The effect of temperature on rates of dissolved organic carbon (DOC) release by the kelp <i>Ecklonia radiata</i> (phylum Ochrophyta): Implications for the future coastal ocean carbon cycle

Dissolved organic carbon (DOC) released by macroalgae is an intrinsic component of the coastal ocean carbon cycle, yet knowledge of how future ocean warming may influence this is limited. Temperature is one of the primary abiotic regulators of macroalgal physiology, but there is minimal understanding of how it influences the magnitude and mechanisms of DOC release. To investigate this, we examined the effect of a range of temperatures on DOC release rates and physiological traits of Ecklonia radiata, the most abundant and widespread kelp in Australia that represents a potentially significant contribution to coastal ocean carbon cycling. Juvenile sporophytes were incubated at eight temperatures (4-28°C) for 14 days, after which time, DOC concentrations and physiological traits (growth, photosynthesis, respiration, F_v/F_m, photosynthetic pigment content, and carbon, and nitrogen content) were analyzed using thermal performance curves (TPCs) or regression analyses. Thermal optima were 15.63°C for growth and 25.84°C for photosynthesis, highlighting vulnerability to future ocean warming. Dissolved organic carbon concentrations increased when the temperature was above ~22°C, being greatest at the highest temperature tested (28°C), which was likely driven by photosynthetic overflow and thermal stress. Mean F_v/F_m, total chlorophyll, and total fucoxanthin content were lowest at 28°C. The C:N ratio of blades increased linearly with temperature from 23.9 ± 1.30 at 4°C to 33.0 ± 1.22 at 28°C. We demonstrate increased DOC release by E. radiata under elevated seawater temperatures and discuss potential implications for coastal carbon cycling under future ocean warming given the complex and uncertain fate of macroalgal DOC in the marine environment.