Ocean Acidification, Not Warming, May Reduce Pteropod Abundance in the Mediterranean Sea

Shelled pteropods represent an excellent sentinel for indicating exposure to ocean acidification (OA). Here we characterise pteropod distribution throughout the Mediterranean Sea which has been identified as a climate change hot-spot. The presence of a West-East natural biogeochemical gradient makes this region a natural laboratory to investigate how the variability in environmental parameters may affects pteropod distribution. Results show that pteropod abundance is significantly higher in the eastern Mediterranean Sea, where Ωar is higher, suggesting that, even in saturated waters, relatively small declines in Ωar can affect pteropod population. We also observed a resilience of pteropods to higher temperatures and low nutrient conditions, including phosphorus limitation, which indicates that a warming ocean may offset the negative impacts associated with OA. The higher abundance of pteropods in ultraoligotrophic conditions (eastern Mediterranean Sea) suggests that this organism can play an important role as the prime calcifying zooplankton within specific oligotrophic regions.