Elevated temperatures create bottlenecks in the life history of bull kelp (Nereocystis luetkeana, Phaeophyceae)

Abstract

The bull kelp Nereocystis luetkeana has declined in parts of the Salish Sea due to local and global change factors, including rising sea surface temperatures. As a foundation species, its loss has broad impacts on biodiversity and nearshore ecosystem function, underscoring the need to identify and address mechanisms of decline. Elevated temperatures affect all stages of the bull kelp life cycle, but it remains unclear whether some stages are more vulnerable than others. To assess stage-specific sensitivity, I examined the effects of four temperatures (10, 12, 15, and 18°C) on the development of gametophytes and young sporophytes cultured from field-collected sori over four weeks. The density of female gametophytes declined significantly at 15°C and 18°C, while the density of male gametophytes remained consistent across temperatures, leading to greatly reduced sex ratios over time. Gametophyte reproductive success was severely limited at 18°C, suggesting a thermal threshold for successful recruitment. Sporophyte production was reduced at 15°C and nearly absent at 18°C, driven by the reductions in the density of female gametophytes and impaired reproductive success at elevated temperatures. These findings reveal critical bottlenecks in bull kelp maturation and recruitment, with serious implications for population persistence and ecosystem resilience in a warming ocean.