Including filter-feeding gelatinous macrozooplankton in a global marine biogeochemical model: model–data comparison and impact on the ocean carbon cycle

Filter-feeding gelatinous macrozooplankton (FFGM), namely salps, pyrosomes and doliolids, are increasingly recognized as an essential component of the marine ecosystem. Unlike crustacean zooplankton (e.g., copepods) that feed on prey that are an order of magnitude smaller, filter feeding allows FFGM to have access to a wider range of organisms, with predator-over-prey size ratios as high as 105:1. In addition, most FFGM produce carcasses and/or fecal pellets that sink 10 times faster than those of copepods. This implies a rapid and efficient export of organic matter to depth. Even if these organisms represent < 5 % of the overall planktonic biomass, their associated organic matter flux could be substantial. Here we present a first estimate of the influence of FFGM on the export of particulate organic matter to the deep ocean based on the marine biogeochemical model NEMO-PISCES (Nucleus for European Modelling of the Ocean, Pelagic Interaction Scheme for Carbon and Ecosystem Studies). In this new version of PISCES, two processes characterize FFGM: the preference for small organisms due to filter feeding and the rapid sinking of carcasses and fecal pellets. To evaluate our simulated FFGM distribution, we compiled FFGM abundance observations into a monthly biomass climatology using a taxon-specific biomass–abundance conversion. Model–observation comparison supports the model's ability to quantify the global and large-scale patterns of FFGM biomass distribution but reveals an urgent need to better understand the factors triggering the boom-and-bust FFGM dynamics before we can reproduce the observed spatio-temporal variability of FFGM. FFGM substantially contribute to carbon export at depth (0.4 Pg C yr−1 at 1000 m), particularly in low-productivity regions (up to 40 % of organic carbon export at 1000 m), where they dominate macrozooplankton biomass by a factor of 2. The FFGM-induced export increases in importance with depth, with a simulated transfer efficiency close to 1.