As a general rule, species of seaweeds with a wide geographical range are eurythermal - that is, they are able to tolerate (or adapt to) a wide range of temperature conditions. However, the kelp species of Western Europe have relatively limited geographical ranges, which suggests that these species are stenothermal - unable to tolerate large fluctuations in temperature on either a temporal or a geographical basis.

The life cycles of kelp plants consist of two phases: the large, upright sporophyte phase and the microscopic, prostrate gametophyte phase. For some species, the temperature tolerance ranges for one or both life phases are known experimentally and, for others, the tolerances may be inferred from their geographical distribution with respect to seasonal ocean isotherms. There are a number of temperature constraints on each phase of the life cycle, and lethal temperatures and temperatures that limit vegetative growth and reproduction have been reported for several kelp species (e.g. table below). From a metabolic point of view, there are upper and lower tolerance limits for growth in both phases of the life cycle, but there are also temperature restrictions for successful gametophyte settlement, spore production by the sporophyte and gamete production by the gametophyte. All these temperature constraints combined may explain some of the different geographical distributions of different kelp species. Where local conditions result in a persistent patch of the kelp forest that is warmer or cooler than the surrounding area, the potential exists for different species of kelp to exploit that area. Localised temperature niches on the south coast of England have enabled several new species of kelp to become established in UK waters in recent times - notably L. ochroleuca and most recently, Undaria pinnatifida.

Upper and lower temperature tolerances for UK species are shown in Table 9. Species of Laminaria have been shown to have different temperature tolerances (with consequent effects on sporophyte growth) at different times of the year. Increased temperatures during the winter months are less well tolerated than increased temperatures during the summer months (Lüning, 1990). Some species (e.g. L. saccharina) form temperature-adapted ecotypes, in which the temperature tolerance of the species varies with location depending on the local conditions to which the plant population has adapted (Davison, 1987).

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