Microzooplankton grazing in the oceans: impacts, data variability, knowledge gaps and future directions

Abstract

Here we review all published data on phytoplankton growth and microzooplankton grazing using the dilution technique to better understand the role of this group of grazers in different regions of the oceans, and to identify the knowledge gaps that require future efforts. A total of 1525 data points assimilated from 110 studies were included and grouped using the biogeographic subsets defined by Longhurst et al. [(1995) An estimate of global primary production in the ocean from satellite radiometer data. J. Plankton Res., 17, 1245–1271]. Total median phytoplankton growth rates in each of the subsets varied between 0.15 (Polar Southern) and 0.83 day−1 (Trades Atlantic), with the corresponding microzooplankton grazing rates ranging between 0.07 (Polar Southern) and 0.48 day−1 (Trades Indian). The median percentage of primary production (PP) grazed by microzooplankton was relatively constant among the regions and ranged from 49 to 77%, with the lowest percentage found in the Westerlies Southern and the highest in the Coastal Indian. Despite strong evidence that microzooplankton grazing impacts were highly variable, no global drivers for this variability (temperature, chlorophyll concentration) were identified in the analysis. As a global carbon budget, the overall consumption of phytoplankton for all regions was 31.3 Gt C year−1, which accounted for 62.4% of the total PP grazed daily. The amount of carbon ingested by micro- and mesozooplankton varied proportionally to the PP and the consumption was five times higher for micro- than for mesozooplankton. As concluding remarks of the study, we present some suggestions that may improve the quality of the data obtained with the dilution technique.