Trait-based modeling of marine mesozooplankton feeding strategies at the global-scale

Marine mesozooplankton gather planktonic animals between 0.2 and 20 mm. They are one of the most studied zooplankton size classes and are essential in marine food webs and biogeochemical cycles. In most ocean biogeochemical models, zooplankton are generally represented as size classes, with micro- (<0.2 mm) and meso-zooplankton, overlooking the rest of the functional diversity of marine zooplankton. Yet, studies have shown the key role this diversity can play in ecosystem dynamics. This argues for the need to develop a more precise representation of zooplankton functional diversity by explicitly taking into account additional functional traits, i.e. individual characteristics of organisms that impact their fitness. Among these functional traits, feeding strategy is a poorly studied but key trait that relates to energy intake, predation risk, energetic losses and mate finding.

In this study, we implemented several feeding strategies of mesozooplankton in the ocean biogeochemical model PISCES. Three typical mesozooplankton functional types (PFTs) were considered: cruisers (active swimming feeding on suspension particles), ambushers (passive, relying on a sit-and-wait strategy) and flux-feeders (passively feeding on particle flux). Instead of a classic suspension-feeding (cruisers and ambushers), flux-feeders favor feeding on rapidly sinking particles that would otherwise be transported at depth, directly acting on carbon transfer from the euphotic zone to the mesopelagic. 

Simulations have been performed using the NEMO-PISCES model at global scale. Our results highlight the distinct global, regional and vertical distributions of the different groups, with suspension feeders being dominant in the surface layers and flux-feeders being more abundant below the euphotic layer. The different contributions to biogeochemical fluxes is also presented, in particular flux-feeders play a major role in the global carbon cycle, directly impacting the carbon export to deep waters. This work contributes to better understanding the ecology of mesozooplankton at global scale and the role of different feeding strategies in the oceans. Thus, our findings offer new insights on the link between plankton diversity and marine ecosystem functioning and emphasize the necessity for a better integration of mesozooplankton trophic strategies within global biogeochemical models.