Interactions between invasive ctenophores in the Black Sea: assessment of control mechanisms based on long-term observations

International audienceInvasion of the carnivorous ctenophore Mnemiopsis leidyi in the Black Sea in the 1980s disrupted the ecosystem, which started to recover with the arrival of the predatory ctenophore Beroe ovata in 1997. We used the results of 25 yr of field observations and experiments in the northeastern Black Sea to assess 3 hypotheses that should explain most of the population dynamics of M. leidyi and B. ovata. The first hypothesis is that since its arrival, B. ovata has controlled the period of the year during which M. leidyi was present in sizable concentrations. This hypothesis is supported by the observation that M. leidyi abundance was sizable almost year-round (spring, summer, autumn) before the arrival of B. ovata but was sizable only for a period of 1.3 to 3.1 mo (mostly summer) after its arrival. The second hypothesis is that the same sequence of predator prey mechanisms that led B. ovata to shorten the duration of a sizable M. leidyi population occurred every year irrespective of interannual environmental variability. This is supported by the repetition of the same reproductive sequences of the 2 ctenophores yearly since 1999 despite differences in environmental factors. The third hypothesis (i.e. environmental conditions influenced the joint abundances of the 2 species) is supported by the observed covariability between the 2 species every year. Experimental and field results identified temperature, food and wind as the key factors influencing M. leidyi, which suggested that the interannual environmental variations that affect M. leidyi abundance cause proportional changes in B. ovata abundance. Some aspects of these hypotheses have been previously examined in the literature, but this is the first study in which they are assessed using a consistent set of data