The boreal kelp Laminaria digitata dominates the low intertidal and upper subtidal zones of moderately exposed rocky shores in north-western Europe. Due to ocean warming, this foundation species is predicted to disappear from French coasts in the near future. Photosynthesis of L. digitata sporophytes was surveyed in situ during spring and summer tidal cycles with emersion periods around midday during hot, sunny weather. The net production (NP) of whole individuals (i.e. the difference between their gross primary production and respiration) was assessed by measuring carbon fluxes inside a closed chamber. Photosynthetic performance of thalli was assessed using pulse-amplitude modulated (PAM) fluorescence parameters, the effective (ΦPSII) and optimal (Fv/Fm) quantum yields of photosystem II. Content in pigments involved in the xanthophyll cycle, a photoprotective mechanism, was measured in thalli to evaluate the de-epoxidation ratio (DR). NP shifted from positive values (reaching 140 μmol C gDW-1 h-1) during morning immersion to negative values uduring emersion (reaching -37 μmol C gDW-1 h-1), and did not return to positive values during the following immersion period when respiration was exacerbated. ΦPSII decreased during emersion (down to 0.01), but recovered during afternoon immersion. Fv/Fm decreased during emersion (down to 0.18) indicating severe photoinhibition. High DR values (up to 0.70) showed the effectiveness of the photoprotective mechanism, which appeared nevertheless insufficient to prevent photodamage during emersion stress. Among the environmental factors contributing to this emersion stress, repeated heat shocks over consecutive tide cycles likely play a leading role. These repeated heat shocks appear to further exacerbate the detrimental effects of warming events on this marginal population of L. digitata.