Influence of top-down control in the plankton food web on vertical carbon flux: a mesocosm study in the Chesapeake Bay

The effects of predation on carbon export in planktonic food webs are poorly known, but likely play a key role in the biological pump. Gelatinous zooplankton (GZ) dominate the zooplankton community in the Chesapeake Bay during summer months, exerting considerable top-down control on the planktonic food web. The medusa Chrysaora quinquecirrha preys upon the ctenophore Mnemiopsis leidyi, which in turn is a major predator of the omnivorous copepod Acartia tonsa. This trophic cascade is known to significantly affect copepod abundance in Chesapeake Bay, but the resulting changes to particulate organic carbon (POC) flux are unknown. We hypothesized that additions or exclusions of GZ predators would result in changes in both total POC flux and the composition of exported particles (e.g., phytoplankton aggregates, fecal pellets). We conducted mesocosm experiments in the York River tributary of Chesapeake Bay during summer and fall, 2015 to quantify the cascading effects of GZ blooms on POC flux. The mesocosms contained a natural assemblage of phytoplankton and microzooplankton, and A. tonsa copepods, and received one of four treatments of GZ: 1) a control with no GZ added, 2) addition of ctenophores, 3) addition of medusae, and 4) addition of both ctenophores and medusae. POC flux from each mesocosm was measured over multiple 2-day experimental runs and grazing rates of GZ on each other and on copepods were calculated. There were no significant differences in total POC flux between treatments, but the composition of both the final zooplankton assemblage and exported organic matter differed between treatments. As a result of grazing on copepods by ctenophores, treatments which included GZ had lower final copepod abundances and a corresponding decrease in flux of copepod fecal pellets. We discuss how this change in composition of exported material as a result of cascading trophic interactions may affect the efficiency of the biological pump and benthic processes.