Executive Summary Brown kelps of the order Laminariales are the most important habitat structuring macroalgae species along temperate to polar rocky coastal ecosystems, growing in dense forests and supporting different marine communities. At most sites several kelp species co-occur but often one species is dominant. Latitudinal biographic distribution of seaweeds depends on temperature requirements and temperature tolerance for growth and reproduction, while the major factor for determination of depth zonation was found to be the usceptibility of kelp spores to irradiance, especially UV radiation. However, the character and intensity of interspecific competitive interactions, either by using more effectively the available resources or by direct interactions with allelochemicals, are also very important in the process of formation and functioning of any seaweed community. This study focused on competitive interactions between gametophytes and sporophytes of two cold-temperate kelp species from Kongsfjorden (Spitsbergen): Alaria esculenta and Laminaria digitata. Two laboratory experiments were performed at three different temperatures (5, 10 and 15°C). Gametophytic and sporophytic development was followed during two months by determining developmental stages (vegetative gametophytes, oogonia, egg cells and sporophytes) under the microscope, measuring sporophytic size and weighing fresh and dry sporophytic biomass. During this study, it was quantitatively documented a clear interspecific sporophytic resource competition at 5°C, in which A. esculenta displaced L. digitata when these two kelp species were cultured together under low light condition (12±1 μmol photons/m2s). In addition, intraspecific sporophytic resource competition was also verified at 5°C, whereas interspecific gametophytic interference competition, probably through allelochemicals, was qualitatively observed. A. esculenta gametophytes developed faster at 10°C than 5°C, while its gametogenesis was drastically retarded at 15ºC. L. digitata gametophytes developed the fastest at 10°C, then 5°C and then 15°C, while its sporophytes developed faster at 15°C than 5°C. Since these kelp species grow differently depending on the temperature, the rising of the sea water temperature may change their distribution, causing new interspecific interactions and competition with other seaweeds that could also influence the marine environment. Moreover, other factors such as life cycle stage, nutrient concentration, temperature and light intensity can regulate the intensity of interspecific competition.These results demonstrate that interspecific kelp interactions are complex and variable, while not much is known about this topic. Thereby, future multifactorial and field studies are needed to draw more accurately final conclusions.