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

Abstract. 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 have shown how important the diversity of the functional traits of marine zooplankton is in driving ecosystem dynamics and biogeochemistry.

Here, we use such a functional trait-based approach by modelling the effect of various mesozooplankton feeding strategies on the ocean carbon cycle globally, using the ocean biogeochemical model PISCES. Three new mesozooplankton functional types (PFTs) and their associated trade-offs were integrated in 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 for cruisers was implemented 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 share the epipelagic zone, with ambushers being the dominant group at global scale (0.11 GtC/yr, 54.8 % of total mesozooplankton in the top 150 m) and cruise feeders (0.03 GtC/yr) prevailing in the productive regions near the poles, whereas flux-feeders (0.06 GtC/yr) dominate in the mesopelagic zone of coastal regions. We also 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 provides more knowledge on functional diversity of mesozooplankton and emphasizes the necessity for a better integration of the trophic strategies of this planktonic compartment within global biogeochemical models.