Top-predator effects of jellyfish Odessia maeotica in Mediterranean salt marshes

Jellyfish can act as planktonic top predators, and their effects may cause drastic changes in the plankton structure of marine and freshwater systems. However, the top predator effects may not necessarily be the same in species-poor habitats as they are in species-rich habitats. The present study analyses the effects of the small lacunae jellyfish Odessia maeotica in a species- poor habitat, Mediterranean salt marshes in the wetlands of Emporda (NE Iberian Peninsula). A field experiment was carried out in March 2008 to assess the direct and indirect effects of O. maeo- tica on plankton composition. Our results show that the presence of O. maeotica changed the plank- ton composition through top-down effects. Changes were strong in zooplankton, because O. maeot- ica can suppress almost the entire trophic level of large zooplankton (> 50 µm). Weak indirect effects on phytoplankton composition were observed as well. When O. maeotica was present, changes in the relative abundance of the phytoplankton species were found, but there was no net increase in phytoplankton biomass. Our results suggest that these weak indirect effects may be the result of trophic cascade effects coupled with the oligotrophic conditions of these salt marshes. Thus, trophic cascade effects lead to an increase in ciliate biomass, and these ciliates would feed on small algae (jellyfish-copepods-ciliates-small algae), while oligotrophic conditions would prevent increases in algal biomass.