Subtle effects of the water soluble fraction of oil spills on natural phytoplankton assemblages enclosed in mesocosms

Four mesocosm experiments were conducted to evaluate the effect of episodic oil spills on coastal marine phytoplankton assemblages. The experimental design was selected to simulate the Prestige oil spill, which occurred in Galician coastal waters (NW Iberia) in November 2002. The empirical results indicate that no significant direct effects of the water soluble fraction of oil (20–60 μg l−1 of chrysene equivalents) on phytoplankton biomass and production were observed immediately after oil additions. Despite this, subtle negative effects on primary production were detected using a modelling approach, being the impact lower on phytoplankton communities dominated by diatoms. Consistent with the reduced direct effect of oil additions on phytoplankton biomass and photosynthesis-related variables, no indirect trophic cascading effects, previously reported in microcosm experiments, were detected. This shows that the effect of punctual inputs of the water accommodated fraction of oil on natural phytoplankton communities was very subtle, undetectable on some occasions, and of much lower magnitude than the effects recorded in microcosm experiments. This suggests that the initial composition of the phytoplankton community determines the degree of response and that the experimental approach adopted could explain the different, and sometimes contradictory, reported responses of the planktonic community to the input of oil into the marine environment.