Abstract

Giant kelp (Macrocystis pyrifera) covers large coastal areas along the Comau Fjord (Northern Patagonia), across different environmental gradients that determine its structural complexity. In the present study, we compared the morphological (thallus length and wet biomass, holdfast diameter, blade morphology, etc.) and photochemical characteristics based on PAM chlorophyll fluorescence (Effective Quantum Yield and P-E curve parameters) of six populations along the Comau Fjord distributed in three sectors: Lilihuapi Island (1 population), Cahuelmo (2 populations) sector and Comau Fjord inside (3 populations). The main results indicated that, at an individual level, different structural conformations of M. pyrifera populations where only observed at the mouth of the fjord, where a well-established “kelp forest” was identified. In contrast, in areas inside the fjord, populated by “patches" and narrow “kelp belts”, no differences in biomass and thallus length were recorded. However, the morphological differences between individuals of these populations were related with the size of blades, with blade area being larger in the inner parts of the fjord compared to the other sites. Regarding the photochemical characteristics, in general, the algae showed shade adapted features, with algae from Cahuelmo exhibiting higher ETR values and light requirements (Ek) compared to the other sites. These findings indicating large blade area and lower light demands for photochemistry could be related to a photo-acclimation strategy to respond to the light gradients along the fjord, marked by high water column stratification. Here, the presence of high mountains, especially towards the fjord head, determines the degree of exposure and availability of light for photosynthesis. This study is the first morphological and photochemical characterization of natural populations of M. pyrifera in this area of Northern Patagonia and underlines the importance of morpho-functional traits of kelps to endure environmental variability in an understudied area threatened by global climate change and also local anthropogenic activities. © The Author(s), under exclusive licence to Springer Nature B.V. 2025.