Effects of dispersed and adsorbed crude oil on microalgal and bacterial communities of cold seawater.

: Mesocosm facilities consisting of five 3.5 m(3) stainless steel tanks filled with seawater from the St Lawrence Estuary (Québec, Canada) were used to conduct a 2 month experiment under the natural conditions prevailing at the end of the winter in subarctic environments, with seawater temperatures ranging from-1.5°C (surface ice cover) to 3°C. Various oil treatments were simulated in mesocosms: Forties crude oil was chemically dispersed, adsorbed onto an immersed substrate and spilled without any treatment. Total oil concentrations ranged from <1 mgl(-1) (untreated oil) to 44.6 mgl(-1) (dispersed oil). Contrasting with the parent crude oil and dispersed oil, the dissolved phase was enriched with low molecular weight polycyclic aromatic hydrocarbons (PAH). As revealed by pH variations, chlorophyll a contents and degraded pigments, the phytoplankton growth was inhibited early in tanks contaminated with dispersed and adsorbed oil. Although global measurements showed a recovery of the microalgal activity while the dispersed oil was diluted in a flow-through cascade system, the specific composition was quite different from the control, with an increase in small microflagellate species and a marked decline in the diversity of centric diatoms. Small microflagellates also dominated in the heavily oil-contaminated surface microlayer. The growth of viable heterotrophic bacteria (VHB) was immediately stimulated by both dispersed and adsorbed oil (10(4)-10(5) colony forming units per ml) and oil-degrading bacteria (ODB) reached maximum densities (10(2) CFU ml(-1)) later in the experiment. The adaptation of the indigenous community was assessed using the ODB/VHB ratio, which increased by ten times in the seawater contaminated with dispersed oil. No significant bacterial enhancement was observed in the tank that received untreated oil. No bacterial enrichment was found in the surface microlayer. In sediment traps, the bacterial density increased with the amount of total settling matter and oil residues.