Thermal performance curves identify seasonal and site-specific variation in the development of Ecklonia radiata (Phaeophyceae) gametophytes and sporophytes

Rapid ocean warming is affecting kelp forests globally. While the sporophyte life stage has been well studied for many species, the microscopic life stages of laminarian kelps have been understudied, particularly regarding spatial and temporal variations in thermal tolerance and their interaction. We investi-gated the thermal tolerance of growth, survival, development, and fertilization of Ecklonia radiata gametophytes, derived from zoospores sampled from two sites in Tasmania, Australia, throughout a year, over a temperature gradient (3-30 degrees C). For growth we found a relatively stable thermal optimum at-20.5 degrees C and stable thermal maxima (25.3-27.7 degrees C). The magnitude of growth was highly variable and depended on season and site, with no consistent spatial pattern for growth and gametophyte size. Survival also had a relatively stable thermal optimum of -17 degrees C, 3 degrees C below the optimum for growth. Gametophytes grew to single cells between 5 and 25 degrees C, but sporophytes were only observed between 10 and 20 degrees C, indicating reproductive failure outside this range. The results reveal complex effects of source population and season of collection on gametophyte performance in E. radiata , with implications when comparing results from material collected at different localities and times. In Tasmania, gametophytes grow considerably below the estimated thermal maxima and thermal optima that are currently only reached during summer heatwaves, whereas optima for survival (-17 degrees C) are frequently reached and surpassed during heatwaves, which may affect the persistence and recruitment of E. ra-diata in a warmer climate.

Automated summary

Rapid ocean warming is impacting kelp forests globally. This study examined the thermal tolerance of Ecklonia radiata gametophytes and found that growth and survival had stable thermal optima while development and fertilization were limited to specific temperature ranges. The results highlight the complex effects of source population and season on gametophyte performance, with implications for the persistence and recruitment of E. radiata in a warmer climate.