Including different mesozooplankton feeding strategies in a biogeochemical ocean model impacts global ocean biomass and carbon cycle

Mesozooplankton present a wide range of functionally diverse heterotrophic organisms ranging from 200 µm to 2 cm that are essential to marine ecosystems and biogeochemical cycles. In most ocean biogeochemical models, mesozooplankton are represented as a single compartment along with microzooplankton (<0.2 mm), thereby overlooking their large functional diversity. Yet, observational and modelling studies relying on functional trait-based approaches showed how important the functional traits diversity of marine zooplankton is in driving ecosystem dynamics and biogeochemical cycles. Here, we use such a functional trait-based approach by modelling the effect of various mesozooplankton feeding strategies on the ocean carbon cycle, using the global ocean biogeochemical model PISCES. Three new mesozooplankton functional types (PFTs) and their associated trade-offs were integrated into PISCES: cruisers (active swimmers feeding on suspension particles), ambushers (passive suspension feeder, relying on a sit-and-wait strategy) and flux-feeders (passively feeding on particles). An additional foraging effort was implemented for cruisers to account for the optimization of their active behaviour. Our new configuration shows that these functional groups have distinct latitudinal and vertical distributions: the two suspension feeding groups (cruisers and ambushers) share the epipelagic zone, with ambushers being the dominant group globally (0.11 Gt C yr−1, 54.8 % of total mesozooplankton in the top 150 m) and cruise feeders (0.03 Gt C yr−1) prevailing in the productive regions near the poles. Meanwhile, flux-feeders (0.06 Gt C yr−1) dominate in the mesopelagic zone of coastal regions. The change of parameters, thus trade-offs, in our sensitivity experiments also shows how we can modulate and even reverse the latitudinal pattern of suspension feeders. Finally, we demonstrate how the deep-dwelling flux-feeders directly affect carbon export at depth more strongly by consuming the particles that would otherwise be transported to deeper layers (the carbon export increases by 40.8 % when flux-feeders are removed). This study emphasizes the necessity for a better integration of the trophic strategies of this planktonic compartment within global biogeochemical models.