Shifting seasonal timing of peak abundance of two invading ctenophore populations in the Black Sea during the period 1991-2017

Abstract

Factors affecting zooplankton dynamics, especially in shelf-sea ecosystems such as the Black Sea influenced by eutrophication, overfishing, climate variability, and biological invaders, merit special attention for successful development and management. We used coherence functions (phase angle) to analyze time series (918 samples collected every 10 days) of the two invading ctenophores Mnemiopsis leidyi and Beroe ovata and mesoplankton (seven species) in 1991-2017. We observed a constant behavior of focal mesoplankton taxa (no shift of phase angle along the time axis) against such environmental benchmarks as surface temperature and surface chlorophyll concentration. Conversely, development of the ctenophore populations progressively accelerated (phase angle decreased) against both mesoplankton and environmental benchmarks. Mnemiopsis was leading this racing and followed by Beroe: (i) in 1991-1998 (before the Beroe invasion) peaks of Mnemiopsis were lagging by ∼1½ months temperature peaks; (ii) in 1998-2002 (Beroe appeared) Mnemiopsis peaks were shifting earlier in time and (iii) in 2002-2008 they became leading Beroe peaks by ∼1½ months; (iv) in 2009-2013, Beroe peaks were also shifting earlier in time, and (v) in 2013-2017, Beroe has shifted by ∼1½ months and became nearly in-phase with Mnemiopsis. Our results provide a deeper insight into adaptive strategies of invading ctenophores and enable prognoses of ctenophore blooms.