sectors. Traits measured in blades and individual thalli contributed differently to the total variability within the giant kelp populations. The individuals from the intertidal muddy flats from Buque Quemado differed in many traits, especially biomass allocation along the thallus and bio-optics, with respect to the subtidal rocky assemblages from San Gregorio and especially Bahía Buzos. Photosynthetic characteristics revealed shade adaptation with E_k values normally ≤400 μmol m⁻² s⁻¹. In San Gregorio, a site with lower water transparency, light requirements coincide with irradiances at depths between 11 and 4 m, while E_k values estimated for Bahía Buzos indicated photosynthesize at depths >20 m.