Thermal performance curves for Lessonia corrugata

Kelps are in global decline due to climate change, including ocean warming. To identify vulnerable species, we need to identify their tolerances to increasing temperatures and whether tolerances are altered by co-occurring drivers such as inorganic nutrient levels. This is particularly important for those with restricted distributions, which may already be experiencing thermal stress. To identify thermal tolerance of the range restricted kelp Lessonia corrugata, we conducted a laboratory experiment on juvenile sporophytes to measure performance (growth, photosynthesis) across its thermal range (4 – 22 °C). We found the upper thermal limit for growth and photosynthesis to be ~ 22 – 23 °C, with an optimum of ~ 16 °C. To determine if elevated inorganic nitrogen availability could enhance thermal tolerance, we compared performance of juveniles under low (4.5 µmol/day) and high (90 µmol/day) nitrate conditions at and above the thermal optimum (16 – 23.5 °C). Nitrate enrichment did not enhance thermal performance at temperatures above the optimum but did lead to elevated growth rates at the thermal optimum 16 °C. Our findings indicate L. corrugata is likely to be extremely susceptible to moderate ocean warming and marine heatwaves. Peak sea surface temperatures during summer in eastern and northeastern Tasmania can reach up to 20 – 21 °C and climate projections suggest that L. corrugata’s thermal limit will be regularly exceeded by 2050 as south-eastern Australia is a global ocean-warming hotspot. By identifying the upper thermal limit of L. corrugata we have taken a critical step in predicting the future of the species in a warming climate.

受气候变化（包括海洋变暖）影响，全球海带类（kelps）种群正经历全球性衰退。为甄别易受胁迫的物种，我们需明确其对温度升高的耐受能力，以及共存的环境驱动因子（如无机营养盐浓度）是否会改变该耐受能力。对于分布范围受限的物种而言，这一点尤为关键——这类物种或许已经面临热胁迫。为明确分布受限的昆布属物种皱叶昆布（Lessonia corrugata）的热耐受能力，我们针对其幼孢子体开展了室内控制实验，在其适宜温度区间（4℃至22℃）内测定其生理表现（生长速率、光合能力）。实验结果显示，该物种生长与光合作用的最高热耐受极限约为22℃至23℃，最适温度约为16℃。为探究无机氮有效性提升是否能增强其热耐受能力，我们在热最适温度（16℃）及以上温度区间（16℃至23.5℃）内，分别设置低浓度（4.5 μmol/天）与高浓度（90 μmol/天）硝酸盐环境，对比幼孢子体的生理表现。结果表明，硝酸盐富集无法在高于热最适温度的区间内改善其热生理表现，但在热最适温度16℃下可显著提升其生长速率。本研究结果显示，皱叶昆布极有可能极易受适度海洋变暖和海洋热浪的影响。塔斯马尼亚州东部与东北部海域夏季表层海温峰值可达20℃至21℃，而由于澳大利亚东南部是全球海洋变暖热点区域，气候预测显示，到2050年，该海域海温将经常性突破皱叶昆布的热耐受极限。通过明确皱叶昆布的最高热耐受极限，我们为预测该物种在气候变暖背景下的未来生存状况迈出了关键一步。