Abstract
Due to the importance of brown algae, such as kelp (Laminariales, Phaeophyta), within most cool nearshore environments, any direct responses of kelp to multiple global changes could alter the integrity of future coastal marine systems. Fifty‐five‐day manipulation of carbon dioxide (CO 2 ) and ultraviolet light (UVB) within outdoor sea‐tanks, approximating past, present and two predicted future levels, examined the direct influences on Saccharina latissima (= Laminaria saccharina ) and Nereocystis luetkeana development and biochemistry, as well as the indirect influences on a marine herbivore ( Tegula funebralis ; Gastropoda, Mollusca) and on naturally occurring intertidal detritivores. Kelp species displayed variable directional (negative and positive growth) and scale responses to CO 2 and UVB manipulations, which was influenced by interactions. Kelp phlorotannin (phenolic) production in blade tissues was induced by elevated UVB levels, and especially enhanced (additively) by elevated CO 2 , further suggesting that some actively growing kelp species are carbon limited in typical nearshore environments. Negative indirect effects upon detritivore consumers fed CO 2 ‐manipulated kelp blade tissues were detected, however, no statistical relationships existed among UVB‐treated tissues, and test herbivores did not distinguish between phlorotannin‐altered CO 2 : UVB‐treated kelp blade tissues. Results suggest that past and future conditions differentially benefit these kelp species, which implies a potential for shifts in species abundance and community composition. Higher CO 2 conditions can indirectly impede marine decay processes delaying access to recycled trace nutrients, which may be disruptive to the seasonal regrowth of algae and/or higher trophic levels of nearshore ecosystems.