Did the closure of the Central American Seaway induce a major reorganisation of the planktic ecosystem?

During the Pliocene the planktic ecosystem, for the first time in its evolutionary history, experienced a separation of the tropical seaways. At the same time, low latitude planktic foraminifers reached sizes not seen for millions of years. We set out to examine whether the closure of the Central American Seaway (CAS) led to the reorganisation of the planktic food web and enabled this growth. As many plankton are not well preserved in the fossil record, we applied a trait-based ecosystem model for plankton – ForamEcoGEnIE – to an open and closed CAS Pliocene environment. ForamEcoGEnIE is an extension of the size-structured 3-D plankton ecosystem model, “EcoGEnIE” that includes non-spinose planktic foraminifers as a new functional group based on the costs and benefits of key traits (e.g. growth, grazing, calcification). We test whether the planktic food web and planktic foraminiferal physiology responded to this change in paleogeography by quantifying changes in plankton biomass and size. In large regions of the ocean, we observe no change in phytoplankton, zooplankton, or foraminiferal biomass in response to a closed CAS. However, we note an increase in biomass at the eastern equatorial Pacific and a decrease in the North Atlantic in response to the closure of the CAS. ForamEcoGEnIE predicts an increase in non-spinose foraminiferal body size at the eastern equatorial Pacific and a small decrease in the North Atlantic. We attribute the Pacific response to increased upwelling due to the closure of the CAS and in the North Atlantic we suggest the reduction in biomass and size is linked with the reorganisation of surface ocean currents. As much of the ocean shows no response, we tentatively conclude that the closure of the CAS did not induce a major reorganisation of the planktic ecosystem.