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Climate change is affecting marine ecosystems through warming temperatures and ocean acidification. From 2014-2016, an extensive marine heatwave extended along the west coast of North America and had devastating effects on numerous species during this period, including bull kelp (Nereocystis luetkeana). Bull kelp is an important foundation species in coastal ecosystems, but the impacts of marine heat waves and ocean acidification on specific parts of the bull kelp life cycle have not been fully investigated. To determine the effects of changing temperatures and carbonate levels on Northern California's bull kelp populations, we collected sporophylls from mature bull kelp individuals in Point Arena, CA. At the Bodega Marine Laboratory, we released spores and proceeded to culture microscopic bull kelp gametophytes in a common garden experiment with a fully factorial design crossing modern conditions (11.63±0.54°C and 7.93±0.26pH) with projected climate change conditions (15.56±0.83°C and 7.64±0.32pH). Our results found that both increasing temperatures and decreasing pH influenced growth and egg production of bull kelp microscopic stages. We found that increased temperature and decreased pH had interacting effects on both sporophyte growth and egg production. Increased temperature generally resulted in decreased gametophyte survival and offspring production. In contrast, decreasing pH had less of an effect, but generally resulted in increased gametophyte survival and offspring production. Additionally, we found that increased temperature significantly impacted the rate of production by causing female gametophytes to produce offspring earlier than under low temperature conditions. Our findings inform better predictions of the impacts of climate change on coastal ecosystems as well as provide key insight into environmental dynamics regulating the bull kelp lifecycle.

Climate change is affecting marine ecosystems in many ways, including raising temperatures and leading to ocean acidification. From 2014 to 2016, an extensive marine heat wave extended along the west coast of North America and had devastating effects on numerous species, including bull kelp (Nereocystis luetkeana). Bull kelp is an important foundation species in coastal ecosystems and can be affected by marine heat waves and ocean acidification; however, the impacts have not been investigated on sensitive early life stages. To determine the effects of changing temperatures and carbonate levels on Northern California's bull kelp populations, we collected sporophylls from mature bull kelp individuals in Point Arena, CA. At the Bodega Marine Laboratory, we released spores from field‐collected bull kelp, and cultured microscopic gametophytes in a common garden experiment with a fully factorial design crossing modern conditions (11.63 ± 0.54°C and pH 7.93 ± 0.26) with observed extreme climate conditions (15.56 ± 0.83°C and 7.64 ± 0.32 pH). Our results indicated that both increased temperature and decreased pH influenced growth and egg production of bull kelp microscopic stages. Increased temperature resulted in decreased gametophyte survival and offspring production. In contrast, decreased pH had less of an effect but resulted in increased gametophyte survival and offspring production. Additionally, increased temperature significantly impacted reproductive timing by causing female gametophytes to produce offspring earlier than under ambient temperature conditions. Our findings can inform better predictions of the impacts of climate change on coastal ecosystems and provide key insights into environmental dynamics regulating the bull kelp lifecycle.