Effects of Beroe cf ovata on gelatinous and other zooplankton along the Bulgarian Black Sea Coast

Our study revealed a high inter-annual variability of the mesozooplankton and of Mnemiopsis after the arrival of Beroe ovata. The period 1998–2001 can be considered as transitional, with the zooplankton still vulnerable to Mnemiopsis predation pressure and under adjustment to the new prey-predator couple Beroe-Mnemiopsis. Mnemiopsis decreased relatively compared to the period before Beroe(1994–1996), which could be attributed to control by B. cf ovata after 1998. Despite this inter-annual variability, the recent features of mesozooplankton performance, viz. species number, numerical abundance and biomass, show signs of recovery. These are widely believed to reflect a reduced top-down control by Mnemiopsis. Still, Mnemiopsis remains a key control on zooplankton growth, especially in summer, as illustrated by our PCA analysis.