Responses of natural phytoplankton assemblages to anthropic perturbations: the case of a low concentration oil spill scenario

Phytoplankton, playing a key role in marine food webs and carbon fluxes between the atmosphere and the ocean, can be negatively affected by anthropogenic contamination such as hydrocarbon spills. Hydrocarbon spills from catastrophic events as well as from chronic, limited leaks, represent one of the main pollution sources in oceans. Low concentration inputs, mostly from highly toxic refined oils, are less studied than crude oil spills, although their effects could be significant for marine organisms’ phytoplankton ones. The main objective of the present study was to examine the responses on the biomass, density and taxonomic/pigments composition of natural phytoplankton assemblages exposed to a low-concentration diesel pollution scenario, using an experimental mesocosms approach. One control and two kinds of exposures were simulated, one with a single addition (20 mL) of diesel (Ci = 0. 845 mg L-1) during the phytoplankton bloom, and the other one with the same diesel concentration, but “split” twice and added respectively during the pre-bloom and the bloom periods of phytoplankton succession. The mainly significant effects were detected during the post-bloom in both exposure cases. Diesel exposures significant decline both phytoplankton biomass (derived from Chl a and pigment concentration or carbon estimation) and cell density of the main functional groups identified by different independent techniques (flow cytometry, image analysis, microscopical observation). One of the main results highlighted a shift in phytoplankton composition under diesel exposure from a diatom-dominated phytoplankton assemblage to a small-dominated one (nano-flagellates and cyanobacteria). Changes from assemblages dominated by large cells to other dominated by small cells in response to diesel addition may affect the pelagic food webs in the area, resulting in faster carbon cycling in the ocean surface with the consequent decrease in the biological pump. Our study suggests that Lower Saint Lawrence’s estuarine ecosystem remains a fragile environment, which would be strongly impacted by the increase in global transportation of oil products