Climate change is fundamentally altering global ocean living conditions for marine ecosystems, both by long-term trends and by increasingly extreme perturbations of physical conditions. Oceanic phytoplankton communities can mainly respond to these stresses through the evolutionary adaptation of resident taxa and/or the migration of better-acclimatized organisms to other parts of the ocean. Quantifying the relative importance of these two mechanisms is critical for assessing the overall impact of climate change and to better predict how marine ecosystems will respond. Here we present work that shows how estimates of surface-ocean connectivity can be used to calculate the migration timescales of warm-tolerant planktonic organisms and be compared with predicted rates of change of sea surface temperature. We find that for the majority of the ocean migration timescales are short enough to compensate for long-term warming, suggesting that the immigration of novel types will be the key mechanism by which marine planktonic communities adapt.  

The major exception to these findings are regions, mainly in the tropical ocean, that are warmer than surrounding waters and therefore experience local temperature maxima. Here warm-tolerant organisms have to travel long distances over several years to supplant the local population with genetic material that is better adapted to new conditions. The Red Sea and the Persian Gulf (one of the warmest Large Marine Ecosystems - LMEs) currently experiencing the highest temperatures act as a critical seed bank of warm-tolerant species. These temperature-defined ecosystem niches will see a greatly expanded spatial niche by 2100 (increasing from 0.15% to 38% of the global surface ocean). However, these presently warm areas are also at the highest risk of extirpation and will likely rely on evolutionary adaptation to maintain viable populations.  

This potentially increasing dependency of regions already experiencing conditions expected in climate change scenarios to act as a source for temperature-adapted organisms suggests a significant risk of a tipping point of marine ecosystems and is of great concern from a management and conservation perspective. It is of critical importance to better understand these ecosystems and assess their resilience, especially in light of the environmental stress that the Red Sea and Persian Gulf experience today.