Kelp in hot water: I.Warming seawater temperature induces weakening and loss of kelp tissue

Recent declines and losses of highly productive and diverse kelp beds have been observed worldwide and linked to increases in ocean temperature. We investigated the impacts of 4 temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of the dominant kelp species in Nova Scotia: Saccharina latissima, Laminaria digitata and Agarum clathratum. Growth rates of A. clathratum were reduced at 18°C over 3 wk of exposure, and all species experienced negative net changes in length at this temperature. Exposure to 21°C led to tissue loss at least twice that observed at 11°C and mortality within the first 2 wk of exposure. Exposure to 21°C for 1 wk reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% in S. latissima and L. digitata, and all 3 species exhibited reduced strength after 3 wk exposure to 18°C. Histological examination of the blade tissue showed temperature-induced damage to the cellular structure of blades of S. latissima and L. digitata. A. clathratum displayed limited tissue damage and was less susceptible to temperature-induced tissue weakening and loss. Our findings provide a mechanism by which rising temperatures could contribute to observed population declines of kelp species.