Field study on natural phytoplankton throughout “Bizerte City” oil spill on the south-western coast of the Mediterranean Sea

Oil spills are recurrent worldwide. Assessing the response of phytoplankton—the basis of marine food webs—at the early stages of an oil spill and throughout its evolution is crucial to improve our understanding of the impact of oil spills on the marine environment. Field data collected 1, 4, 8, and 18 days after the “Bizerte City” oil spill showed that phytoplankton responded differentially over time. In the short term (1–8 days), picophytoplankton biomass and abundance increased, possibly owing to reduced grazing. In contrast, nano- and microphytoplankton biomass decreased, probably owing to inhibited growth of species sensitive to polycyclic aromatic hydrocarbons (PAHs)—the most toxic components of oil. After 18 days, the dispersal of oil and its decreasing negative effect were accompanied by outbreaks of all size fractions. Accordingly, the phytoplankton size structure shifted throughout the oil exposure level from a prevalence of microphytoplankton after a few days toward picophytoplankton dominance. Oil pollution influenced the species composition and significantly decreased diversity indexes. In the first days, nanophytoplankton was dominated by Cryptophyceae (mainly Hillea fusiformis and H. marina), while microphytoplankton was mostly represented by the pennate diatoms Pseudo-nitzschia and Nitzschia, suggesting a better resistance of these genera to oil. Algal recovery after 18 days was associated with high proliferation of nano-sized Chaetoceros and micro-sized Astrionellopsis glacialis diatoms. These results improve our knowledge of the impact of oil pollution on coastal phytoplankton communities and reinforce the idea of using them as bioindicators.