Autumnal bottom-up and top-down impacts of Cyanea capillata: a mesocosm study

Jellyfish are effective predators on mesozooplankton and release large amounts of dissolved organic matter. Nevertheless, jellyfish initiated trophic cascades and bottom-up influences impacting lower trophic levels have received limited attention. We conducted a mesocosm experiment to quantify simultaneous top-down and bottom-up effects of a common jellyfish, Cyanea capillata, in a natural plankton community during autumn. Treatments were 0, 2 or 5 jellyfish per 2.5 m(3) mesocosm, four replicates each, with initial additions of inorganic nutrients. Primary and bacterial production, species abundance and composition of several trophic levels and nutrient and carbon dynamics were followed during the 8-day experiment. Multivariate statistics and generalized additive mixed modelling were applied to test whether jellyfish carbon concentration (0- 1.26 mg jellyC L-1) in the mesocosms affected the variables monitored. Unexpected negligible predatory impact of jellyfish on mesozooplankton was observed, potentially related to jellyfish senescence. Community compositions of bacteria, phytoplankton and mesozooplankton changed with time, but did not differ between treatments. However, nutrient regeneration by jellyfish was evident, and jellyfish had a positive impact on total and specific bacterial production, total primary production and the >10 mu m chlorophyll a fraction. Bottom-up influences from abundant jellyfish could thus stimulate productivity in nutrient depleted autumnal surface waters.