Effect of the water-soluble fraction of diesel oil on bacterial and primary production and the trophic transfer to mesozooplankton through a microbial food web in Yangtze estuary, China

We studied the influence of the water-soluble fraction (WSF) of diesel fuel oil on bacterial and photosynthetic producers and on carbon transfer to higher trophic organisms in an aquatic plankton ecosystem by means of a mesocosm experiment in the Yangtze estuary, China. In the oil-enriched mesocosm, average bacterial production increased 3.9 times compared with that in the control mesocosm. WSF addition also increased the abundance of bacterivorous heterotrophic nanoflagellates, but it decreased the abundance of other higher trophic organisms (micro- and mesozooplankton) due to the toxic effects of the WSF. The amount of carbon transfer from bacterial production to mesozooplankton was of nearly similar magnitude in the oil-enriched and control mesocosms, though bacterial production was much larger in the former. The relative reachable efficiency of bacterial production to mesozooplankton in the oil-enriched mesocosm was only about 36% of that in the control. This lower efficiency in the oil-enriched mesocosm was attributed to decreased mesozooplankton abundance rather than decreased abundance of trophic intermediates in microbial food webs. We found only a slight difference between photosynthetic production in the oil-enriched and control mesocosms. A complementary batch-incubation experiment showed that the WSF may have considerably decreased the photosynthetic activity of an algal assemblage, however, so the photosynthetic production in mesocosms was regulated by nutrient-limitation rather than by the toxic effect of WSF. The carbon transfer from photosynthetic production to mesozooplankton in the oil-enriched mesocosm was comparable to that in the control, though WSF addition decreased the abundance of mesozooplankton in the oil-enriched mesocosm. This may have resulted from the occurrence of the diatom Skeletonema costatum, which is much preferred by mesozooplankton (copepods) as prey, in the oil-enriched mesocosm, whereas the dinoflagellate Prorocentrum dentatum was overwhelmingly dominant in the control mesocosm.