Understanding how climate change may influence marine ecosystems depends substantially on its effects on important foundation species. The giant kelp (Macrocystis pyrifera), is one such foundation species that provides critical food and habitat for ecologically and economically important coastal communities and may be locally adapted along its Western Hemisphere range to temperature and pH conditions. However, the potential for local adaptation in this species is poorly understood despite its broad distribution along strong pH and temperature gradients and strong barriers to dispersal. To test this possibility, we exposed giant kelp early life stages sourced from genetically disparate populations – three in California and four in Chile – to fully factorial common garden experiments. We used two temperature and two pH treatments representing the range of in situ conditions experienced in each hemisphere. Experiments were carried out with the California populations at the Bodega Marine Laboratory (California) and with the Chilean populations at the Universidad de Los Lagos (Puerto Montt, Chile). We measured growth and development at weekly intervals throughout the experiment and preserved all tissue for later transcriptomic and isotopic analysis.
Results/Conclusions
All populations displayed significantly different growth and rates of development in treatment conditions indicating population-specific tolerance ranges to temperature and pH change. Across populations, overall growth and proportion of sporophytes after five weeks was greatest in high pH, low temperature treatments and least in high pH, high temperature treatments. All populations were negatively affected by high temperature with temperature-induced mortality greatest in the Chilean populations, suggesting a greater vulnerability to climate- or ENSO-driven warming events among Southern Hemisphere populations. This is the first study to explicitly test for population-level differences in giant kelp response to environmental change and the only to test such a wide latitudinal gradient. Results have important implications for understanding and modeling the future range shifts of giant kelp under a changing climate with projected declines in pH and projected increases in sea surface temperatures.