Interactions between the invading ctenophores Mnemiopsis leidyi (A. Agassiz) and Beroe ovata Mayer 1912, and their influence on the Pelagic ecosystem of the Northeastern Black Sea

Abstract

The state of the northeastern Black Sea, affected by an invasion of Beroe cf ovata in 1999-2002, is compared with the period of the M. leidyi invasion, before and after the arrival of Beroe. Development of Beroe considerably depressed Mnemiopsis leidyi, which had destabilized the Black Sea ecosystem for over a decade. The relationship between the seasonal populations of the two comb-jellies is discussed. Changes at all trophic levels after the arrival of M. leidyi and after that of Beroe are analyzed. With a reduction of M. leidyi a near-immediate recovery of the main components of the pelagic ecosystern-zooplankton (including meroplankton), phytoplankton, dolphins and fish as well as their eggs and larvae occurred. Microplankton biomass became even higher than under the presence of M. leidyi. The M. leidyi invasion caused cascading effects: bottom-up effects included a decreasing zooplankton stock, linked to collapsing planktivorous fish, to vanishing dolphins. Top-down effects included a decrease in zooplankton stock, causing an increase in phytoplankton, relaxed from grazing pressure, and increasing bacterioplankton triggering increases in zooflagellates and infusoria. The development of Beroe reversed the situation: promptly, the ecosystem began to recover, at all trophic levels.