Modeling the response of top-down control exerted by gelatinous carnivoreson the Black Sea pelagic food web

Recent changes in structure and functioning of the interior Black Sea ecosy stem are studied by a series of simulations using a one-dimensional, vertic ally resolved, coupled physical-biochemical model. The simulations are inte nded to provide a better understanding of how the pelagic food web structur e responds to increasing grazing pressure by gelatinous carnivores (medusae Aurelia aurita and ctenophore Mnemiopsis leidyi) during the past 2 decades . The model is first shown to represent typical eutrophic ecosystem conditi ons of the late 1970s and early 1980s. This simulation reproduces reasonabl y well the observed planktonic food web structure at a particular location of the Black Sea for which a year-long data set is available from 1978. Add itional simulations are performed to explore the role of the Mnemiopsis-dom inated ecosystem in the late 1980s. They are also validated by extended obs ervations from specific years. The results indicate that the population out breaks of the gelatinous species, either Aurelia or Mnemiopsis, reduce meso zooplankton grazing and lead to increased phytoplankton blooms as observed throughout the 1980s and 1990s in the Black Sea. The peaks of phytoplankton , mesozooplankton, Noctiluca, and gelatinous predator biomass distributions march sequentially as a result of prey-predator interactions. The late win ter diatom bloom and a subsequent increase in mesozooplankton stocks are ro bust features common to all simulations. The autotrophs and heterotrophs, h owever, have different responses during the rest of the year, depending on the nature of grazing pressure exerted by the gelatinous predators. In the presence of Mnemiopsis, phytoplankton have additional distinct and pronounc ed bloom episodes during the spring and summer seasons. These events appear with a 2 month time shift in the ecosystem prior to introduction of Mnemio psis.