Are changes in the structure of nematode assemblages reliable indicators of moderate petroleum contamination?

- We assessed short-term responses of nematode assemblages to a diesel oil spill.
- The experimental oil spill had no effect on total density and number of nematode taxa.
- Exposed assemblages were similar to those in controls from before to after the spill.
- Nematode taxa were resilient and able to survive in moderately contaminated areas.

This study assesses through a multiple before-after-control-impact (MBACI) design the effects of diesel oil on the structure of nematode assemblages in unvegetated tidal flats of a subtropical estuary. Oil-exposed treatments were contrasted with controls for a duration of four successive days before and after an experimental spill in three distinct areas of the Paranaguá Estuarine Complex (Southern Brazil). No significant differences were observed in nematode total density, number of taxa and the overall assemblage structure between the control and impact treatments from before to after the experimental spill. This reinforces the idea that, despite being good indicators of environmental stress, free-living marine nematodes are able to tolerate low concentrations of hydrocarbons and to survive in moderately contaminated areas. We also show that robust experimental designs are useful to avoid confounding expected natural variability with the effects of a mild impact.