Novel insights into the impact of the oil spill on phytoplankton-protozooplankton trophic interactions and carbon fluxes in the SW Mediterranean coast: A field and modeling study
Résumé
The effects of the oil spill, that occurred in SW Mediterranean, on phytoplankton growth and protozooplankton grazing were assessed using field dilution experiments conducted 1, 4, 8, and 18 days after the incident. The ecological impact on carbon flux and planktonic food web structure was analyzed by linear inverse model approach (LIM-MCMC). In the short-term (1-8 days), growth rates and biomasses of nano-and microphytoplankton significantly decreased, due to oil toxicity and/or light reduction. Their grazing by protozooplankton also declined, coinciding with a sharp drop in oil-sensitive heterotrophic dinoflagellates (e.g., Gyrodinium, Protoperidinium) and ciliates (e.g., Strombidium) that feed on them. In the long-term (day 18), oil dispersion and toxicity attenuation allowed regrowth of large phytoplankton and increased protozooplankton grazing, mainly by mixotrophic dinoflagellates (e.g., Heterocapsa) and ciliates (e.g., Strombidium, Tintinnopsis). Picophytoplankton growth was unaffected, but their biomass increased reaching high value on day 18 due to release from protozooplankton grazing, coinciding with the disappearance of heterotrophic nanoflagellates from day 8. LIM-MCMC analysis showed that several carbon fluxes, including primary and bacterial production, zooplankton consumption, respiration, carbon loss and carbon throughput were affected by the oil spill. Food web structure shifted from the herbivorous pathway (day 1) to a multivorous configuration (days 4/8), and toward a microbial pathway on day 18, implying major changes in carbon export efficiency. Our results show that combining field experiments with modeling provides a robust framework for assessing the oil spill impacts on trophic interactions, ecosystem functioning, and management of sensitive coastal areas.