Effect of Non-Native Ctenophore Beroe ovata on Invader Mnemiopsis leidyi and Mesozooplankton in the South-Western Caspian Sea

Invasive species are among the most threatening factors to the Caspian Sea’s diversity and unique aquatic resources. After the appearance of the ctenophore predator Beroe ovata in 2019, the dynamics of the previous ctenophore invader Mnemiopsis leidyi and mesozooplankton stocks in the south-western Caspian Sea were investigated in December, April, July and November 2020–2021. The invasion of the Caspian Sea by the voracious zooplanktivorous M. leidyi by the end of the 1990s had devastated trophic levels of life in this enclosed ecosystem causing the complete disappearance or severe decreases of many zooplankton species. Since its first appearance, the number of B. ovata increased 1.5-fold between 2019, 2020 and 2021. Conversely, the average number of M. leidyi had decreased from an average (abundance ± standard deviation) 314 ± 104 ind. m–3 (n = 485) for the period from 2001–2019 to 141 ± 152 ind. m–3 (n = 48) in 2020 and 2021 which is the lowest reported average value for M. leidyi. Simultaneously, both the species number and abundance of mesozooplankton in 2020 and 2021 were observed to increase compared to previous years. From one Copepoda species in the 2001–2018 period, three species; Acartia tonsa, Calanipeda aquaedulcis and Halicyclops sarsi increased in 2020 and 2021. After years of disappearance, the Cladocera Evadne ananyx typica, Podonevadne camptonyx and the endemic Podonevadne trigona were again present in the samples. In this study, abundances of several species (such as the Copepoda A. tonsa, the Cladocera Podon polyphemoides), Bivalvia larvae and Cirripedia larvae) were observed to increase substantially (between 2 to 7 fold) compared to the 2001–2018 period. The mass appearance of B. ovata in the southern Caspian Sea since 2019, through its predation on M. leidyi, has resulted in the rise of species numbers and mesozooplankton abundance in the south-western Caspian Sea. Our results here support earlier results of observations performed in for the Black Sea where the ctenophore predator Beroe ovata had positive cascading effects on mesozooplankton in particular and on the ecosystem in general.