PHYSIOLOGICAL RESPONSES OF ECKLONIA RADIATA (LAMINARIALES) TO A LATITUDINAL GRADIENT IN OCEAN TEMPERATURE1

We tested the ability of sporophytes of a small kelp, Ecklonia radiata (C. Agardh) J. Agardh, to adjust their photosynthesis, respiration, and cellular processes to increasingly warm ocean climates along a latitudinal gradient in ocean temperature (～4°C). Tissue concentrations of pigment and nutrients decreased with increasing ocean temperature. Concurrently, a number of gradual changes in the metabolic balance of E. radiata took place along the latitudinal gradient. Warm‐acclimatized kelps had 50% lower photosynthetic rates and 90% lower respiration rates at the optimum temperature than did cool‐acclimatized kelps. A reduction in temperature sensitivity was also observed as a reduction in Q₁₀‐values from cool‐ to warm‐acclimatized kelps for gross photosynthesis (Q₁₀: 3.35 to 1.45) and respiration (Q₁₀: 3.82 to 1.65). Respiration rates were more sensitive to increasing experimental temperatures (10% higher Q₁₀‐values) than photosynthesis and had a higher optimum temperature, irrespective of sampling location. To maintain a positive carbon balance, E. radiata increased the critical light demand (E_c) exponentially with increasing experimental temperature. The temperature dependency of E_c was, however, weakened with increasing ocean temperature, such that the critical light demand was relaxed in kelp acclimated to higher ocean temperatures. Nevertheless, calculations of critical depth limits suggested that direct effects of future temperature increases are unlikely to be as strong as effects of reduced water clarity, another globally increasing problem in coastal areas.