Top-down regulation in ctenophore-copepod-ciliate-diatom-phytoflagellate communities in coastal waters: a mesocosm study

Interactions between the ctenophore Pleurobrachia pileus, copepods, ciliates and phytoplankton in a late spring natural plankton community were studied experimentally using mesocosms consisting of 300 1 polyethene cylinders, Mesocosms were filled with 90 pm filtered nutrient-enriched seawater from Gullmar Fjord on the Swedish Skagerrak coast. Mesozooplankton (primarily copepods and marine cladocerans) or ctenophores were added, either alone or in combination, to examine top-down predation and grazing effects on phytoplankton abundance and composition, Predation impact of ctenophores on copepods appeared to be minimal in that it did not significantly decrease the biomass of the copepods until the last day of the experiment, However, in the presence of ctenophores, ciliates increased compared to controls both with and without added zooplankton, suggesting that ctenophore predation on copepods reduced copepod predation on ciliates, In the absence of ctenophores, ciliates declined precipitously, presumably due to copepod predation. This was particularly severe in treatments with zooplankton added at 10 x the natural abundance but without ctenophores, Copepods heavily grazed on the diatom Skeletonema costatum, and most phytoflagellates increased in mesocosms with copepod additions, suggesting that they were not substantially grazed by copepods. However, copepods did graze to some extent on larger dinoflagellates of the genus Gymnodinium/Karenia, Ciliates were important phytoplankton grazers, especially of phytoflagellates and picoplankton. It appears that the primary top-down effect of ctenophore predation on copepods was to reduce copepod predation on ciliates, thereby increasing ciliate grazing on the small flagellates that dominated the phytoplankton.