Photosynthesis of Laminaria digitata sporophytes was surveyed in situ throughout spring and summer tidal cycles including emersion periods around midday. Photosynthetic performance of thalli was assessed through pulse-amplitude modulated (PAM) fluorescence parameters, the effective (Φ PSII) and optimal (F v /F m) quantum yields of photosystem II. Net production (NP) of entire individuals, the balance between their gross primary production and respiration, was assessed by measuring the carbon fluxes inside a closed chamber. Thalli content in pigments involved in the xanthophyll cycle, a photoprotective mechanism, was measured to evaluate the de-epoxidation ratio (DR). Φ PSII decreased at emersion (down to 0.01) and recovered under afternoon immersion. NP shifted from positive values (reaching 140 μmol C g DW-1 h-1) under morning immersion to negative values under emersion (reaching-37 μmol C g DW-1 h-1) but did not recover under following immersion when respiration was exacerbated. Fv/Fm decreased at emersion (down to 0.18) indicating a severe photoinhibition. High DR (up to 0.70) showed the effectiveness of the photoprotective mechanism which appeared nevertheless insufficient to prevent photodamages under emersion stress. Among environmental factors contributing to this emersion stress, heat shocks repeated over consecutive tide cycles were suspected to play a leading role. This reinforces the expectation of detrimental effect of warming events on this marginal population of L. digitata.