Ecological vs. climate uncertainty in future marine ecosystems: Lessons learned from krill in a major upwelling region

Krill is a central organism in the food web of many marine ecosystems and eastern boundary current upwelling regions specifically. Here, a superensemble of climate and ecological models is used to determine drivers of future change, variability, and uncertainty in krill abundance for the California Current. While krill is projected to slowly decrease throughout the 21st century, the long-term trend consistently exceeds natural variability only under extreme warming. Similarly, unprecedented low krill years are expected to progressively increase, but their frequency of occurrence will depend on background abundances tied to low-frequency climate variability. The relative contributions of warming rate and ecological model formulation to projected uncertainty are comparable and reflect latitudinal changes in the magnitude of climate forcing and availability of empirical data to parameterize krill models. This finding highlights the fact that uncertainty in climate change impacts on coastal upwelling ecosystems may depend as strongly on model formulation as they do on anthropogenic forcing. Furthermore, the increasingly divergent krill model responses outside of the core domain for which they were originally implemented advocate for regionally tailored projections and models to reduce overall uncertainty. By identifying and quantifying uncertainty sources in future krill abundance across relevant time scales, the present study lays the foundation for understanding how the superposition of long-term trends, low-frequency variability, and extreme events may lead to unprecedented ecosystem states, and for assessing their broader impacts on altered presence, distribution, and recovery of species that directly or indirectly depend on krill.